formation and evolution of the tancheng lujiang wrench fault system a major shear system to the northwest of the pacific ocean abstract in the northwestern peri pacific tectonic belt on the continental margin of east asia there is a series of major faults trending ne nne i e the tancheng lujiang wrench fault system this fault system extends more than 5000 km with a width of 800 1000 km in the continent and a general strike of n30 e on the basis of the results of special research on the major faults and a lot of new information about the fault system the authors discuss early mesozoic triassic jurassic period sinistral transcurrent faulting in the different regions it is suggested that in the eastern continent of east asia the e w trending tectonic pattern of the paleozoic and beginning of the mesozoic appear in the nuclei of the sino korean continent massif and the yangtze continent massif these ancient continent massifs accreted both southwards and northwards which finally led to their collision and suturing to the angura continent massif later this tectonic framework was changed greatly by the tancheng lujiang wrench fault system along which left lateral horizontal displacement of 1000 1500 km took place since the mesozoic the dislocation and deformation of horizontal displacement of the tancheng lujiang fault system produced a series of structures trending ne nne in the continental margin which have greatly transformed the ancient structures of the preceding mesozoic this is an important diktyogenese in the continent of east asia during the horizontal displacement motion of these faults the folds nappes and decollement structures pull apart basins and subductions within the continent occurred in the continent at the same time some large sinistral ductile shear zones of horizontal displacement appeared the depth of the deformed horizon of ductile shear which belongs to simple shear was controlled by the tectonic element and distance away from the continental margin whilst the ductile shear deformation occurred the fault dynamo thermal metamorphism and the fault remelting took place and magma activity appeared within the continent in the mesozoic the fault zones used to be high energy zones and recently became the seismic belts within the continent formation of the tan lu fault system was due to the collision between the southern and northern blocks its large sinistral horizontal displacement in the mesozoic was closely related to the kula plate moving northwards therefore the continent disintegrated by shearing which produced some new terranes the shear edge belt was formed between the continent and ocean the change of relative motion between the continent and pacific ocean plate transformed the tan lu fault system into first extension and then compression clarifying the characteristics of the tan lu fault system is an important key in understanding the evolution of the east asia continent since the mesozoic

the geometry and petrogenesis of hydrothermal dolomites at navan ireland the dolomites at navan ireland formed in courceyan peritidal and shallow shelf limestones the dolomite body has a plume like geometry cross cutting both lithological boundaries and diagenetic barriers generated by sea floor cementation and emergence the dolomitizing fluids rose parallel to major faults to diffuse laterally through the succession controlled by variations in permeability that reflect both facies variation on deposition and pre dolomitization diagenesis cathodoluminescent zones reveal three principal stages of dolomite emplacement separated by dissolution surfaces with each stage reflecting several changes in the character of dolomitizing solutions the predominance of dull zones indicates burial rather than surface conditions the dolomites formed some time after burial in response to an areally limited hydrothermally driven flow isotope values 18o of 6 6 to 10 4 and 13c of 0 2 to 2 5 pdb and fluid inclusion data suggest that these fluids had compositions similar to those of carboniferous seawater however these became hotter with time with temperature increasing from 60 to 160 c the navan dolomites are closely associated with europe s largest zinc lead deposit the distribution of the ores follows the same trend as that of the dolomites and paragenetic relationships indicate that dolomitization and mineralization were temporally and genetically related

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

deep shallow coupling response of the cenozoic bohai bay basin to plate interactions around the eurasian plate abstract the bohai bay basin bbb is a meso cenozoic intracontinental petroliferous basin where four pre existing continental scale fault zones intersect in east china the bbb has experienced complex deformational and evolutionary history that records significant information about how the lithosphere there has responded to the coeval plate interactions around the eurasian plate and or their resulting deep crust mantle processes in the cenozoic now both deep and shallow deformational and evolutionary processes of the bbb have been well studied but many geodynamic mechanisms previously proposed for those processes are too general to explain them well based on various data and principles we first summarized both deep and shallow responses and processes of the bbb in the cenozoic and then matched reasonable geodynamic mechanisms for each of them the results show as follows the cenozoic differential reactivation of pre existing continental scale weak zones controlled the coeval deep shallow differential coupling evolution of the bbb the paleogene extension in the bbb was mainly caused by crust mantle processes triggered by the stagnant izanagi slab the cenozoic strike slip faulting was influenced by various plate interactions in which the oblique subduction and anticlockwise rotation of young pacific plate resulted in the complex pre oligocene strike slip history of the tan lu fault zone to the south of the zhang peng fault zone while the india eurasia collision has caused the intense and synchronous strike slip faulting of various faults there since 35 ma which was then weakened by the oblique subduction and clockwise rotation of young philippine sea plate in the neogene the compression resulting from both the india eurasia collision and the philippine sea plate s subduction contributed to the latest paleogene tectonic inversion in the bbb the arrival of mantle flow triggered by both the india eurasia collision and the stagnant slab of the pacific plate resulted in the tectonic reactivation there in the late neogene

origin of massive dolostone the upper knox model abstract previous studies of cambro ordovician dolostones from various sites in the u s midcontinent revealed similar textures and geochemistry prompting similar explanations of their origin before these interpretations can be incorporated in a model to explain other massive dolostones they need verification in a contrasting setting the dupont geohydrological survey dghs well continuously cored through the knox group in west central tennessee provides such a contrast this core provides a complete sample of the knox in the setting of a shallowly buried flat lying structurally and stratigraphically simple platform sequence which contrasts with the relatively deeply buried and structurally complex environments that dominate most prior studies light and luminescence petrography xrd sem eds xrf esr and mass spectrometry were used to document the petrology and geochemistry of the upper knox group lower ordovician from 456 m to 1126 m subsea 2018 ft to 4215 ft below the kelly bushing of the dghs well of the 87 samples 81 are dolostone early e 1 dolomite forms a fine to medium crystalline replacive dolostone that lacks luminescence banding its geochemical signature is 13c 2 83 0 76o oo 18o 5 85 2 01o oo stoichiometry 51 6 1 2 mole ca mnpr 79 27 fe 681 237 ppm mn 79 32 ppm and sr 110 63 ppm e 2 dolomite forms a medium to coarse crystalline replacive dolostone that luminescences with a relatively dull core and brighter red rim its geochemical signature is 13c 2 07 0 32o oo 18o 6 96 1 28o oo stoichiometry 51 0 1 3 mole ca mnpr 74 20 fe 880 843 ppm mn 84 52 ppm and sr 79 29 ppm late l 1 dolomite lines vugs and fractures three limestone beds are unaltered remnants of the precursor to e 2 dolomite calcite is present both as relics in dolomite rhombs and as a vug and fracture fill postdating l 1 dolomite three quartz arenite beds and the presence of 1 to 5 well rounded quartz sand grains in many dolostones denote an intermittent clastic source e 1 dolomite began as syndepositional dolomicrite deposited in an upper intertidal to supratidal environment and it was later modified during shallow burial e 2 dolomite formed when subtidal grainstones and packstones were dolomitized during shallow burial l 1 is a late void fill texture and geochemistry of dolostone in the dghs well is similar to that from coeval appalachian ozark and texas oklahoma units numerous vugs fractures and zones of rubble occur throughout the upper knox core suggesting that multiple exposures occurred during its deposition there is no pattern of change with depth for these features or for any other measured or observed parameter we suggest that massive dolostone formation involving dolomite grain growth calcite replacement and dolomite cement took place in association with a succession of exposure related events the large volume of dolomite thus created required a large amount of magnesium so large in fact that a volume of seawater equal to that of the entire modern ocean must have pumped through collective upper knox exposure surfaces

