calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

did a katian large igneous province trigger the late ordovician glaciation a hypothesis tested with a carbon cycle model abstract the ordovician is known as a period with high temperatures and high sea levels and a cooling event at the end of the period leading to the hirnantian glaciation and the first of the big five mass extinctions of marine life the cause of this cooling event remains unclear several authors correlate it with a drop in atmospheric p co 2 to a threshold permitting the ice accumulation on the gondwana supercontinent however what are the reasons of the atmospheric p co 2 decrease here we follow the hypothesis initiated by barnes 2004 that an ordovician superplume event occurred during the middle to late ordovician such an event would not only have a large impact on the ordovician biodiversification barnes 2004 but it would also be responsible for the climatic upheaval during the late ordovician by the emplacement of a low latitudinal continental basaltic province that had an impact on the atmospheric p co 2 there is no direct evidence of a superplume event or of basaltic traps and the present study is therefore a hypothetical modelling approach where we demonstrate with a numerical box model including carbon alkalinity and phosphorus cycles coupled with a 1d climate model energy balance model ebm francois and walker 1992 that such a scenario allows to explain both the mid ashgill katian global warming event known as the boda event fortey and cocks 2005 and the subsequent late ordovician hirnantian glaciation because silicate weathering is enhanced upon warm and wet climate we try to constrain the size and the latitudinal emplacement of the basaltic province leading to a sufficient consumption of atmospheric p co 2 to the threshold proposed by herrmann et al 2004 a b to initiate a glaciation on gondwana

petrogenesis of post collisional a type granitoids from the urumieh dokhtar magmatic assemblage southwestern kerman iran constraints on the arabian eurasian continental collision abstract three plutons deh siahan bande bagh and baghe khoshk sharghi collectively referred to as the dbb hereafter in southwestern kerman in the southeastern part of the urumieh dokhtar magmatic assemblage udma of the zagros orogenic belt differ from the typical calc alkaline metaluminous i type intrusions of the region the dbb intrusions have a distinct lithological assemblage varying from diorite through monzogranite and monzonite to alkali feldspar syenite and alkali granite the dbb granitoids are metaluminous to slightly peraluminous alkaline to shoshonitic in composition and have high total alkali contents with k2o na2o high feot mgo values and low cao and mgo contents they are enriched in some liles such as rb and th and hfses such as zr y and rees except eu and depleted in sr and ba relative to primordial mantle and have low concentrations of transitional metals these features along with various geochemical discriminant diagrams suggest that the dbb granitoids are post collisional a type granitoids which had not been recognized previously in the udma the chondrite normalized ree patterns of the dbb granitoids show slightly enriched light rees la sm n 2 26 4 13 negative eu anomalies eu eu n 0 19 0 74 and flat heavy ree patterns gd yb n 0 80 1 87 the negative eu anomaly indicates an important role for plagioclase and or k feldspar during fractional crystallization whole rock rb sr isotope analysis yields an isochron age of 33 1 ma with an initial 87sr 86sr value of 0 7049 0 0001 whole rock sm nd isotope analysis gives endt values from 2 56 to 3 62 at 33 ma the positive endt and low isr values of the dbb granitoids together with their tdm of 0 6 0 7 ga suggest their formation from partial melting of a lithospheric mantle source modified by fluids or melts from earlier subduction processes melting of lithospheric mantle occurred via a dehydration melting process at pressures below the garnet stability field as a consequence of lithospheric mantle delamination or break off of a subducted slab and melting of the lithospheric mantle by upwelling of hot asthenosphere on the basis of rb sr age dating and the post collisional geochemical signatures of the dbb granitoids along with extensive pre collisional volcanic eruptions in middle eocene we suggest late eocene for the time of collision between the arabian and central iranian plates this also implies that the calc alkaline i type intrusions in the southwestern kerman and in other parts of the udma may have formed in a post collisional context

regional setting and geochronology of the late cretaceous banatitic magmatic and metallogenetic belt the 1 500 km long banatitic magmatic and metallogenetic belt bmmb of romania serbia and bulgaria is a complex calc alkaline magmatic arc of late cretaceous age it hosts a variety of magmatic hydro thermal cu au mo zn pb and fe deposits including europe s only world class porphyry copper deposits regional metallogeny can be linked to subduction of the vardar ocean during the late cretaceous as part of the closure of the neotethys ocean that had separated europe and africa in the mesozoic porphyry cu au mo and intimately associated epithermal massive sul phides dominate in the central segments of the belt in southernmost banat romania serbia and north west bulgaria these districts are the economically most im portant today including major active cu au mines at moldova noua in romania majdanpek veliki krivelj and bor in serbia and elatsite assarel and chelopech in bulgaria more numerous and mostly mined in the past are fe cu and zn pb skarns which occur mainly at the two ends of the belt in eastern bulgaria and in romania this paper summarises some of the deposit characteristics within the geodynamic framework of terminal vardar subduction heterogeneous terranes of the belt including the apuseni mountains at the western end are aligned parallel to the vardar front following continental collision of the dacia and tisza blocks all available geochronological data numerous k ar and some u pb and re os ages are compiled and are complemented by a new high precision re os date for the dognecea skarn deposit south west romania 76 6 0 3 ma these data indicate that magmatism extended over at least 25 million years from about 90 to 65 ma in each segment of the belt within apuseni mountains and banat where magma emplacement was related to syn collisional extension in the orogenic belt of carpathians ore formation seems to be restricted in time and maybe constrained by a shared tectonic event

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle

petrogenesis of the late cretaceous demirkoy igneous complex in the nw turkey implications for magma genesis in the strandja zone abstract the srednogorie zone in bulgaria and its eastern continuation in nw turkey the strandja zone consist of a late cretaceous magmatic belt comprised of plutonic volcanic and associated volcano sedimentary rocks the demirkoy igneous complex is the biggest cretaceous pluton within the strandja zone and its age is between 71 and 84 ma the demirkoy pluton and additional smaller plutons are intruded into triassic and older metamorphic basement rocks a contact metamorphic aureole with hornblende hornfels facies conditions was developed around the demirkoy pluton the wnw trending demirkoy pluton has an elliptical shape and a concentrically zoned internal structure in this structure gabbro diorites and quartz diorites are only seen in the sw corner of the pluton while granodiorites form the main body of the pluton surrounded by a zone of granite the granodiorites contain dioritic mafic microgranular enclaves small quartz monzonite lenses and or bodies and mafic dykes are also observed in the demirkoy igneous complex geochemically the demirkoy igneous complex has calc alkaline metaluminous and medium k characteristics there is a significant correlation between the lree hree ratios and sio 2 values chondrite normalized ree patterns are relatively flat la n lu n 2 in gabbros this ratio ranges from 3 to 11 for the diorite granodiorites and from 14 to 35 for the granites dioritic enclaves have ree patterns similar to those of the main body and have la n lu n values between 3 and 6 relatively enriched lile sr k rb ba and th and relatively depleted hfse ta nb and ti values indicate the classic subduction related origin of the complex the initial isotopic signatures range from e nd i 0 45 to 2 57 and 87 sr 86 sr i 0 7042 0 7064 for gabbros and e nd i 0 43 to 2 67 and 87 sr 86 sr i 0 7048 0 7059 for granodiorites and granites 18 o values range from 6 18 to 7 80 vsmow for gabbros and from 6 86 to 8 89 vsmow for granodiorites and granites based on geochemical and isotopic characteristics this magmatic complex can be subdivided into 4 groups a gabbro b diorite to alkali granite main body c syenite monzonite and d mafic dykes geochemical data and modelling results favour a typical subduction related enriched lithospheric mantle and lower crustal source for the demirkoy igneous complex slab derived fluids and crustal contamination also played a key role in the petrogenesis of this magmatic suite

mass mortalities on an irish silurian seafloor abstract recurrent deep water benthic communities dominated by diverse tabulate corals colonized soft substrates during the late llandovery transgression in western ireland a number of autochthonous shelly assemblages dominated by brachiopods corals and crinoids occur in the upper parts of the kilbride formation the highest and most distal levels are dominated by in situ tabulate corals that are mantled by a thin layer of volcanic ash most of the coral colonies could cope with limited sedimentation growth ceased after the volcaniclastic surge this catastrophic mode of preservation permits an accurate analysis of an early stage of community development in deep water benthos on a soft llandovery seabed periodically swept by volcaniclastic debris the kilbride faunas have implications for the palaeoenvironments and taphonomy of deep water marine faunas associated with areas of active volcanism

cenozoic k rich adakitic volcanic rocks in the hohxil area northern tibet lower crustal melting in an intracontinental setting it is generally accepted that the cenozoic potassic volcanic rocks of northern tibet were derived from a lithospheric mantle source here we report new chronological geochemical and isotopic data for the miocene ca 18 15 ma k rich adakitic volcanic rocks from the hohxil area of the songpan ganzi block in northern tibet we contend that these rocks were generated by partial melting of the mafic lower crust in an intracontinental setting unrelated to subduction of oceanic crust the hohxil rocks exhibit high sr y and la yb ratios high sr and la contents but low yb and y concentrations similar to adakites formed by slab melting associated with subduction however their relatively low e n d values 2 09 to 4 58 high 8 7 sr 8 6 sr 0 7072 0 7075 th u th ba and rb ba ratios and distinctive potassium enrichments k 2 o na 2 o are very different from the composition of typical adakites in addition those k rich adakitic rocks with the highest sio 2 contents 61 wt exhibit the lowest 8 7 sr 8 6 sr ratios and highest e n d values and are the oldest cenozoic volcanic rocks exposed in the songpan ganzi block suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma taking into account the composition of lower crustal mafic xenoliths in tibet as well as the tectonic and geophysical evidence we conclude that the hohxil adakitic magmas were produced by partial melting of amphibole bearing eclogites with a k rich mafic bulk composition in the lower part 55 km of the thickened northern tibetan crust partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward subducted continental lithosphere

gigantic ordovician volcanic ash fall in north america and europe biological tectonomagmatic and event stratigraphic significance biostratigraphical geochemical isotopic and paleogeographic data suggest that the millbrig k bentonite one of the thickest and most widespread ordovician volcanic ash beds in eastern north america is the same as the so called big bentonite in baltoscandia this is the first time that the same k bentonite has been identified in both north america and europe and it serves as a unique event stratigraphic marker over a large portion of the northern hemisphere this eruption produced at least 340 km sup 3 of dense rock equivalent ash that was deposited in a layer up to 1 2 m thick over several million square kilometers as much as 800 km sup 3 of additional ash may have fallen into the iapetus ocean for a total of 1 140 km sup 3 trace element geochemistry shows that the ash was derived from a felsic calc alkalic magmatic source characteristic of volcanism in a continental crust based destructive plate margin setting this is one of the largest if not the largest ash falls recorded in earth s phanerozoic stratigraphic record but its recognizable effect on faunas and floras was minimal and it did not result in a global extinction event the millbrig big bentonite bed provides accurate time control for sedimentologic paleoecologic and paleogeographic reconstructions across plates positionedmore in tropical laurentia and temperate baltica latitudes during middle ordovician time less

upper ordovician mohawkian carbon isotope δ13c stratigraphy in eastern and central north america regional expression of a perturbation of the global carbon cycle abstract the guttenberg carbon isotope excursion gice documented from eastern north america demonstrates the effects that regional geochemically distinct water masses upwelling and ocean circulation have on the carbon isotope record from carbonate platforms late turinian chatfieldian carbonates from oklahoma kentucky virginia and west virginia record a positive carbon isotope excursion 3 0 the gice excursion the gice excursion has relations to established biostratigraphy beginning in the north american midcontinent phragmodus undatus conodont zone and continuing through the plectodina tenuis zone sequence and event stratigraphy previously established models for positive carbon isotope shifts on carbonate platforms have been tested during the gice excursion where geochemically distinct water masses are defined for the upper ordovician a major eustatic sea level rise before the gice promoted a greater exchange of open ocean waters onto the carbonate platform of laurentia however restricted or sluggish circulation and exchange between water masses within the epeiric seas and the adjacent iapetus ocean were still apparent local variations documented in the gice excursion are directly related to upwelling of nutrient rich isotopically light waters increased primary productivity and the subsequent organic carbon production and burial

early cambrian palaeogeography and the probable iberia siberia connection abstract the end of the proterozoic beginning of the cambrian is marked by some of the most dramatic events in the history of earth the fall of the ediacaran biota followed by the cambrian explosion of skeletonised bilaterians a pronounced shift in oceanic and atmospheric chemistry and rapid climatic change from snowball earth to greenhouse conditions all happened within a rather geologically short period of time these events took place against a background of the rearrangement of the prevailing supercontinent some authors view this as a sequence of individual supercontinents such as mesoproterozoic midgardia neoproterozoic rodinia and early cambrian pannotia assembled in the mesoproterozoic this supercontinent appears to have existed through the neoproterozoic into the early cambrian with periodic changes in configuration the final rearrangement took place during the precambrian cambrian transition with the cadomian and related phases of the pan african orogeny the distribution of early cambrian molluscs and other small shelly fossils ssf across all continents indicates a close geographic proximity of all major cratonic basins that is consistent with the continued existence of the supercontinent at that time subsequently rodinia experienced breakup that led to the amalgamation of gondwana separation of laurentia baltica siberia and some small terranes and the emergence of oceanic basins between them spreading oceanic basins caused a gradual geographic isolation of the faunal assemblages that were united during the vendian early cambrian

environmental magnetic properties used to assess sediment sources and transport patterns in the muddy area along the zhejiang coast china abstract the muddy area along the zhejiang coast contains a record of historical environmental conditions and can facilitate a better understanding of the linkages among terrestrial sediment fluxes littoral sediment dynamics and climate change however the origin and transport of the sediments in this muddy area has been open to debate we present a detailed environmental magnetic study with high density sampling to provide magnetic evidence for the input and transport of the materials in this region our results show that magnetic minerals in the sediments are generally dominated by magnetite and titanomagnetite with minor amounts of maghemite and hematite the magnetic minerals are mainly pseudo single domain psd particles and decrease in concentration seawards based on the coastal current flow and magnetic characteristics we argue that the sediments in this area are mainly derived from the changjiang river the mountainous rivers in the surrounding islands and erosion by coastal currents notably the increase in sediment input from small mountainous rivers and coastal erosion in the zhejiang coastal area is primarily associated with decreased sediment discharge from the changjiang river however there is no evidence of the accumulation of taiwan derived sediment in this area

40ar 39ar and nd sr isotopic characteristics of mid ordovician north american k bentonites a test of early paleozoic laurentia gondwana interactions highly altered tephra layers k bentonites of mid ordovician age are widespread throughout the midcontinent region of the united states and the appalachian taconic foreland basin the numerous ash layers their wide areal extent and their large volume provide evidence that extensive volcanic activity accompanied taconic orogenesis the mineralogical and geochemical characteristics of these widespread ordovician tephra layers can be used to constrain the nature of their source region and thereby help test recent hypotheses that gondwana collided with laurentia in mid ordovician time initial 143nd 144nd and 87sr 86sr ratios of two k bentonites that occur traced throughout the midcontinent are 0 51183 0 51187 and 0 7101 0 7120 respectively isotopic ratios for k bentonites within the taconic basin are 0 51178 0 51201 and 0 70564 0 70905 the isotopic composition of the tephra layers and the ubiquitous presence of proterozoic zircon xenocrysts within the k bentonites is consistent with an origin by crustal anatexis the cooling history of the source is constrained by 842 996 ma 40ar 39ar apparent ages of hornblende xenocrysts contained within two taconic basin k bentonites a possible south american source region of the k bentonites is the arequipa massif of peru this region contains early paleozoic arc related rocks many of which contain mid proterozoic zircon xenocrysts and early proterozoic gneisses which were affected by grenville age granulite grade metamorphism gneisses within the adirondack highlands of the grenville province have similar end 450 ma values and hornblende cooling ages to the k bentonites consistent with this region as a source caradocian k bentonites in sweden have sr isotopic ratios similar to the north american beds if the swedish and north american k bentonites were produced from the same source then baltica and laurentia were proximal in ordovician time precluding a gondwana laurentia link if caradocian k bentonites with north american characteristics are found in western south america support for a laurentia gondwana link would be established if such k bentonites cannot be found considerable doubt would be cast on this model