irish carboniferous conodonts record maturation levels and the influence of tectonism igneous activity and mineralization organic maturation patterns in the lower carboniferous of ireland have been determined from conodont colour alteration indices cai for some 750 localities onshore ireland conodonts record increasing maturation in the carboniferous rocks from the oil window rocks of the north east southwards to the greenschist meta argillite regime of the munster basin in detail the caledonoid trend has a marked influence in the midlands whilst in the south sharp variations in cai are interpreted as being caused by thrusting of the variscan orogeny in the south east low maturities in the wexford half graben indicate that the region was shielded from orogenic stresses probably by the leinster massif igneous centres such as the carlingford complex and croghan hill produce local sites of increased palaeotemperature significant cai variations have also been noted in proximity to major base metal orebodies the cai results highlight north east ireland and the wexford half graben as areas with hydrocarbon potential

feldspar albitization in cretaceous non marine mudrocks gyeongsang basin korea plagioclases and k feldspars in the sandstones and mudrocks of the cretaceous non marine gyeongsang basin korea were partially to completely albitized the preservation of fresh plagioclase grains in early micrite cemented sandstones suggests the diagenetic origin of albite albitization textures in mudrocks were examined using backscattered electron images in contrast to completely albitized plagioclase grains in sandstones those in mudrocks are mostly partially albitized it suggests that mudrocks can be more useful for a provenance study than sandstones by preserving detrital minerals better k feldspar is unaltered to partially albitized in both sandstones and mudrocks in mudrocks albitization starts preferentially along microfractures cleavages and grain margins albitization along grain margins seems to be a characteristic feature in mudrocks where development of microfractures in silt sized feldspar grains by physical compaction is limited by clayey matrix as well as by overpressure the extent of albitization in mudrocks is mainly controlled by composition of the detrital plagioclase mudrocks containing calcic plagioclase grains tend to be more extensively albitized than those containing sodic varieties

biotite stability in peraluminous granitic melts compositional dependence and application to the generation of two mica granites in the south bohemian batholith bohemian massif czech republic abstract crystallization experiments have been conducted in the system na2o k2o mgo feo al2o3 sio2 h2o with 4 normative corundum in order to constrain the stability of biotite as a function of water activity and the mg of biotite mg mg fetotal in equilibrium with peraluminous granitic melts the temperature at which biotite breakdown starts is strongly dependent on the mg of biotite at 500 mpa the temperature of biotite breakdown to form orthopyroxene increases from 750 c to 830 c as the mg of biotite increases from 0 4 to 0 5 considering that the system investigated is relevant for ca poor peraluminous biotite bearing rocks metapelites the biotite dehydration curves obtained are used to discuss the melting reactions and the temperatures that lead to the formation of two distinct types of two mica granites found in the south bohemian batholith specifically the eisgarn and destna granites the phase relationships were determined experimentally for the composition of these two granites in order to constrain the composition of the biotite in equilibrium with the melt in the protoliths we demonstrate that eisgarn granitic melts may have been generated at temperatures in the range 830 850 c from melting reactions involving biotite with a mg up to 0 5 as a reactant in contrast destna granitic melts cannot have been generated from dehydration melting reactions involving biotite

regional dolomitization of the waulsortian limestone in southeastern ireland evidence of large scale fluid flow driven by the hercynian orogeny the lower carboniferous waulsortian limestone has been affected by late diagenetic regional burial dolomitization over an area of 7000 km 2 extending 170 km northeastward from the hercynian front the dolostone is composed of two components 1 a very fine crystalline replacive component and 2 a coarse crystalline baroque component that fills vuggy porosity developed within the replacive dolostone fluid inclusions within the coarse crystalline component of the regional dolostone indicate that the dolomitizing solutions were warm 100 c and moderately saline 10 13 wt nacl equivalent the replacive dolomite displays systematic decreases in crystal size and degree of xenotopic texture northward both components of the regional dolostone display a regular northward increase in mean 18 o values the replacive dolomite has 87 sr 86 sr values that decrease northward these laterally variable characteristics suggest that regional dolomitization resulted from large scale north directed fluid flow of heated brines developed in response to topographic uplift associated with collapse of the continental margin south of ireland during the hercynian variscan orogeny

a pseudo adakite derived from partial melting of tonalitic to granodioritic crust kyushu southwest japan arc abstract cretaceous peraluminous granitic intrusions occur in kyushu in the southwest japan arc as batholiths or stocks accompanied by metaluminous tonalite to granodiorite the peraluminous intrusions are classed as adakitic based on sr y y ppm relationships but they contain abundant k feldspar a phase not common in typical adakites to consider their significance in the arc system we investigated the tsutsugatake stock as representative of the intrusions exposed in central kyushu the tsutsugatake rocks are mainly garnet bearing two mica leuco granitoids their sr y ratios and rare earth element concentrations are appropriate for adakite but k 2 o na 2 o ratios 0 53 1 43 and al 2 o 3 13 7 15 0 wt mgo 0 20 0 55 wt sr 207 462 ppm cr 12 ppm and ni 8 ppm contents differ from those of typical adakite geochemical modeling shows that the petrogenesis of the tsutsugatake stock can be explained by partial melting of arc type tonalite or adakitic granodiorite under middle to lower crustal conditions leaving amphibole and plagioclase as residual phases the differences between the tsutsugatake stock and classical adakites lie in their source rocks and melting conditions the tsutsugatake source was not basaltic and the melting pressure was insufficient to stabilize garnet the tsutsugatake rocks are here designated pseudo adakites such pseudo adakites may be found in both modern and ancient arcs and would play an important role in crustal evolution as a member of the recycled constituent