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

the k ar and 40ar 39ar methods revisited for dating fine grained k bearing clay minerals abstract the strengths and weaknesses of the two ar isotopic methods k ar and 40ar 39ar were evaluated on the basis of respective recent applications mainly on low temperature k bearing illite type clay minerals this review includes a presentation of basic analytical and technical aspects for both methods as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization whenever possible the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates the comparative review examines stratigraphic dating of glauconites indirect dating of low temperature ore deposits dating of burial related illitization and dating of polyphased tectono thermal activity more specifically of fault gouges some pending questions such as the necessary encapsulation due to 39ar recoil and its restoration into step heating patterns are also raised together with the new potential of ar dating of nanometric illite crystals weakness of the k ar method is in its pioneering status that makes many believe that it is no longer accurate because of its traditional analytical aspects and of the k determinations leading to somewhat large uncertainties however precise evaluation of varied applications points to a k ar method having probably larger applicability in sedimentary to diagenetic environments than the 40ar 39ar method the drawbacks become less important if the method is applied to nanometer sized clay minerals in diagenetic to low grade metamorphic environments in this instance the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations if mathematically justified can be reduced by duplicate analyses weakness of the 40ar 39ar method is in its basics such as the 39ar recoil the necessary encapsulation the reintegration of the 39ar into the step heating patterns and the meaning of the step heating patterns that are more suggestive of variable 39ar reservoirs created among the clay particles by irradiation than of meaningful geologic ages if the k ar method is the preferred method for dating diagenetic clay processes such as glauconitization illite crystal nucleation and growth or low temperature hydrothermal activities then the 40ar 39ar method has more potential in dating low temperature tectono thermal activities and in detailing mixtures of multi generation illite

western versus eastern black sea tectonic evolution pre rift lithospheric controls on basin formation abstract we demonstrate the key role of pre rift rheology on the kinematics of basin formation in the western and eastern black sea basins constraints on modelling results are provided by a large data set based on more than 50 000 km of multichannel seismics offshore and onshore wells regional gravity and magnetic surveys refraction seismics and field studies the model supports the presence of important differences in the thickness and in the thermal state of the lithosphere which rifted to form the western middle barremian and the eastern middle paleocene black sea subbasins a 200 km and a 80 km thick pre rift lithosphere appear to have driven the deformation in the western and in the eastern black sea respectively differences in the geometry and in the mechanical properties of the pre rift lithosphere have a strong control on the depth of necking and thus on the basin morphology the model sheds light on palaeotectonic and palaeogeographic reconstructions duration of rifting events location of subsiding areas and erosional surfaces the western and the eastern parts of the black sea appear to be two distinct basins characterised by different evolutionary paths determined by different pre rift conditions

gas reservoir potential of the lower ordovician beekmantown group quebec lowlands canada the beekmantown group in the quebec lowlands was deposited as part of an extensive early ordovician coastal and shallow marine complex on the eastern margin of the north american craton the beekmantown is stratigraphically equivalent to the beekmantown knox arbuckle and ellenburger rocks of the united states and is subdivided into two formations the sandstone rich theresa formation and the overlying dolomite rich beauharnois dolomites of the beekmantown provide an important exploration target in both the autochthon and the overlying thrust sheets of the canadian and u s appalachians the reservoir potential of the autochthonous beekmantown group in the quebec lowlands can be determined from seismic data well logs cuttings and petrographic analyses of depositional and diagenetic textures deposition of the beekmantown occurred along the western passive margin of the iapetus ocean by the late ordovician the passive margin had been transformed into a foreland basin faulting locally positioned upper ordovician utica source rocks against the beekmantown and contributed to forming hydrocarbon reservoirs the largest beekmantown reservoir found to date is the st flavien field with 7 75 bcf of original gas methane in place in fractured and possibly karst influenced allochthonous dolomites within a thrust fault anticline the beekmantown below the thrust sheets forms a northward thinning wedge of peritidal and subtidal deposits seven major depositional units can be distinguished in cuttings and correlated with wireline logs most of these units form northward thinning sediment wedges and were deposited on a gently dipping ramp quartz sandstones dominate updip whereas shallow subtidal pelletal to skeletal limestones dominate downdip a widespread blanket of shaly dolomite is the uppermost unit of the beekmantown but is of poor reservoir quality dolomites in the beekmantown contain vuggy moldic intercrystalline and fracture porosity early porosity formed at the top of the major depositional units in peritidal dolomites however much of this porosity was later filled by late stage calcite cement after hydrocarbon migration thus a key to finding gas reservoirs in the autochthonous beekmantown is to define ordovician paleostructures in which early and continuous entrapment of hydrocarbons prevented later cementation

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

a new age model for the ordovician sandbian k bentonites in oslo norway abstract during the late ordovician large explosive volcanic eruptions deposited worldwide k bentonites including the millbrig and deicke k bentonites in north america and the kinnekulle k bentonite in scandinavia we have studied a classical locality in oslo containing one of the most complete sections of k bentonites in europe in a 53 m section of sandbian age we discovered 33 individual k bentonite beds the most notable beds being the kinnekulle and the upper grimstorp k bentonite magnetic susceptibility ms measurements on two intervals show significant periodicity peaks interpreted as milankovitch cycles and thus astronomically forced changes in sediment supply and composition these cycles fit remarkably well with both the expected milankovitch periodicities for the ordovician as well as the radiometric ages presented in this study and may represent one of the most convincing demonstrations of milankovitch cycles from the lower paleozoic so far five of the k bentonites have been dated by high precision chemical abrasion thermal ionization mass spectrometry ca tims u pb zircon geochronology where the kinnekulle k bentonite gives an age of 454 06 0 43 ma we have integrated the new data and calculated an age model showing the sedimentation rates through the section and thereby the ages of each of the 33 k bentonites using the age model we further present a new age for the sandbian katian stage boundary the section in oslo provides the highest resolution window into the upper ordovician k bentonite succession so far and helps shed more light on the chronology of one of the most intense volcanic periods of the paleozoic and the relationship with the global carbon cycle changes that followed

phanerozoic large igneous provinces lips heatt haline euxinic acidic thermal transgression episodes and mass extinctions abstract the phanerozoic consists of three mutually exclusive climate states icehouse greenhouse and hothouse the greenhouse is the default state representing 70 of the phanerozoic it is characterized by a weak thermal mode pole driven deep water formation with little or no continental sea ice and weakly oxic cool water sinking near the poles nitrogen limitation characterizes greenhouse oceans in which ammonium may be the dominant bioavailable form of nitrogen the steady state greenhouse default must be forced to produce either of the other two states continent continent collision produces the intense chemical weathering that draws down atmospheric co2 and results in the icehouse state that comprises 20 25 of the phanerozoic icehouses have ice caps in contact with the ocean that produce icebergs and foster sea ice formation sea ice formation fills the ocean with highly oxic phosphorus limited icewater except for a thin layer of warm surface water in the tropics of the 23 most conspicuous mass extinctions of the phanerozoic including the late ediacaran 4 are associated with ice volume related regressions and 3 are problematic we suggest that the other 16 mass extinctions occurred when greenhouse climate was forced to hothouse climate large igneous provinces lips can trigger the heatt haline euxinic acidic thermal transgression mechanism which can force greenhouse climate to a hothouse climate state the rapid voluminous co2 emissions of lips cause a series of events that result in very slow but pronounced haline mode circulation mid latitude driven deep water formation that produces warm saline bottom water at very dry hot subtropical and middle latitudes this water upwells at the poles causing warm and moist equable climate hothouse development promotes deep sea anoxia that is generated in concert with co2 buildup and warming the spread of anoxia favors reduced nitrogen over nitrate and limited iron dust curtails nitrogen fixation this leads to dominance of primary productivity by primitive green algae and or cyanobacteria progression to euxinia that reaches the photic zone severely curtails primary productivity via poisoning even as availability of usable nitrogen plummets the result is eukaryotic mass extinction we suggest that the 16 phanerozoic mass extinctions referred to above resulted at least in part from the heatt process evidence for this relationship at 11 of the most severe extinctions is convincing data sets for the other 5 are strongly suggestive of heatt related mass extinction but do not yet have enough documentation to reliably characterize them as heatt episodes

age and tectonic setting of bocsa and ocna de fier dognecea granodiorites southwest romania and of associated skarn mineralisation the granodiorite intrusion at ocna de fier dognecea in the western south carpathians romania triggered the formation of a classic fe pb zn skarn deposit the intrusive is related to the larger composite bocsa laccolith five kilometres north that is part of the regional banatite suite previous work indicated a k ar age of 65 57 ma and postulated an andean type subduction related tectonic setting for the intrusions we report ion probe u pb zircon ages of 79 6 2 5 ma for the bocsa laccolith and 75 5 1 6 ma for the ocna de fier pluton which date their emplacement fission track dating on titanite gives slightly younger ages 78 4 ma for bocsa and 73 4 ma for ocna de fier together with zircon and apatite data from the same samples average cooling rates of 52 c ma and 83 c ma are calculated for the bocsa and ocna de fier intrusives respectively a post collision tectonic setting is proposed on the basis of field evidence the timing of intrusions in the context of regional tectonic evolution and trace element geochemistry

the influence of amphibole fractionation on the evolution of calc alkaline andesite and dacite tephra from the central aleutians alaska petrologic studies of tephra from kanaga adak and great sitkin islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc alkaline andesite and dacite magmas in the central aleutians amphibole is ubiquitous in tephra from kanaga and adak islands whereas it is present only in a basaltic andesite pumice from great sitkin dacitic tephra from great sitkin do not contain amphibole hornblende dacite tephra contain hb plag ox opx cpx phenocrysts with simple zoning patterns suggesting that the dacites evolved in isolated magma chambers andesitic tephra from adak contain two pyroxene and hornbelende populations and reversely zoned plagioclase indicating a more complex history involving mixing and fractional crystallization mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole bearing cumulate xenoliths both formed during the evolution of high al basalts the presence of amphibole in andesitic and dacitic tephra implies that aleutian cale alkaline magmas evolve in the mid to lower crust under hydrous 4 wt h2o and oxidizing ni nio conditions amphibole bearing andesites and pyroxene bearing dacites from great sitkin indicates fractionation at several levels within the arc crust despite its absence in many calc alkaline andesite and dacite lavas open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on adak island neither open nor closed system fractionation involving a pyroxene bearing assemblage is capable of explaining the trace element concentrations or ratios found in the adak suite we envision a scenario where amphibole was initially a liquidus phase in many calc alkaline magmas but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust the mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma whereas the trace element chemistry provides evidence for a earlier amphibole bearing mineral assemblage

simulating the dispersal of tephra from the 1991 pinatubo eruption implications for the formation of widespread ash layers abstract puff and hazmap two tephra dispersal models developed for volcanic hazard mitigation are used to simulate the climatic 1991 eruption of mt pinatubo puff simulations indicate that the majority of ash was advected away from the source at the level of the tropopause 17 km several eruptive pulses injected ash and so2 gas to higher altitudes 25 km but these pulses represent only a small fraction 1 of the total erupted material released during the simulation comparison with toms images of the so2 cloud after 71 and 93 h indicate that the so2 gas originated at an altitude of 25 km near the source and descended to an altitude of 22 km as the cloud moved across the indian ocean hazmap simulations indicate that the pinatubo tephra fall deposit in the south china sea was formed by an eruption cloud with the majority of the ash concentrated at a height of 16 18 km results of this study demonstrate that the largest concentration of distal ash was transported at a level significantly below the maximum eruption column height 40 km and at a level below the calculated height of neutral buoyancy 25 km simulations showed that distal ash transport was dominated by atmospheric circulation patterns near the regional tropopause in contrast the movement of the so2 cloud occurred at higher levels along slightly different trajectories and may have resulted from gas particle segregations that took place during intrusion of the pinatubo umbrella cloud as it moved away from source

the tectonics of the strandja massif late variscan and mid mesozoic deformation and metamorphism in the northern aegean the strandja massif is a mid mesozoic orogenic belt in the balkans build on a late variscan basement of gneisses migmatites and granites new single zircon evaporation ages from the gneisses and granites indicate that the high grade metamorphism and plutonism is early permian in age 271 ma the late variscan basement was unconformably overlain by a continental to shallow marine sequence of early triassic mid jurassic age during the late jurassic early cretaceous oxfordian barremian the lower mesozoic cover and the basement were penetratively deformed and regionally metamorphosed in greenschist facies possibly due to a continental collision an rb sr biotite whole rock age from a metagranite dates the regional metamorphism as late jurassic 155 ma deformation involved north vergent thrust imbrication of the basement and the emplacement of allochthonous deep marine triassic series over the jurassic metasediments the metamorphic rocks of the strandja massif are unconformably overlain by the cenomanian shallow marine sandstones during the senonian the northern half of the strandja massif formed a basement to an intra arc basin and to a magmatic arc generated above the northward subducting tethyan oceanic lithosphere the srednogorie arc closed during the early tertiary through renewed northward thrusting of the strandja massif

ordovician k bentonites in the upper plate active margin of western gondwana famatina ranges stratigraphic and palaeogeographic significance abstract numerous thin bedded tabular pale yellowish clay bands are interlayered with black shales in a biostratigraphically constrained early ordovician volcano sedimentary succession at famatina western argentina this region was part of a fairly continuous upper plate convergent volcanic chain that fringed western gondwana mineralogy on both clay and non clay fractions whole rock chemistry and field observations on these distinctive event beds indicate that they originated as relatively coarse fallout tephras altered first into bentonites and later through burial metamorphism into k bentonites metabentonites these tephras were deposited as single crystals and glassy dust or pumiceous fragments in a restricted subtidal environment the region of famatina has previously been inferred as the source of abundant distal k bentonites recorded in the adjacent lower plate allochthonous precordillera terrane however these k bentonites within the proximal arc site were unknown and rather unexpected since they are generally better preserved like distal deposits associated either with central vent plinian ultraplinian eruptions or with accompanying co ignimbrite ash clouds their chemistry and comparison with those k bentonites in the precordillera allow tracing an evolution from volcanic arcs into continental crust k bentonites described in this paper are much older than those recorded in the adjacent precordillera terrane and seem to be associated with a first eruptive period along western gondwana that has no counterpart in the argentine precordillera suggesting a significant longitudinal separation between these two regions by the early ordovician

the kapanbogazi formation a key unit for understanding late cretaceous evolution of the pontides n turkey abstract the pontides forming the south western continental margin of the black sea consist of two tectonic units the istanbul zone in the west and the sakarya zone in the central and eastern parts the sinop basin in the sakarya zone is filled from base to top by hauterivian to albian turbidites cenomanian turonian red pelagic sediments turonian campanian magmatic arc and related deposits and by the uppermost campanian to middle eocene post magmatic units developed on the southern passive margin of the black sea based on nannofossil dinoflagellate foraminifera and radiolaria data we describe the kapanbogazi formation a cenomanian turonian unit in the sinop basin represented by red calcareous siliceous pelagic shales limestones and cherts passing gradually from the albian black shales these sediments possibly represent deepest depositional conditions of the basin during the cenomanian turonian interval and also reflect the transition from an anoxic to an oxic palaeoenvironmental setting the istanbul zone to the west was emerged during the deposition of the kapanbogazi formation in the sakarya zone in the pontides red pelagic sediments were deposited at different times during the cenomanian maastrichtian interval because the kapanbogazi formation was deposited only in the sakarya zone and because it is present in limited outcrops due to structural reorganization and thick overlying volcanoclastic pile most previous authors assumed cenomanian turonian hiatus herein we describe detailed palaeontological data from this unit and discuss their importance to the interpretation of depositional history and tectonics of the black sea region as well as climatic and eustatic implications

origin alteration and geochemical correlation of late permian airfall tuffs in coal measures sydney basin australia abstract the late permian illawarra coal measures of the southern sydney basin contain tuffs derived from altered pyroclastic airfall material these units represent ideal stratigraphic time planes because as the products of airfall volcanic ashes they are distributed over wide areas in short periods of geological time geochemical correlation of the tuffs of the illawarra coal measures with similar units in the stratigraphically equivalent newcastle coal measures offers a unique opportunity to understand the temporal and spatial relations during deposition of coal bearing units in the sydney basin thirty five core samples comprising 12 samples from each of the burragorang and farmborough claystone members and 11 samples from the huntley claystone member were analysed for major and trace elements by x ray fluorescence and instrumental neutron activation and the results were subjected to stepwise discriminant analysis the elements that served as the best discriminators between tuffs were in order of atomic number ti v sn hf and th samples for basin wide correlation were taken from thick interseam tuffs of the newcastle coal measures and treated as unknowns in the discriminant model the burragorang claystone member shows a strong geochemical correlation with the awaba tuff and the farmborough claystone member correlates with the warners bay tuff the huntley claystone member however has proved difficult to correlate with the nobby s tuff the proposed stratigraphic equivalent in the newcastle coal measures petrographic data for tuffs of the illawarra coal measures indicate that they were deposited by airfall mechanisms and consist mainly of kaolinite and mixed layer illite smectite clays quartz and plagioclase crystal fragments lithic fragments and secondary calcite and siderite zr tio2 and nb y ratios tectonic discrimination diagrams and chondrite normalised ree patterns la yb 3 3 to 11 9 are identical to those for tuffs of the newcastle coal measures and are interpreted as being derived from magma that was calc alkaline rhyodacitic to rhyolitic in composition and derived from a continental volcanic arc tectonic setting the source of the tuffs is interpreted to be an active volcanic arc to the east of the present coastline currarong orogen but a lack of evidence from below the continental shelf makes it difficult to make concrete conclusions on its likely extent and composition