nonmonotonic variation of seawater 87 sr 86 sr across the ivorian chadian boundary mississippian osagean evidence from marine cements within the irish waulsortian limestone abstract detailed analysis of compositionally unaltered marine fibrous cements mfc from a single core through the mississippian irish waulsortian limestone indicates that the variation of seawater 87sr 86sr is nonmonotonic across the ivorian chadian boundary this nonmonotonic variation has not been recognized by previous studies furthermore marine cements yielded 87sr 86sr ratios lower than previously reported values for the ivorian chadian osagean marine fibrous cements are interpreted to be compositionally unaltered on the basis of nonluminescent character and stable isotope c o composition comparable to previous estimates of mississippian marine calcite the isotope chemistry c o sr and cathodoluminescent character of the marine fibrous cements therefore remained intact during their conversion from high mg calcite to low mg calcite microdolomite a conversion that probably took place in marine water during precipitation of zone 1 calcite cement the oldest non mfc cement high stratigraphic resolution was obtained by restricting the sample set to a single core 429 m long thereby eliminating chronostratigraphic correlation errors the mfc cements are pre arundian based on previous study and are interpreted to be in stratigraphic order and to be approximately the same age as the host limestones the core is estimated to represent about 9 8 million years of waulsortian limestone deposition 87sr 86sr ratios of pristine mfc samples range from a high of 0 707908 in the early ivorian to a low of 0 707650 in the late ivorian and mid chadian with an early chadian maximum at 0 707800 the maximum rate of change in seawater 87sr 86sr is 0 00012 ma comparable in magnitude to tertiary values our data document the presence of fine scale seawater 87sr 86sr modulations for the ivorian chadian in contrast to the previously published monotonic seawater 87sr 86sr curve for this interval and emphasize the importance of well characterized intraformational isotopic baselines

early cretaceous adakitic granites in the northern dabie complex central china implications for partial melting and delamination of thickened lower crust abstract to date few adakitic rocks have been reported in direct association with contemporary intra continental extensional structures which has cast doubt on genetic models involving partial melting of the lower crust this study presents early cretaceous 143 129 ma new sensitive high resolution ion microprobe shrimp zircon u pb ages adakitic granites which are directly associated with a contemporary metamorphic core complex i e the northern dabie complex in the dabie area these granites exhibit relatively high sr contents negligible to positive eu and sr anomalies high la yb and sr y ratios but very low yb and y contents similar to subducted oceanic crust derived adakites they are also characterized however by very low mgo or mg and ni values and nd sr isotope compositions end t 14 6 to 19 4 and 87sr 86sr i 0 7067 0 7087 similar to triassic continent derived eclogites subducted in the dabie sulu orogen additionally late granitic dikes in the adakitic intrusions exhibit low sr contents clearly negative eu and sr anomalies low la yb and sr y ratios but relatively high yb and y contents similar to 118 105 ma granites in the northern dabie complex based on composition and geochronology data of neoproterozoic amphibolites and orthogneisses triassic high to ultra high pressure metamorphic rocks and early cretaceous mafic ultramafic intrusive rocks and the constraints provided by experimental melt data for tonalites metabasaltic rocks and eclogites we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile bearing eclogitic lower crust as a consequence of triassic middle jurassic subduction and thrusting the late dikes probably originated from plagioclase bearing intermediate granulites moreover we suggest that late mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of early cretaceous mafic ultramafic intrusive rocks in the dabie area and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite asthenospheric upwelling in response to post collisional delamination of lithospheric mantle was likely to have provided the heat source for the cretaceous magmatism

episodic bimodal magmatism at an active continental margin due to paleo pacific plate subduction a case study from the southern segment of the tan lu fault zone eastern china abstract volcanic rocks in the southern part of the tan lu fault zone tlfz eastern china provide key evidence for the connection between spatially and temporally related pulses of cretaceous magmatism activity on the tlfz and the dynamics of the paleo pacific plate we identify two stages of bimodal volcanism in the chaohu lujiang segment southern tlfz based on lithological assemblages and precise isotopic ages the early cretaceous 125 121 ma volcanic rocks are fractionated aluminous a type rhyolites and basalts whereas the late cretaceous volcanic rocks 97 83 ma range in composition from porphyritic andesites and trachydacites to rhyolites the early cretaceous rhyolites display strongly negative nd hf isotopic compositions end t 16 3 to 16 1 ehf t 24 1 to 17 9 the most likely source rocks of the fractionated aluminous a type rhyolites are metaigneous rocks of the feidong complex the early cretaceous potassic basalts k2o na2o 1 5 3 4 end t 9 0 exhibit more enriched isotopic compositions than the late cretaceous sodic porphyritic andesites k2o na2o 0 6 0 8 end t 5 6 ehf t 15 1 to 6 8 indicating derivation from heterogeneously enriched lithospheric mantle this heterogeneity is ascribed to derivation from more fertile and isotopically enriched mantle sources with lower solidus temperatures modeling of selected elements reveals that the late cretaceous trachydacites end t 12 2 to 8 2 ehf t 16 5 to 6 4 originated by mixing between coeval mafic and felsic magmas the most negative end t 18 8 to 17 1 ehf t 20 5 to 4 7 isotopic compositions along with neoproterozoic inherited zircons within the late cretaceous rhyolites indicate derivation from lower crust of the south china block we suggest that the early cretaceous bimodal volcanic rocks formed in an extensional backarc setting induced by retreat of the subducting izanagi slab however the tlfz became a focus of late cretaceous bimodal volcanism due to weak far field extension caused by the slow northward motion of the paleo pacific plate moreover the high velocity and low angle of subduction of the izanagi slab were the main driving mechanisms behind an early cretaceous transpressional event that separated the two stages of bimodal volcanism

late miocene adakites and nb enriched basalts from vizcaino peninsula mexico indicators of east pacific rise subduction below southern baja california a typical slab melt association was emplaced from 11 to 8 ma in the santa clara volcanic field vizcaino peninsula baja california sur it includes adakitic domes and associated pyroclastic flow deposits together with lava flows of niobium enriched basalts the trace element and isotopic sr nd pb signatures of adakites are consistent with melting of altered mid ocean ridge basalts and the sources of the nb enriched basalts contain an enriched mantle wedge component such associations commonly form at depths of 70 80 km during low dip subduction of very young oceanic crust however the santa clara field is relatively close 100 km to the paleotrench which suggests that the genesis of its adakites and nb enriched basalts occurred in a very high thermal regime linked to the subduction of the then active guadalupe spreading center of the east pacific rise our data suggest that the asthenospheric window documented below northern baja california also developed beneath the south of the peninsula during the neogene this hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