late ordovician cooling event evidence from the siberian craton abstract the upper ordovician of the tungus basin on the siberian platform is represented by cool water carbonates onset of cool water conditions starts at the darriwilian and it is associated with widespread phosphatization especially in the volginian kirensko kudrinian and chertovskian regional stages mid darriwilian lower sandbian the shift from tropical type to temperate type carbonates was preceded by destruction of the warm water carbonate factory and a large input of siliciclastic material baykit sandstone during the vikhorevian and mukteian regional stages lower to mid darriwilian wide distribution of temperate type carbonates across the siberian platform located in the low latitudes in the ordovician time can be explained by upwelling of cold oceanic waters and their penetration into epicontinental seas the same situation was recorded for the upper ordovician of the north american platform newly discovered k bentonite beds in the uppermost sandbian katian of the tungus basin are also associated with cool water carbonates the k bentonite beds point to extensive volcanism on or near the western in present day orientation margin of the siberian craton in the late ordovician time timing of the volcanism is surprisingly close to the period of volcanic activity of the taconic arc near the margin of laurentia both upwelling and volcanism seem to be activated by the same plate tectonic reorganization and both these processes seem to play an important role in a global later ordovician cooling terminated by the hirnantian glaciation

lower ordovician iron ooids and associated oolitic clays in russia and estonia a clue to the origin of iron oolites abstract on the baltic platform a lower llanvirn ordovician iron oolite can be traced for a distance of 1200 km from norway to the east of lake ladoga in russia this oolite is usually thin seldom exceeding 0 5 m and is dominated by goethite limonite type ooids the easternmost part of the oolite from tallinn to ladoga is examined here the oolitic limestone is intercalated with oolitic clay beds the mineralogical chemical and isotopic composition and other indicators point to volcanic ash being the source for the clay similarities in ree distribution patterns and immobile element contents between ooids and the oolitic clay suggest that the ooids were also formed from volcanic ash

tectonics of the banat srednogorie rift abstract the banat srednogorie rift is located on the balkan peninsula in the northeastern part of the alpine system it is superimposed on a continental type basement which cuts across older structures the rift is filled with a 3 8 km thick layer of upper cretaceous sedimentary and volcanogenic rocks the volcanic and plutonic formations belong to four series calc alkaline tholeiitic potassic alkaline and peralkaline the rift was formed in three stages 1 1 the formation of the sedimentary trough 2 2 subsidence accompanied by intense volcanic activity and tephroidal flysch sedimentation 3 3 inversion expressed in the laramide deformation the epicontinental rifting occurred in the preorogenic stage of the geotectonic development of the balkanide dinarides after the closure of the vardar paleo ocean and the formation of an embryonal orogenic arc

quaternary volcanism near the valley of mexico implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions the valley of mexico and surrounding regions of mexico and morelos states in central mexico contain more than 250 quaternary eruptive vents in addition to the large composite volcanoes of popocatepetl iztaccihuatl and nevado de toluca the eruptive vents include cinder and lava cones shield volcanoes and isolated andesitic and dacitic lava flows and are most numerous in the sierra chichinautzin that forms the southern terminus of the valley of mexico the chichinautzin volcanic field cvf is part of the e w trending mexican volcanic belt mvb a subduction related volcanic arc that extends across mexico the crustal thickness beneath the cvf 50 km is the greatest of any region in the mvb and one of the greatest found in any arc worldwide lavas and scoriae erupted from vents in the cvf include alkaline basalts and calc alkaline basaltic andesites andesites and dacites both alkaline and calc alkaline groups contain primitive varieties that have whole rock mg mgo and ni contents and liquidus olivine compositions fo90 that are close to those expected of partial melts from mantle peridotite primitive varieties also show a wide range of incompatible trace element abundances e g ba 210 1080 ppm ce 25 100 ppm zr 130 280 ppm data for primitive calc alkaline rocks from both the cvf and other regions of the mvb to the west are consistent with magma generation in an underlying mantle wedge that is depleted in ti zr and nb and enriched in large ion lithophile k ba rb and light rare earth la ce elements extents of partial melting estimated from ti and zr data are lower for primitive calc alkaline magmas in the cvf than for those from the regions of the mvb to the west where the crust is thinner the distinctive major element compositions low cao and al2o3 high sio2 of the primitive calc alkaline magmas in the cvf indicate a more refractory mantle source beneath this region of thick crust in contrast primitive alkaline magmas from the cvf and other regions of the mvb show compositional similarities to intraplate type alkali basalts erupted behind the arc in the mexican basin and range province these similarities are consistent with the hypothesis that slab induced convection in the mantle wedge beneath the mvb causes advection of asthenospheric mantle from behind the arc to the region of magma generation trace element systematics of primitive magmas in the mvb reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub alkaline basaltic melts

trace element and isotopic evidence for two types of crustal melting beneath a high cascade volcanic center mt jefferson oregon mt jefferson is an andesite dacite composite volcano in the cascade range the locus of andesite and dacite dominated volcanism for at least 1 million years a large trace element data set for mt jefferson and its surrounding mafic volcanic platform effectively rules out any fractionation based model fc or afc for the generation of mt jefferson andesites several incompatible element zr nb y concentrations decrease in the range from basalt to andesite and then increase in the range from andesite to rhyodacite others ba rb la th remain constant or show a slight increase in the basalt to andesite range with modest increases from andesite to rhyodacite systematic variations in highly incompatible element ratios such as ba la and rb th suggest magma mixing dominates the trace element signatures rhyodacites are isotopically uniform 87sr 86sr 0 70325 0 70343 206pb 204pb 18 75 18 85 18o 6 3 0 3 whereas andesite and dacite are more variable 87sr 86sr 0 70291 0 70353 206pb 204pb 18 59 18 86 18o 6 0 0 6 typical basaltic andesite has 87sr 86sr 0 70326 0 70358 206pb 204pb 18 78 18 85 and 18o 5 9 0 4 sr rich 1 000 ppm basaltic andesite is more variable 87sr 86sr 0 70300 0 70360 206pb 204pb 18 70 18 89 18o 5 9 0 4 the data define mixing arrays with one end member at 87sr 86sr 0 7029 206pb 204pb 18 59 another at rhyodacite and a third at 87sr 86sr 0 7036 206pb 204pb 18 89 the first end member is defined by sr rich 800 1 200 ppm andesite with high al2o3 and low k2o ba and rb th the third one by k2o and very sr rich 2 000 ppm shoshonite isotopic data for basalts in northern oregon preclude any fractionation relationship between basalt and either rhyodacite or sr rich andesite e g the minimum 206pb 204pb ratio in basalt is 18 83 considered in light of geophysical models for the cascades these data suggest two types of crustal melting beneath the arc rhyodacite may be generated at 25 30 km depth by partial melting of arc basalt like amphibolite at 850 900 c sr rich andesite may be formed by partial melting of depleted morb like mafic granulite at 35 45 km depth at 1 000 1 100 c experimental and ree evidence supports these interpretations as does the restriction of sr rich andesite in the cascades to the area south of the 100 mw m2 heat flow contour between mt jefferson and mt hood thick crust and high heat flow are necessary to produce such andesite

mineral chemistry crystallization conditions and geodynamic implications of the oligo miocene granitoids in the biga peninsula northwest turkey abstract widespread plutonic rocks in nw turkey occur within the southward younging and overlapping magmatic belts across the aegean region post collisional magmatism is represented by a series of granitoidic intrusions and volcanic successions k ar and u pb la icp ms zircon dating of the kazdag and yenice plutons yielded ages between 20 5 0 5 ma and 27 89 0 17 ma late oligocene early miocene the granitoid samples are high k calc alkaline and metaluminous to slightly peraluminous the 87sr 86sr values for the granitoids enclaves and leucocratic rocks range between 0 705168 and 0 708357 the initial 143nd 144nd ratios calculated for the crystallization ages of ca 23 27 ma are between 0 512425 and 0 512614 and the end values vary from 3 5 to 0 2 the nd tdm model ages range between 0 73 and 1 13 ga these samples are enriched in liles and lree and depleted in hfses with negative eu anomalies indicating that the melts were derived from an enriched lithospheric mantle modified by subducted slab derived melts energy constrained assimilation and fractional crystallization ec afc modelling indicates that fractional crystallization and crustal assimilation modified the parent magma s composition during its residence in the upper crust the mineral chemistry of amphiboles pyroxenes biotites and feldspars is used to constrain the pressure p temperature t oxygen fugacity log o2 and water contents h2omelt during the crystallization of the magmas in the studied granitoids the clinopyroxene temperatures are in the range of 823 910 45 c the amphibole temperatures for the studied plutonic rocks are in the range of 707 926 c mean 798 45 c and the crystallization depths are estimated to be in the interval of 1 02 10 2 km the nw anatolian plutonic rocks can be considered to have been equilibrated at the oxygen fugacities of calcic amphiboles log o2 between 8 99 and 13 96 bars mean 12 11 bar and h2omelt contents between 1 63 and 6 79 mean 4 15 meanwhile the biotites which display consistent oxygen fugacity values 10 65 to 13 22 suggest their reliability with the typical values of calc alkaline magma crystallization these values suggest a relatively higher oxidation state during crystallization and are related to arc magmatism all of the calculated values indicate that all of the plutons were emplaced at shallow crustal levels it can be inferred that the oligo miocene magmatism in nw turkey may be linked to crustal thinning that resulted from slab roll back and a back arc extensional regime after the collision between the sakarya zone and anatolide tauride platform

gigantic volcanic eruptions and climatic change in the early eocene 23 layers of altered volcanic ash bentonites originating from the north atlantic igneous province have been recorded in early eocene deposits of the austrian alps about 1 900 km away from the source area the austrian bentonites are distal equivalents of the main ash phase in denmark and the north sea basin we have calculated the total eruption volume of this series as 21 000 km3 which occurred in 600 000 years the most powerful single eruption of this series took place 54 0 million years ago ma and ejected ca 1 200 km3 of ash material which makes it one of the largest basaltic pyroclastic eruptions in geological history the clustering of eruptions must have significantly affected the incoming solar radiation in the early eocene by the continuous production of stratospheric dust and aerosol clouds this hypothesis is corroborated by oxygen isotope values which indicate a global decrease of sea surface temperatures between 1 and 2 c during this major phase of explosive volcanism

alkalic ocean island basalt type and calc alkalic volcanism in the mexican volcanic belt a case for plume related magmatism and propagating rifting at an active margin the mexican volcanic belt has been traditionally regarded as a classic case of subduction related calc alkalic volcanism however a series of geologic geophysical and petrological arguments makes this simple relationship doubtful a seismic gap beneath the belt a large scale mantle anomaly a graben triple junction domain and the presence of volumetrically important oceanic island basalt oib volcanism throughout the belt suggest a more complex tectonic scenario involving plume and subduction related processes we here propose a model involving the development of a propagating rift opening from west to east in response to plume activity the process started in miocene time within the western sector of the belt guadalajara and gave rise to a graben triple junction and oib type and calc alkalic volcanism extension and volcanism proceeded to the east giving rise to progressively younger ages for the initiation of oib type volcanism 1 miocene in the west e g guadalajara 2 pliocene in the central zone e g michoacan guanajuato and 3 quaternary farther east e g chichinautzin geochemical evidence suggests that part of the modern calc alkalic volcanism e g chichinautzin may be derived from magma mixing between the oib mafic magmas and silicic crust derived magmas however we do not preclude some influence of the subducting slab in the generation of other e g jorullo calc alkalic volcanic rocks our model suggests a currently unrooted upper plume attached to the subcontinental lithosphere which defines a hot zone beneath the mexican volcanic belt

geochemical and tight reservoir characteristics of sedimentary organic matter bearing tuff from the permian tiaohu formation in the santanghu basin northwest china abstract tuff reservoirs have been found in petroliferous basins but have not been sufficiently studied the discovery of the permian tiaohu formation reservoir in the santanghu basin in northwest china has offered an excellent opportunity to further enhance our knowledge of the tuff s geochemical and reservoir characteristics the tuff reservoir has the peculiar property of being a sedimentary organic matter bearing tuff in this study an integrated analysis of the organic geochemistry elemental and mineral compositions quartz crystallinity pore types and tight reservoir characteristics was conducted based on samples from the 20 30 m thick tuffs the extracted tuff samples have total organic carbon toc values of 0 5 1 0 wt total hydrocarbon yield values of 2 6 mg g and hydrogen index values of 20 336 mg hc g toc the organic matter consists predominantly of type iii and ii 2 kerogens and the temperature of the maximum yield of pyrolysate varies from 420 to 450 c which reflects the oil generating capacity of the rock the physical properties of the tuff are characterised by high porosity varying from 10 to 25 and low permeability the air permeability mainly ranges from 0 01 to 0 50 md the devitrification of vitreous textures within the tuff primarily dictates the reservoir s characteristics i e the greater the degree of devitrification the higher the porosity of the reservoir vitric tuffs have reservoirs with higher porosity than crystal vitric tuffs and the degree of devitrification is generally controlled by the burial depth temperature and organic matter organic acid content vitric tuffs with high porosity and permeability are usually located far from the crater and within the central lower part of the reservoir this research may influence geologists to pay more attention to the exploration value of this special type of reservoir in other regions

late vendian early palaeozoic tectonic evolution of the baltic basin regional tectonic implications from subsidence analysis abstract subsidence analysis was performed on 43 boreholes penetrating the upper vendian lower palaeozoic sedimentary succession of the baltic basin the results were related to lithofacial and structural data to elucidate subsidence mechanisms and the regional tectonic setting of basin development tectonic subsidence patterns are consistent throughout the basin for the time period studied an extensional tectonic subsidence event possibly of two phases is indicated from the late vendian to the beginning of the middle cambrian this event is seen in the southwestern part of the baltic basin peri tornquist zone until the earliest cambrian after which it is also observed in the sw ne trending baltic depression part of the basin basin development during this time is interpreted as recording the latest stages of break up of the precambrian super continent rodinia and ultimately the formation of the tornquist sea the late middle cambrian to middle ordovician tectonic subsidence pattern of the baltic basin is characteristic of post rift thermal subsidence of the newly formed passive continental margin of baltica developed along its southwestern edge a gradual increase in subsidence rate is observed from the middle late ordovician and throughout the silurian particularly for ludlow and pridoli times creating subsidence curves with convex shapes typical of foreland basin development the rate of late silurian tectonic subsidence increases significantly towards the southwest margin of the baltic basin adjacent to the present location of the north german polish caledonides the baltic basin therefore appears to have developed primarily as a flexural foreland basin during silurian oblique collision of baltica and eastern avalonia a foreland setting is supported by the influx of distal turbidites into the basin from southwest sources in the late silurian compressional deformation structures of early devonian lochkovian age are seen in seismic sections in the central part of the baltic basin lithuania these together with a change in subsidence pattern mark the end of the caledonian stage of basin development of the baltic basin

the geochemistry of the sedimentary rocks from the huadi no 1 well in the wufeng longmaxi formations upper ordovician lower silurian south china with implications for paleoweathering provenance tectonic setting and paleoclimate abstract the late ordovician early silurian transition was a critical interval in earth s history marked by mass extinction the gondwana glaciation and volcanic events here we present mineralogical and geochemical characteristics across the ordovician silurian boundary wufeng guanyinqiao and longmaxi formations from the huadi no 1 well in the sichuan basin in order to explore the paleoweathering provenance tectonic setting and paleoclimatic conditions of the source area thin sections xrd x ray diffraction data and major element results reveal that the rocks were abundant in clay minerals quartz minor feldspar pyrite and carbonate minerals the samples have similar chondrite normalized ree patterns to those of paas which are characterized by slight lree enrichments and accompanied by flat hree trends with weakly negative eu anomalies paleoweathering indices cia and pia suggest that the sediments experienced weak to moderate chemical weathering in the source area cia values in the lower wufeng formation range from 67 11 to 72 17 indicating a warm and humid climate the cia values in the upper wufeng formation decrease from 70 35 to 66 15 and the cia values in the guanyinqiao and lower longmaxi formation are persistently low 56 64 62 87 suggesting a cold and dry climate which is consistent with the widespread presence of glaciation in the hirnantian period further upward in the upper longmaxi formation the cia returns to a high value 65 59 which may reflect that the climate started getting warm and glaciation had finished fluctuations in cia values reflect that the climate shifted from warm to cold and then to warm again during the end ordovician early silurian time period the a cn k triangular diagram and sio2 al2o3 tio2 tio2 zr and la th hf bivariate diagrams indicate that the provenance of the rocks from the three formations was primarily felsic igneous rocks discriminant function diagrams and la th sc th co zr 10 and th sc zr 10 triangular diagrams show that the clastic sediments of the wufeng longmaxi formations were derived most likely from an active continental margin finally across the late ordovician early silurian tectonic activity and the end of the ordovician glaciation dominantly controlled the deposition of black shales and siltstones in the wufeng longmaxi formations of south china