the ti saturation surface for low to medium pressure metapelitic biotites implications for geothermometry and ti substitution mechanisms the ti content of biotite can serve as a geothermometer for graphitic peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4 6 kbar the relationship between ti content temperature and mg mg fe value was calibrated empirically using an extensive natural biotite data set 529 samples from western maine and south central massachusetts in combination with the petrogenetic grid of spear et al 1999 the calculated ti saturation surface is curved such that for a given mg mg fe value ti concentration increases as a function of temperature in a nonlinear fashion and for a given temperature ti concentrations decrease with an increase in mg mg fe the fit to the ti saturation surface can be reformulated as the geothermometric expression t ln ti a c x mg 3 b 0 333 in which t is temperature in degrees celsius ti is the number of atoms per formula unit apfu normalized on the basis of 22 o atoms x mg is mg mg fe a 2 3594 b 4 6482 10 9 and c 1 7283 the calibration range for this expression is x mg 0 275 1 000 ti 0 04 0 60 apfu and t 480 800 c precision of the ti in biotite geothermometer is estimated to be 24 c at the lower temperature range and improves to 12 c at higher temperatures application of the ti in biotite geothermometer to ilmenite or rutile bearing graphitic peraluminous metapelites equilibrated at 3 6 kbar is generally consistent with independent temperature determinations but with some deviations that represent local reequilibration consequently the ti systematics in biotite can also serve as the basis of a very sensitive indicator of chemical equilibrium or lack thereof application of the geothermometer to metapelites not containing the requisite mineral assemblages can lead to minor to significant errors in estimated temperatures biotite ti substitution mechanisms are controlled by several factors based on the biotite calibration data set magnesian biotites x mg 0 65 incorporate ti in accordance with the exchange vector tial2r 1si 2 where r is the sum of the divalent cations mg fe mn this substitution mechanism is primarily a response to misfit of the octahedral and tetrahedral layers in magnesian biotites intermediate biotites x mg 0 65 particularly at higher temperatures exhibit enhanced ti concentrations most consistent with the ti deprotonation tio2r 1 oh 2 exchange vector dominance of ti deprotonation substitution is largely a function of reduction of h2o activity at higher metamorphic grades supplementary biotite data from metaluminous amphibolites and mafic granulites metamorphosed isothermally with variable h2o activities reveal that low al biotite incorporates significantly higher concentrations of ti relative to peraluminous biotite as a result of a combination of the exchange vectors tio2r 1 oh 2 and rsial 2 substituting in roughly an 8 1 ratio

petrologic and geochemical constraints on the origin of subsurface dolomite enewetak atoll an example of dolomitization by normal seawater dolomite is present 1 250 1 400 m below sea level in eocene strata of enewetak atoll petrographically the deep enewetak dolomite postdates brittle compaction of rigid grains in the host eocene strata the 87 sr 86 sr ratio of these dolomites 0 70865 0 70901 indicates that they formed at one or more times between the middle miocene and the present since the top of the lower miocene is more than 900 m above the deep dolomitic interval the dolomite must have formed at a minimum burial depth of 900 m stable oxygen isotope determinations suggest dolomite precipitation from cold marine water lower miocene and eocene carbonate strata on the atoll are apparently in open communication with cold modern ocean water suggesting that those same strata were in open communication with ocean water during dolomitization at a depth of about 1 000 m modern pacific ocean water becomes undersaturated with respect to calcite but is still supersaturated with respect to dolomite therefore it is proposed that the deep enewetak dolomite precipitated from cold deep ocean water undersaturated with respect to calcite at a burial depth of more than 900 m

dolomitization of the waulsortian limestone lower carboniferous in the irish midlands the waulsortian limestone lower carboniferous of the southern irish midlands is dolomitized pervasively over a much larger region than previous studies have documented this study indicates a complex multistage multiple fluid history for regional dolomitization partially and completely dolomitized sections of waulsortian limestones are characterized by finely crystalline 0 01 0 3 mm planar dolomite planar replacive dolomite is commonly followed by coarse 0 5 mm nonplanar replacive dolomite and pervasive void filling saddle dolomite cement is frequently associated with zn pb mineralization planar dolomite has average 18o and 13c values pdb of 4 8 and 3 9 respectively these are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the waulsortian 18o 2 2 13c 3 7 mean c and o isotope values of planar replacive dolomite are also distinct from those of nonplanar and saddle dolomite cement 7 0 and 3 3 7 4 and 2 4 respectively fluid inclusions indicate a complex history involving at least three chemically and thermally distinct fluids during dolomite cementation the petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin possibly in equilibrium with modified carboniferous sea water where the waulsortian was exposed to hydrothermal fluids 70 280 c planar dolomite underwent a neomorphic recrystallization to a coarser crystalline planar and nonplanar dolomite characterized by lower 18o values void filling dolomite cement is isotopically similar to nonplanar replacive dolomite and reflects a similar origin from hydrothermal fluids this history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the irish midlands and provides a framework upon which to test competing models of regional vs localized fluid flow

golden grove dolomite barbados origin from modified seawater abstract the largest dolomite occurrence on barbados part of a roadcut at golden grove is about 40 m long and 0 5 2 5 m thick dolomite at golden grove constitutes about 10 40 vol of the host limestone an algalforaminiferal facies of late pleistocene age with local patches consisting of up to 100 vol dolomite almost all dolomite is a replacement of matrix and predominantly mg calcite skeletal grains an average of less than 0 1 vol of the rock is fibrous to bladed radial and radiaxial dolomite cements with calcite banding the partially dolomitized algal foraminiferal facies is unconformably overlain by scleractinian coral reef limestones replacive dolomitization appears to have been very localized at the stratigraphic level of golden grove the petrographic data suggest that replacive dolomitization took place after deposition of the algal foraminiferal facies and prior to deposition of the overlying coral reef facies 18o 1 1 to 4 4 pdb values of the replacive dolomites suggest seawater with variable temperature and or slight degrees of evaporation as the principal agent of dolomitization in addition variable and highly negative 13c values of the replacive dolomites 23 5 to 8 913c the main source of this 13c depleted carbonate probably was oxidized methane with a possible but very minor contribution of carbonate derived from bacterial gsulfate reduction methane may have been supplied by bacterial methanogenesis relatively close to the site of dolomitization or by upward seepage of thermogenic methane derived from underlying tertiary hydrocarbon reservoir s it is possible albeit speculative that the localized availability of methane via fractures and or its localized oxidation somehow induced dolomitization at golden grove furthermore the 13c 18o and 87sr 86sr ratios 0 70913 0 70919 of replacive dolomites of golden grove calcites 13c 14 0 to 3 9 pdb 18o 5 0 to 2 2 pdb and of groundwaters from the present coastal mixing zone and inland carbonate aquifers 87sr 86sr 0 70911 0 70924 13c 123 to 5 9 pdb 18o 5 0 to 2 smow effectively role out that replacive dolomitization at golden grove was caused in a typical coastal freshwater seawater mixing zone or that dolomitization was influenced by meteoric waters to any significant degree the small amounts of banded dolomite cements however may have formed in a typical coastal mixing zone