absence of cocos plate subduction related basic volcanism in southern mexico a unique case on earth the relationship between volcanism and subduction of the cocos plate is examined on the basis of new as well as published sr nd and pb isotopic and geochemical data on late miocene to holocene ca 9 0 ma basic volcanic rocks from southern mexico and central america basic rocks with sio 2 52 were chosen in order to minimize the effects of crustal assimilation processes and therefore to test the contribution from deeper sources the subducted cocos plate and the mantle wedge by using fluid mobile to relatively fluid immobile elements and radiogenic isotope ratios for such rocks i show that the subduction of the cocos plate does not contribute to the basic volcanism in all of southern mexico as opposed to that in central america from guatemala to northwestern costa rica south mexican volcanism is related to ongoing rifting processes inferred from field geology seismology gravity tectonics and volcano alignments this lack of subduction relationship probably represents the first case on earth where the ongoing subduction of an oceanic plate does not give rise to basic volcanism such as is present throughout southern mexico

magmatism and metallogeny of the ridanj krepoljin belt eastern serbia and their correlation with northern and eastern analogues in the southwestern carpathians of sw romania and e serbia calc alkaline upper cretaceous paleogene subduction related magmatic rocks occur in two main zones the ridanj krepoljin belt e serbia represents the southern part of the western zone in this belt early maastrichtian dacites and subordinate andesites occur but there are indications of an unexposed granodioritic magmatism about 60 ma old pb zn ag and only subordinate cu mineralization is associated with this igneous activity the timok magmatic complex e serbia composed of upper cretaceous paleocene andesites and analogue intrusives represents the eastern zone south of the danube associated with this magmatism are cu au and very subordinate zn pb mineralization to the north sw romania in both zones mainly intrusive bodies diorites quartz diorites granodiorites monzonites of campanian paleocene age are exposed porphyry copper mo and in north pb zn cu mineralization are related to those intrusives the petrological and geochemical features of all these calc alkaline rocks are very similar it appears however that the associated polymetallic and the copper mineralization are both spatially and vertically separated

adakites the key to understanding lile depletion in granulites abstract for more than 20 years it has been argued that granulites of the lower continental crust are depleted in fluid mobile elements such as cs rb u and th either because they were removed by melting of the lower crust or through dehydration we argue that there is little evidence for a simple relationship between granulite facies metamorphism and element depletion and propose that for felsic orthogneisses the depletion may be a primary feature of crust generation processes modern adakites show a range of fluid mobile element ratios and adakites from the austral volcanic zone avz chile show fluid mobile element ratios very similar to those found in lower crustal felsic granulites this suite of adakites has a strong slab derived component we propose that modern adakites of the type found in the austral volcanic zone are analogous in their genesis to lower crustal felsic granulites and that their mafic precursors were already depleted in fluid mobile trace elements prior to melting an incremental melting model in which fluid mobile elements are released from an eclogitic slab prior to partial melting can explain the fluid mobile element behaviour of both modern adakites and lower crustal felsic granulites

quartz crystal size distribution of the ordovician millbrig k bentonite abstract crystal size distribution csd analysis has been applied to quartz crystals of the ordovician millbrig k bentonite which represents one of the largest known fallout ash deposits in the phanerozoic era to establish crystal growth histories and conditions in the magma chamber prior to eruption specific csds were examined for crystal growth conditions all millbrig samples exhibit lognormal concave down shapes in agreement with previously reported csds on large silicic systems but in contrast to more mafic systems characterized by linear csds concave down csds are possibly an intrinsic property of phenocryst growth in large volume silicic magmas as documented in the literature bindeman i n 2003 crystal sizes in evolving silicic magma chambers geology 31 367 370 quartz crystals of the millbrig must have been produced by surface controlled size dependent growth with a minor ostwald ripening component which could have easily changed to supply controlled growth due to a limited supply of nutrients as the crystals grew larger csds were also used to examine the possible origin of multiple ash beds in the millbrig these ash beds appear to be a product of a series of separate eruptions that represent separate magma layers or batches each with slightly different crystal growth conditions

fracture zones in the north atlantic from combined geosat and seasat data we present a new tectonic fabric map of the northern and central north atlantic ocean floor from 10 s to 66 n based on combined geosat and seasat altimetry data short wavelength vertical gravity along track was derived by hilbert transforming and high pass filtering geosat and seasat deflection of the vertical profiles the along track gravity data were used in conjunction with magnetic anomaly data to locate a large number of fracture zones as well as extinct spreading ridges and v shaped structures we traced many fracture zones more continuously and closer to adjacent continental margins than was previously possible we also mapped a number of unnamed fracture zones and fracture zone segments many of which were previously uncharted we have evaluated the agreement between the mapped fracture zones and synthetic flow lines as predicted from recent plate models for the opening of the north central north and south atlantic most fracture zones in this area correlate quite well with synthetic flow lines supporting the proposition that geosat seasat altimetry data serve as a useful tool to map fracture zones where insufficient bathymetric data are available many fracture zones including some small offset fracture zones in the central north atlantic have been stable and follow tectonic flow lines for tens of millions of years however many other fracture zones do not reflect a railroad track pattern of plate motions the geometry of many small and medium offset fracture zones indicates that the ridge axis discontinuities that produce them can migrate along the ridge causing off axis traces that deviate from tectonic flow lines velocity space diagrams constructed using a recent relative and absolute present day plate motion model indicate that some low angle v shaped troughs and ridges are likely formed by migration of the plate boundary relative to a mesospheric reference frame other low angle v shaped structures reflect slow migrations oscillations of small offset ridge axis discontinuities that are distinct from both tectonic flow lines and v shapes reflecting absolute plate motions high angle v shaped structures are the result of fast migrating ridge axis discontinuities propagating rifts

trace element and isotope sr nd geochemistry of porphyry and skarn mineralising late cretaceous intrusions from banat western south carpathians romania geochemical and isotopic strontium neodymium data have been obtained from 24 samples of seven late cretaceous intrusions associated with skarn tincova bocsa ocna de fier dognecea and porphyry copper molybdenum skarn deposits oravi a ciclova sasca and moldova nou in the south carpathians of romania the intrusions cut the supragetic getic nappes and form a 100 km long nne ssw trending lineament in the banat metallogenic province the samples range in composition from gabbro to granodiorite and define in major and trace element variation diagrams a calc alkaline to high potassium calc alkaline trend ranging from 50 to 67 sio2 they are magnetite bearing i type granitoids enriched in lile lree and depleted in nb ta and zr common features for magmas associated with subduction zones the lack of cumulate textures and of positive europium anomalies seems to indicate that the samples represent liquid compositions the observed trend thus approximates a liquid line of descent resulting from fractional crystallisation of parental magmas with similar major and trace element compositions initial strontium isotopic ratios sri vary from 0 7042 to 0 7058 and end t values range from 3 9 to 0 2 variations of sri and end t values with sio2 show minor crustal assimilation the generally positive end t values together with moderate sri indicate that the parental magmas result from partial melting of a rubidium enriched and lree depleted source such as a heterogeneous lithospheric mantle or a young mafic lower crust derived from it selective rubidium enrichment probably occurred during a previous subduction event by metasomatism of the overlying mantle wedge either in the early cretaceous or earlier as suggested by tdm model ages ca 600 ma the cu mo pb zn fe deposits are intimately associated with the intrusions and the banat romania region can be subdivided in two metallogenic zones in northern banat fe cu pb zn skarns occur whereas in southern banat porphyry style copper and molybdenum deposits predominate these differences result from a combination of several parameters 1 magma composition whereby copper and molybdenum rich deposits tend to be associated with calc alkaline compositions 2 an increase of the present day erosion level from south to north as indicated by the presence of large equigranular plutons in northern banat and of porphyritic cupolas and apophyses associated with porphyry style mineralisation in southern banat 3 the nature of the host rocks with skarns preferentially developed in calcareous host rocks and 4 local variations of conditions controlling the infiltration of fluids and the precipitation of ore minerals

organic matter accumulation of the wufeng longmaxi shales in southern sichuan basin evidence and insight from volcanism abstract the wufeng longmaxi formations wf lm fms in the sichuan basin are sets of graptolite bearing shale strata containing high frequency bentonite layers in order to investigate the influence of volcanism on the organic matter om accumulation in shales detailed observations of several typical profiles and wells were performed also analyses including total organic carbon toc contents and mineral compositions were conducted on 126 shale and 5 bentonite core samples major and trace element analyses were conducted on the selected 25 shale and 5 bentonite samples in the wf lm fms of well lu a results show that wf and lm1 4 members are the bentonites concentrated sections while lm5 6 are the bentonites relatively concentrated sections and no bentonite is found in lm7 9 members the high potassium k content of bentonites have the typical characteristics of intermediate acid calcium alkaline volcanic rocks such as andesite and dacite which probably originated from intraplate collision or volcanic arc structural backgrounds related to the collision belt of the cathaysia block to the yangtze block productivity indicators p content excess si p al cu al and ni al in the wf and lm1 3 members are all significantly higher than those in the lingxiang lx and lm4 5 members the higher biological productivity during the ordovician silurian o s transition was attributed to the massive and intensive volcanism the fertilization by volcanic ash brought abundant biologically relevant nutrient elements n p si fe and al into the ocean thus triggering the phytoplankton bloom and enhancing primary biological productivity all the redox condition indexes moef uef nief vef mo toc corg p and u th positively correlate with toc content during the o s transition the intensive and massive volcanic eruptions triggered global cooling sea level changes late ordovician mass extinction and primary productivity increases thus promoting anoxic and possibly intermittently euxinic bottom conditions in addition the deposited bentonite layers had a strong capacity for oxygen isolation volcanism should be considered when modeling the om accumulation of the wf lm shales

factors controlling organic matter accumulation in the upper ordovician lower silurian organic rich shale on the northeast margin of the upper yangtze platform evidence from petrographic and geochemical proxies abstract organic rich shales are widespread in the upper ordovician lower silurian wufeng longmaxi formations on the northeastern margin of the upper yangtze platform and these shales have been recognized to be the hydrocarbon source rocks and shale gas reservoirs with the greatest development potential in china however organic matter accumulation in these shales needs further research especially in the areas which experienced an extensive upwelling and volcanism here this work integrates detailed petrographic and geochemical proxies namely redox sensitive trace elements rstes and ratios of si al ti al from two ordovician silurian sections on the northeast margin of upper yangtze platform in order to understand the depositional environment and identify the major factors controlling the organic matter accumulation during the ordovician silurian transition data of si al and ti al ratios indicate that the deposition of black gray siliceous mudstones in the lower wufeng formation commenced with a transgression triggered by the boda warming event which led to a reduced detrital input based on rstes the black carbonaceous shales of the upper wufeng formation are organic rich owing to an elevated primary productivity caused by seasonal upwelling driven by the vigorous thermohaline circulation abundant nutrients supplied by the post glacial transgression and a mass biotic recovery of marine microorganisms stimulated productivity and further strengthened the depletion of oxygen in the water column which lead to high concentrations of organic matter in lower longmaxi formation although the water column conditions were not conducive to the accumulation of organic matter it was possible that the increased productivity from upwelling resulted in relatively high organic matter contents in the middle longmaxi formation the rstes especially cd mo co mn reveal that the organic matter accumulation of the wufeng longmaxi formation during ordovician silurian transition may have been primarily controlled by the preservation conditions and also influenced by the marine primary productivity and the detrital input

middle permian early triassic magmatism in the western pontides nw turkey geodynamic significance for the evolution of the paleo tethys abstract permian triassic magmatic rocks are widely exposed in different tectonic terranes and continental blocks in the aegean region this study is focussed on the istanbul zone and the istranca strandja massif which are located in the western pontides of nw turkey the kirklareli and the tepecik plutons intruded into the metamorphic basement rocks of the istranca massif and the sancaktepe pluton is emplaced into the istanbul paleozoic sedimentary sequences we present new geochemical data for these plutons in order to interpret their petrogenesis and time of emplacement and use this information for geodynamic interpretation the zircon u pb crystallization ages are 268 3 2 1 ma for the kirklareli pluton 249 4 1 5 ma for the tepecik pluton and 257 3 1 5 ma and 253 7 1 75 ma for the the sancaktepe pluton they are high k calc alkaline to shoshonitic granitoids with aluminum saturation index asi values between 0 9 and 1 3 which define them as peraluminous and slightly metaluminous 87sr 86sr i values for the studied plutons range between 0 701123 and 0 707704 143nd 144nd i ratios calculated for crystallization ages are between 0 512052 and 0 512431 and end i values vary from 4 71 to 2 22 tdm model ages range between 0 83 and 1 41 ga these plutons are enriched in liles and lree and depleted in hfses with negative eu anomalies indicating that the melts were derived from mafic crustal sources our new data suggest that the kirklareli pluton was generated in a subduction related magmatic arc and the highly fractionated sancaktepe and tepecik plutons were formed in a back arc setting in response to the northward subduction of the paleo tethys ocean during the mid permian early triassic

3 geochronology and geodynamics of late cretaceous magmatism and cu au mineralization in the panagyurishte region of the apuseni banat timok srednogorie belt bulgaria abstract the apuseni banat timok srednogorie abts belt europe s most extensive belt of calc alkaline magmatism and cu au mineralization is related to the subduction of the tethys ocean beneath the european continental margin during the late cretaceous phase of the alpine himalayan orogeny economic deposits are restricted to certain segments along the belt and all major porphyry style and high sulphidation ore deposits in bulgaria are aligned on the panagyurishte corridor a narrow zone obliquely crossing the abts belt this paper reviews the geology geochemistry and geochronology of igneous and hydrothermal events along this corridor in a profile extending from the european continental basement in the north to the accretionary complex of the rhodopes in the south extensive u pb dating of zircons from subvolcanic intrusions and major plutons supplemented by published age data for magmatic rocks and hydrothermal ore deposits obtained by other methods reveals a general younging of magmatism from ca 92 ma in the north elatsite to ca 78 ma in the south capitan dimitrievo cu au deposits are restricted to the northern and central parts of the profile ranging in age from 92 to 86 ma while the southernmost part exposes more deeply eroded mid crustal plutons devoid of economic mineralization the age progression correlates with a north to south geochemical trend of decreasing crustal input into mantle derived magmas magmatism and ore formation in individual magmatic hydrothermal complexes along the profile is much shorter lived for example subvolcanic intrusions and porphyry cu au mineralization at the elatsite deposit and andesitic volcanism and high sulphidation au as cu mineralization at chelopech all occurred within a time span of about 1 million years the age progression of calc alkaline magmatism within the panagyurishte region bulgaria from north to south is explained as a consequence of slab retreat during oblique subduction this led to transtensional block faulting and subsidence and thus to the preservation of near surface magmatic hydrothermal products including economic cu au deposits accretion of continental fragments in the rhodopes south of the abts belt led to greater uplift of the youngest late cretaceous intrusions this removed any ore deposits that might have been formed in this area

biotic response to explosive volcanism ostracod recovery after ordovician ash falls abstract the impact of two ordovician ash falls of different intensities was studied in order to determine the recovery patterns of benthic ostracods the studied sections are of sandbian age in nw estonia at poosaspea the kinnekulle k bentonite 40 cm derived from a major volcanic event and at ristna the grimstorp bentonite 6 cm in poosaspea important faunal changes are observed the three pre crisis dominant species i e tetrada memorabilis steusloffina sp nov and olbianella braderupensis were not recovered above the bentonite and at least two of them became extinct the post crisis ecosystem shows strong perturbations of diversity and abundance for a long period of time in ristna the impact of the ash fall is less dramatic all of the dominant pre crisis species survived although some changes in abundance and diversity are observed these results show that significant ash falls lead to marked rearrangement of assemblages and extinction of some taxa while less prominent volcanic episodes only result in temporary changes in the assemblage structure in both cases recovery follows several steps post event ecosystem with survival faunas very low abundance low diversity one two species opportunistic taxa e g sigmoopsis rostrata circulinella nuda dominate the assemblage early stages of recolonization with recovery faunas increasing abundance and diversity recolonization by pre event and immigrant species after a short disturbance period return to climax assemblages with specialized taxa high abundance high diversity although assemblages recovered rapidly after the crises the recovery periods still seem much longer than observed in recent foraminiferal assemblages causal links between the volcanic events and subsequent diversifications could not be demonstrated