an example of mixing zone dolomite middle eocene avon park formation floridan aquifer system abstract a late formed dolomite cement in a core of the middle eocene avon park formation peninsular florida provides an example of dolomite cement from a mixing zone and illustrates how dolomite textural alteration and stabilization can occur at earth surface conditions the avon park formation is a pervasively dolomitized peritidal platform carbonate 400 m thick in the floridan aquifer system typical avon park dolomite is inclusion rich fine grained 40 mm noncathodoluminescent highly porous average 20 and formed during the eocene by normal to hypersaline seawater 18o 3 7 pdb 13c 2 0 87sr 86sr 0 70778 sr 167 ppm in a 20 m interval in a core from southwest florida inclusion free cathodoluminescent dolomite overgrows the early formed noncathodoluminescent marine dolomite the cathodoluminescent dolomite cement profoundly alters the texture of avon park dolomite from typical cenozoic like porous poorly crystalline dolomite to hard dense low porosity highly crystalline paleozoic like dolomite the dolomite cement is not a replacement of limestone but an overgrowth of early formed marine dolomite and pore occluding cement from analyses of samples containing both the early marine dolomite and the late dolomite cement quantitative estimates of pure cathodoluminescent dolomite cement are 13c 0 5 and 18o 1 7 87sr 86sr 0 7085 sr 225 ppm the stable isotope compositions are intermediate between avon park marine dolomite and dolomite predicted to be in equilibrium with floridan aquifer freshwater estimated 18o 0 to 0 5 p b indicating the dolomite precipitated from waters intermediate in composition between seawater and freshwater due to the rise in seawater 87sr 86sr since the middle eocene the 87sr 86sr composition of the cathodoluminescent dolomite cement demonstrates that it must have formed after the middle miocene incorporating radiogenic sr from seawater or rocks of middle miocene age or younger a water sample was taken from this same core from an interval in the avon park formation in the dilute part of the modern mixing zone approximately 60 m above the first occurrence of altered dolomite fluids from this interval have 3400 ppm tds and are a mixture of 94 floridan aquifer freshwater and 6 normal seawater these pore fluids are sr rich 24 ppm and so4 rich 755 ppm due to extensive avon park gypsum dissolution and have a 87sr 86sr composition 0 7078 in equilibrium with avon park host rock the salinity of the fluid that precipitated the cathodoluminescent dolomite cement is quantitatively calculated with a fluid fluid mixing model using 1 the sr concentration 24 ppm and 87sr 86sr composition 0 7078 of the von park pore fluid sample from the core 2 the sr concentration 8 ppm and 87sr 86sr compositions of modern 0 7092 and late miocene 0 7089 seawater and 3 the 87sr 86sr composition 0 7085 of the cathodoluminescent dolomite cement these calculations indicate that the dolomite cement precipitated from a mixed marine meteoric fluid with salinity approximately 75 seawater this same fluid would be both calcite supersaturated and contain about 12 ppm sr such a fluid could precipitate dolomite cement with 225 ppm sr using a reasonable kd 0 05 this accounts for the higher sr concentrations in the mixing zone dolomite cement 225 ppm relative to avon park marne dolomite 167 ppm this study demonstrates that 1 dolomite precipitated from a 75 seawater mixing zone fluid that was both calcite saturated and sulfate rich and 2 dramatic textural maturation and stabilization in dolomite can occur in the near surface environment without elevated temperature and burial conditions

40ar 39ar dating of strike slip motion on the tan lu fault zone east china abstract ne trending sinistral ductile shear belts in the southern part of the tan lu fault zone east china are exposed along the eastern edge of the northern dabie belt and southern zhangbaling belt three hornblende three phengite and 12 biotite separates from greenschist facies mylonites in the shear belts were dated by the 40ar 39ar method in this study a hornblende plateau age of 143 3 ma is interpreted as representing the time of sinistral motion on the zhangbaling part of the tan lu fault zone seven biotite samples from the same part yield 40ar 39ar cooling ages from 137 to 125 ma a phengite plateau age of 138 8 ma from the eastern edge of the dabie belt is considered as being close to the time of deformation in the tan lu strike slip zone five biotite 40ar 39ar ages from 120 to 110 ma and one phengite age of 121 ma from the same area represent cooling ages after the sinistral faulting it is concluded on the basis of these 40ar 39ar ages that the sinistral ductile shear belts in the southern part of the tan lu fault zone were produced in earliest early cretaceous this sinistral motion on the tan lu fault zone is considered to be related to high speed highly oblique subduction of the pacific plate at this time

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

87sr86sr isotopic evolution of lower carboniferous seawater dinantian of western europe abstract the chemical and isotopic evolution of ocean water mirrors the dynamics of the earth system the original seawater signature can be measured only indirectly for the palaeozoic samples with the greatest utility for such an approach are the secondary layers of articulate brachiopods for a correct interpretation of isotope data samples that have suffered alteration by diagenetic processes must be avoided trace element analysis is frequently utilized as an analytical tool the present study deals with the delineation of a detailed 87 sr 86 sr curve for the dinantian this in turn enables us to search for causative factors of the observed isotopic oscillations a total of 175 lower carboniferous dinantian brachiopod shells from a variety of western european locations have been analysed for their 87 sr 86 sr ratios for 152 of these shells quantitative geochemical tests on diagenetic alteration based on trace element repartitioning during recrystallisation have been performed the concentrations of ca mg sr mn and fe were routinely measured by inductively coupled plasma atomic emission spectroscopy icp aes analysing the diluted phosphoric acid that remained after carbonate dissolution for stable isotope gas preparation in addition 71 samples have been analysed by proton induced x ray emission spectroscopy micro pixe despite large differences in the analysed masses 3 6 mg for icp aes as low as 10 ng for pixe both methods yield comparable data for the sr and mn contents any discrepancies can be related to inhomogeneities of the shells e g punctae of these shells 73 have sr and mn contents of 600 ppm and the sr isotope record of the dinantian seawater is characterised by a decline in 87 sr 86 sr ratio from 0 7082 at the devonian carboniferous transition to 0 7076 in the mid visean superimposed on this trend are higher order fluctuations with a periodicity in the ma range the dinantian seawater curve may potentially serve as a geochronological and correlation tool particularly for the hastarian to lower chadian interval where the attainable resolution is 1 ma this is better than the resolution available by biostratigraphy the higher order wiggles for the most part can be generated by changing the riverine flux of sr or its isotopic ratio within reasonable geological scenarios some spikes however are too large and too short to be explained by the above phenomena these apparent spikes may reflect instead the fact that their duration has been underestimated due to unreliable geochronology and or the presence of unrecognised hiatuses

mesozoic adakitic rocks from the xuzhou suzhou area eastern china evidence for partial melting of delaminated lower continental crust abstract adakitic rocks in the xuzhou suzhou area eastern china consist of dioritic and monzodioritic porphyries and were dated at 131 132 ma by the shrimp u pb zircon method these rocks have high mgo content 1 47 5 73 high mg values 0 49 0 61 and high la yb and sr y ratios these features are similar to rocks derived from partial melting of a subducted oceanic slab however their high initial 87sr 86sr 0 7053 0 7075 and low end t values 4 43 to 13 14 are inconsistent with the origin from slab melting these rocks often contain garnet residual crystals and eclogite garnet clinopyroxenite and garnet amphibolite xenoliths petrographical characteristics and estimated p t conditions of these xenoliths indicate that they were once deeply subducted and subsequently underwent rapid exhumation in the early mesozoic garnet residual crystals from the porphyries show similar chemical compositions to garnets from garnet clinopyroxenite and garnet amphibolite xenoliths ages of the inherited zircons of the xenoliths and their host rocks likely indicate that sources for the adakitic magma and protoliths of the eclogite and garnet clinopyroxenite xenoliths in the study area were from precambrian basement of the north china craton the data also suggest that the lower continental crust in the eastern north china craton was thickened during the early mesozoic and delaminated in the early cretaceous the high mg adakitic magma resulted from partial melting of this delaminated lower continental crust and its subsequent interaction with the mantle during upward transport leaving garnet as the residual phase

application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

isotope stratigraphy of the european carboniferous proxy signals for ocean chemistry climate and tectonics abstract carboniferous brachiopods from western europe and the former ussr have been utilized as carriers of isotopic proxy signals for paleoceanography and paleoclimatology during the assemblage of pangea this is of particular interest because this time interval coincides with the transition from greenhouse to icehouse conditions the 18 o values 3 ma running means show an overall 18 o increase from about 7 1 to 3 1 during the carboniferous the rise in the 18 o is stepwise with major shifts in the mid tournaisian and at the visean serpukhovian transition the comparable 13 c record shows a tournaisian peak at 5 a visean oscillation at about 2 5 1 a serpukhovian rise of more than 3 a plateau around 5 during the bashkirian and moscovian a 4 drop during the kasimovian and another rise of about 3 during the gzhelian the 87 sr 86 sr record shows an early carboniferous decline from 0 7083 to a mid visean minimum of 0 7077 followed by a rapid serpukhovian rise to a bashkirian and moscovian plateau at 0 7087 and a slight decline during the kasimovian and gzhelian the covariance of isotopic trends and the coincidence with geological phenomena indicates that the long term variations in ocean chemistry and climate may be coupled to the tectonic evolution of the earth system the namurian witnesses the onset of hercynian orogeny initiation of the main phase of carboniferous glaciation formation of the extensive coal deposits and the rise in 87 sr 86 sr 13 c and 18 o all this suggests that a tectonically controlled climate change resulting in cooling and increased carbon storage in the organic reservoir was the major triggering factor for all the above phenomena the late carboniferous glacial episode was interrupted by a warm interval in the stephanian the carbon and oxygen isotope data indicate that this episode may have been restricted to the early kasimovian a proposition at odds with the poorly dated record of glacial deposits that are interpreted as indicating a warm climate for the entire stephanian in addition the pattern of short lived peaks in 13 c and 18 o in the tournaisian and possibly visean may be interpreted as a reflection of short lived glacial episodes a proposition consistent with geologic records

early cretaceous granitoids and their implications for the collapse of the dabie orogen eastern china shrimp zircon u pb dating and geochemistry abstract two stages of early cretaceous post orogenic granitoids are recognized in the dabie orogen eastern china which recorded processes of extensional collapse of the orogen the early stage granitoids 132 ma are foliated hornblende quartz monzonites and porphyritic monzogranites they are of high k calc alkaline series and metaluminous to weakly peraluminous with high k 2 o and low mgo contents mg values 32 0 46 0 they contain high sr low y and heavy rare earth elements hree and have high sr y and la yb n ratios without clear negative eu sr and ti anomalies the early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite rich plagioclase and thus indicate the existence of an over thickened 50 km crustal root beneath the orogen at 132 ma the later stage granitoids 128 ma are undeformed fine grained monzogranites fine grained k feldspar granites and coarse grained k feldspar granite porphyry they belong to a peraluminous and high k calc alkaline to shoshonite series and display a flat hree pattern and have strong negative eu sr and ti anomalies with low sr y and la yb n ratios the late stage granitoids are equilibrated with residues rich in anorthite rich plagioclase hornblende ilmenite titanite and poor in garnet indicating that the crust of the dabie orogen became thinner

geochronological and geochemical constraints on genesis of the adakitic rocks in outang south tan lu fault belt northeastern yangtze block abstract the outang intrusion was newly found near the south tan lu fault belt stlf of the northeastern margin of yangtze block due to the excavating construction of the beijing shanghai high speed railway observations show that lithology of the outang intrusion is intermediate rock series mainly composed of monzodiorite and quartz monzonite major and trace elements zircon u pb dating and hf isotope were analyzed two ages of 124 6 2 9 ma mswd 0 60 n 29 and 129 2 4 1 ma mswd 1 5 n 21 are obtained showing that the regional igneous activity was in early cretaceous being consistent with the massive yanshanian magmatic events in eastern china the quartz monzonite was identified as a high mg adakitic rocks showing geochemical features of high si high k and low sr which indicates that the igneous genesis may be results of partial melting of the delaminated lower crust the negative e hf t values with much older t dm2 ages indicate that the original magma may derive from remelting of ancient continental crust a tectonic model has been proposed to account for the massive subduction of the pacific plate beneath eastern china in jurassic with effect of delamination and thinning from which we conjecture that the archean materials could compose the basement of the stlf caused by massive subduction of the pacific plate beneath the eastern china continent

titanium substitution in biotite an empirical model with applications to thermometry o2 and h2o barometries and consequences for biotite stability abstract an empirical model for solution of ti in biotite is developed and calibrated on the basis of various equilibria among biotite and ti saturating assemblages sources of data used for the calibration include both experimental products and published analyses of natural assemblages ti solution in biotite can be satisfactorily modelled by means of the exchange component tife 2 whose non ideal behavior can be approximated empirically by means of a linear function of the relative concentrations of ti and octahedral al in biotite within assemblages which fix the chemical potential of the exchange component tife 2 the concentration of this component in biotite is very sensitive to temperature and oxygen fugacity making it possible to use biotite oxide assemblages to obtain reliable t and f o 2 estimates in igneous and metamorphic rocks in assemblages which also contain alkali feldspar it is possible to use breakdown equilibria of ti additive components in biotite to estimate h 2 o fugacity such uses are likely to find important applications in rocks in which other assemblages sensitive to temperature f o 2 and or f h 2 o are absent in general rocks containing a single oxide phase plus biotite as the only mafic silicate phase alkali feldspar the empirical model for solution of ti in biotite also allows to calculate the loci of various terminal reactions for the assemblage titanian biotite quartz which by comparison with equivalent equilibria in ti free systems provide insight into the effect of ti on the stability of biotite solubility of ti in biotite increases with temperature in a strongly non linear fashion as a consequence calculations predict that the effect of ti on the stability of biotite is very small up to conditions typical of the beginning of the granulite facies of metamorphism but beyond those conditions ti concentration increases markedly and biotite is rendered very refractory at mid to deep crustal pressures 10 kbar the vapor absent solidus of biotite quartz in rocks containing ti saturating phases is likely to exceed 900 c even in bulk compositions with subequal concentrations of fe and mg

slab melting as an explanation of quaternary volcanism and aseismicity in southwest japan two volcanoes on the quaternary volcanic front in southwest japan have erupted medium k andesite and dacite which are chemically similar to adakites i e high al 2 o 3 sr y low y and steep rare earth element ree patterns with no negative eu anomalies unlike other subduction related volcanoes in japan earthquake data and the extrapolated position of the philippine sea plate from its near trench position indicate that the leading edge of the plate is 75 100 km deep beneath these volcanoes melting of the plate at this depth would leave a garnet eclogite residual accounting for the y and heavy ree depletion the lack of seismic data and active volcanism in southwest japan can be attributed to the attenuated seismic response of the plate due to melting and to the atypical nature of magma generation

geochronology geochemistry and hf isotopic composition of late cretaceous laojunshan granites in the western cathaysia block of south china and their metallogenic and tectonic implications abstract sn in and w rich granites are identified in laojunshan area in the western cathaysia block south china including coarse grained two mica monzogranite cmg and fine grained two mica monzogranite fmg laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating of cmg and fmg are from 85 6 0 8 ma to 90 37 0 7 ma and 89 9 1 4 ma respectively all samples have relatively high total alkalis contents 5 74 wt to 9 73 wt and al saturation indices 0 95 to 1 79 indicating that most of granites belong to the strongly peraluminous granites the granites are enriched in p rb cs sn li be and u and are depleted in ti mg co ni sr ba zr and hf the fractional crystallization of plagioclase and k feldspar was the principal process of magmatic differentiation that controlled rb sr ba and eu concentrations while rare earth elements were fractionated by accessory minerals such as apatite monazite or ilmenite these geochemical features suggest that most of rocks are highly fractionated s type granites the ehf t values of the zircon samples from cmg and fmg range from 12 88 to 1 19 with the two stage hf model ages ranging from 1 2 ga to 1 9 ga the geochemical and isotopic data show that the laojunshan granites are derived from a crystalline basement that has metamorphosed pelitic rocks from the mes paleoproterozoic continental crust the upwelling of the asthenosphere caused by a strong lithospheric extension led to the partial melting of the metamorphosed pelitic rocks to form widespread late cretaceous s type granites in the laojunshan area lithospheric extension and cretaceous magma in se yunnan are probably related to the palaeo tethys ocean crustal delamination which subducted beneath the indochina block

origin of mesozoic adakitic intrusive rocks in the ningzhen area of east china partial melting of delaminated lower continental crust to the best of our knowledge modern adakites have not been documented in a nonarc environment we report geochemical and isotopic data for early cretaceous anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction the anjishan adakitic intrusive rocks which are exposed in the ningzhen area of east china have high sr y and la yb ratios coupled with low yb and y as well as relatively high mgo contents and mg numbers mg 0 4 0 6 similar to products from slab melting however low n d t values 6 8 to 9 7 and high 8 7 sr 8 6 sr i 0 7053 0 7066 are inconsistent with an origin by slab melting the tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust high sr y and la yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting implying that the crustal thickness exceeded 40 km in the early cretaceous the present thickness of the crust in the ningzhen area is only 30 km and therefore the crust appears to have been thinned by at least 10 km since the early cretaceous the relatively high mgo contents and mg of the anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks possibly coinciding with lower crustal delamination which would also account for the observed thinning

numerical modeling of reflux dolomitization in the grosmont platform complex upper devonian western canada sedimentary basin the upper devonian grosmont platform in the western canada sedimentary basin is a pervasively dolomitized giant heavy oil reservoir with reserves of 317 billion bbl of bitumen the principal type of grosmont platform dolomite formed early and on the basis of stratigraphic and geochemical evidence is interpreted as early diagenetic reflux dolomite we use a numerical ground water flow model to investigate the viability of reflux to dolomitize the grosmont platform we simulate reflux at four key stages of platform evolution incorporating the transient effects of changes in platform architecture rock properties and the salinity of platform top waters the pattern and magnitude of reflux is critically controlled by permeability and the distribution of platform top brines which are concentrated up to gypsum saturation reflux flow is focused in the relatively permeable carbonates of the grosmont formation and is from the platform interior toward the platform margin the 120 m thick shales of the ireton formation that separate the grosmont and cooking lake formations restrict cross formational flow and brine transport during a 100 k y period of relative sea level rise and platform top drowning brines of reflux origin continue to sink and entrain platform top waters latent reflux where the intervening aquitards are thin or absent reefs of the leduc formation capture reflux brines from the overlying grosmont platform and focus cross formational brine transport lateral contrasts in salinity are sufficient to drive a series of free convection cells in the relatively permeable reefs of the leduc formation computed distributions of fluid flux in conjunction with magnesium mass balance calculations that incorporate the range of uncertainty particularly in permeability support the suggestion that the reflux of gypsum saturated brines could have formed much if not most of the dolomite in the grosmont formation in the 1 6 m y available

complexity in earthquake sequences controlled by multiscale heterogeneity in fault fracture energy 1 a series of dynamic rupture events under constant tectonic loading is simulated on a fault with multiscale heterogeneity and a stochastic rupture initiation process the fracture energy of the fault plane is assumed to have multiscale heterogeneous distribution using fractal circular patches the stochastic rupture initiation process as a function of the accumulated stress is introduced in order to take account of unknown smaller scale heterogeneity and variability five realizations of a statistical spatial distribution of fracture energy fault heterogeneity maps are tested for the simulations of earthquake sequences during a few seismic cycles the diversity of earthquake sequences is principally controlled by the spatial distribution of the patches the effect of dynamic rupture appears in the residual stress after the characteristic events because of their directivity and this localizes the subsequent sequences although the characteristic earthquakes occur rather regularly in time and similarly in different seismic cycles some irregular behavior is found based on the heterogeneity maps and the randomness of the preceding earthquake sequence leading to a visible anomaly in the seismicity such anomalies are not predicable but understandable through the analysis of the considered earthquakes during the cycle the similarity and the diversity simulated in this study governed by the structure of an inherent distribution of multiscale heterogeneity suggest the importance of the pre existing heterogeneity field along the fault for the appearance of earthquake sequences including those that are characteristic

distinct lateral variation of lithospheric thickness in the northeastern north china craton abstract a detailed knowledge of the thickness of the lithosphere in the northeastern north china craton ncc is important for understanding the significant tectonic reactivation of the craton in the mesozoic and cenozoic time we achieve this goal by applying the newly proposed wave equation based migration technique to the s receiver functions recently collected in the region distinct negative signals are identified below the moho in all the s receiver function migrated images and stacks which we interpret as representing the s to p conversions from the lithosphere asthenosphere boundary lab the imaged lab is as shallow as 60 70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior these observations indicate widespread lithospheric thinning in the study region in comparison with the 180 km lithospheric thicknesses typical of most cratonic regions the revealed topography of the lab generally agrees with the lateral variation in upper mantle seismic anisotropy previously measured through sks splitting analysis in particular a sharp lab step of 40 km is detected at the triple junction of the basin and mountains at almost the same place where an abrupt change from nw se to ne sw in fast polarization direction of shear waves was found these findings suggest a close correlation between the seismic anisotropy and hence deformation of the upper mantle the lithospheric thickness and the surface tectonics of the northeastern ncc while the thinned lithosphere and the nw se fast shear wave polarizations in the east areas probably are related to the dominant nw se tectonic extension in the late mesozoic cenozoic time the thicker lithosphere and the ne sw fast polarization direction in the west mountain ranges may reflect earlier contractional deformations of the region synthetic tests indicate that the lab beneath the northeastern ncc is a well defined zone 10 20 km thick combined with seismic tomography results and geochemical and petrological data this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton

epigenetic dolomitization and the origin of xenotopic dolomite texture abstract xenotopic texture which is commonly observed in pre cenozoic rocks is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and usually undulatory extinction xenotopic dolomite texture is similar in appearance to neomorphic limestone textures idiotopic dolomite texture euhedral to subhedral crystals with straight intercrystalline boundaries contrasts with xenotopic texture and is commonly observed in both cenozoic and more ancient dolomites texture may be controlled by the temperature at which crystals grow crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites this results in faceted crystals and euhedral to subhedral crystal mosaics above a critical roughening temperature crt a rough surface is energetically favored surface nucleation does not require dislocations and atoms are randomly added to the crystal surface resulting in nonfaceted growth and an anhedral crystal mosaic it is hypothesized that a critical roughening temperature exists for dolomite above 50 c xenotopic dolomites are produced by dolomitization of limestone and or neomorphic recrystallization of dolomite at elevated temperature above the crt after burial idiotopic dolomites are produced below the crt by near surface processes and are occasionally produced above crt if growth inhibiting effects of the solution or impurities stabilize crystal faces calcite has a crt below 25 c and therefore develops anhedral crystal mosaics characteristic of neomorphic texture both at near surface and at elevated temperatures synthetic xenotopic dolomite was produced in the laboratory at 250 c and 300 c by dolomitization of aragonite and calcite skeletal fragments and by recrystallization neomorphism of nonstoichiometric cenozoic dolomites xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic hueco dolomite permian texas near the marble canyon intrusion at temperatures between 350 c and 600 c hydrothermal dolomitization of periclase calcite marble near the intrusion also resulted in a xenotopic texture xenotopic dolomite in the galena group ordovician wisconsin was produced by neomorphism of a preexisting dolomite during the emplacement of lead zinc sulfides at temperatures between 80 c and 227 c in the trenton limestone ordovician michig n xenotopic dolomite replaced limestone during the migration of hot 50 c fluids along fracture systems xenotopic dolomite was not observed in cenozoic dolomites that were subjected to temperatures near surface values only

mio pliocene adakite generation related to flat subduction in southern ecuador the quimsacocha volcanic center abstract new geochemical and geochronological data on the miocene pliocene quimsacocha volcanic center qvc have led to the recognition of adakitic lavas generated by slab melting related to the flat slab subduction in southern ecuador and northern peru the qvc located in the presently inactive southern part of the ecuadorian arc was built up during three distinctive volcanic phases the first phase generated a basal edifice with mainly andesitic lava flows while the second phase is characterized by the emplacement of cryptodomes domes and related outflow breccias comprised of andesites and some dacites the last phase released rhyolitic ignimbrites associated with the formation of a large caldera which was later partly filled by dacitic rhyolitic domes geochemical data for the qvc indicate higher al2o3 tio2 na2o zr and sr contents and lower fe2o3 mgo y mree and hree abundances compared to other eruptive rocks of the plio quaternary volcanic front of ecuador such geochemical features as well as the frequent presence of an associated epithermal gold deposit are characteristic of the involvement of slab melts also known as adakites 1 2 in the generation of these magmas after a calc alkaline arc magmatism phase slab horizontalization in response to the subduction of a buoyant oceanic plateau results in increased involvement of a slab melting component in the magmas produced however pristine adakites were generated and emplaced during a relatively short period as indicated by zircon fission track ages then volcanic activity stopped and a volcanic gap formed the identification of these adakites their location and age support a model of slab melting associated with flat slab subduction m a gutscher et al geology 28 2000 535 538

calc alkaline and alkaline miocene and calc alkaline recent volcanism in the southernmost patagonian cordillera chile abstract in the southernmost patagonian cordillera south of lat 54 45 previously unrecorded calc alkaline dacites and andesites and adjacent alkali basalts were dated at 21 and 18 ma k ar respectively in the same area post glacial low k calc alkaline andesites identified at isla cook represent the southernmost recent volcanism recorded so far in south america about 400 km south of monte burney previously known as the southernmost recent volcano the association of contemporaneous calc alkaline and alkali volcanics suggests a subduction related environment for the miocene volcanism and the recent calc alkaline volcanics indicate that currently there is a small component of subduction of the antarctic plate beneath the south american part of the scotia plate

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

genesis of cretaceous igneous rocks and its related large scale porphyry cu au mineralization in chating the middle lower yangtze river metallogenic belt the geochemical constrains abstract the newly discovered large chating porphyry cu au deposit is associated with early cretaceous intrusive rocks in the middle lower yangtze river belt mlyrb eastern china two types of intrusive rocks in the chating area have been identified i e the quartz diorite porphyry related to the cu au mineralization and the diorite porphyry that is ore barren zircon la icp ms u pb dating yields 145 136 ma for the chating intrusions they both belong to high k calc alkaline series with enrichments of large ion lithophile elements liles and light rare earth elements lrees depletions of high field strength elements hfses and heavy rare earth elements hrees and no or slightly negative eu anomalies the quartz diorite porphyry shows high sr contents and sr y ratios low mgo cr and ni contents medium la yb n and mg 34 6 54 6 and low k2o na2o