chemical evolution of magmas in the proterozoic terrane of the st francois mountains southeastern missouri 1 field petrographic and major element data major element chemical data for plutonic and volcanic rocks of the eastern st francois mountains indicates that ring plutons of the butler hill caldera are low silica granites and chemically distinct from the high silica butler hill granite and grassy mountain ignimbrite additionally the ring plutons silvermine stono knoblick and slabtown granites are characterized by abundant mafic inclusions whereas these are almost completely absent in butler hill granite silvermine granite is mostly of a medium grained variety but cliffs along the st francis river expose gradations upward to a high silica fine grained cap rock least squares mixing calculations indicate that the cap rock could have formed by the fractional crystallization of plagioclase hornblende biotite magnetite and sphene all of which are liquidus phases in the medium grained silvermine granite the chemical similarity of the fine grained cap rock to butler hill granite and grassy mountain ignimbrite suggested that silvermine granite might be a sample of the parental magma from which the high silica rocks were derived least squares mixing calculations indicate that butler hill granite magma could have been derived from the composition of the average medium grained silvermine by fractional crystallization of the same liquidus phases indicated above the major element composition of grassy mountain ignimbrite can be derived from the average butler hill granite magma by fractional crystallization of plagioclase alkalic feldspar quartz biotite and magnetite all of which are liquidus phases in butler hill granite textural and chemical variations in butler hill granite from northeast to southwest suggest that the body was internally differentiated and that the present distribution of rock types is due to tilting and erosion the internal zonation of butler hill granite can be accounted for by fractional crystallization of plagioclase alkalic feldspar quartz biotite sphene and magnetite the chemical data and calculations indicate that the primary magma emplaced at the beginning of igneous activity was of silvermine type the magma crystallized fractionally so that the magma chamber became compositionally zoned with high silica magma in the upper part the top most part of this material was erupted to form grassy mountain ignimbrite during formation of the butler hill caldera subsequently more of the high silica magma was emplaced into the volcanic material in the caldera during resurgent doming after formation and resurgent doming of the butler hill caldera some of the parental magma from deeper in the chamber leaked upward into the ring fracture or into arcuate faults within the caldera forming the ring plutons the abundant mafic inclusions in the ring plutons may be earlier crystallized magma carried upward during intrusion one of the ring plutons silvermine granite apparently formed a sill like intrusion and underwent in situ differentiation a body of foliated granodiorite exposed as a giant inclusion in granite at hawn state park east of the principal exposures of the st francois mountains has the mineralogy major element chemistry and trace element chemistry to be a sample of the cumulate resulting from the processes in the magma chamber

geochemistry of hydrothermal fluids from the eastern sector of the sabatini volcanic district central italy abstract this study reports a complete geochemical dataset of 215 water and 9 gas samples collected in 2015 from thermal and cold discharges located in the eastern sector of the sabatini volcanic district svd italy based on these data two main aquifers were recognized as follows 1 a cold ca hco3 to ca na hco3 aquifer related to a shallow circuit within pliocene pleistocene volcanic and sedimentary formations and 2 a deep co2 pressurized aquifer hosted in mesozoic carbonate evaporitic rocks characterized by a ca hco3 so4 to na ca hco3 cl composition a thick sequence of low permeability formations represents a physical barrier between the two reservoirs interaction of the co2 rich gas phase with the shallow aquifer locally producing high tds and low ph cold waters is controlled by fractures and faults related to buried horst graben structures the 18o h2o and d h2o values indicate meteoric water as the main source for both the shallow and deep reservoirs carbon dioxide which is characterized by 13c co2 values ranging from 4 7 to 1 0 v pdb is mostly produced by thermo metamorphic decarbonation involving mesozoic rock formations masking possible co2 contribution from mantle degassing the relatively low r ra values 0 07 1 04 indicate dominant crustal he with a minor mantle he contribution the co2 3he ratios up to 6 1012 support a dominant crustal source for these two gases the 34s h2s values from 9 3 to 11 3 v cdt suggests that h2s is mainly related to thermogenic reduction of triassic anhydrites the 13c ch4 and d ch4 values from 33 4 to 24 9 v pdb and from 168 to 140 v smow respectively and the relatively low c1 c2 ratios

1600 1500 ma hotspot track in eastern australia implications for mesoproterozoic continental reconstructions mesoproterozoic a type magmatic rocks in the gawler craton curnamona province and eastern mount isa inlier form a palaeo curvilinear belt for reconstructed plate orientations the oldest igneous rocks in the gawler craton are the hiltaba granite suite c 1600 1575 ma the youngest in the mount isa inlier are the williams naraku batholiths c 1545 1500 ma the belt is interpreted as a segment of a hotspot track that evolved between c 1600 and 1500 ma this hotspot track may define a quasilinear part of australia s motion between 1636 and 1500 ma and suggests that australia drifted to high latitudes an implication of this interpretation is that australia and laurentia may not have been fellow travellers leading to the formation of rodinia a hotspot model for a type magmatism in australia differs from geodynamic models for this style of magmatism on other continents this suggests that multiple geologic processes may be responsible for the genesis of proterozoic a type magmas

paleomagnetism of the middle proterozoic electra lake gabbro needle mountains southwestern colorado the electra lake gabbro is a small 1 435 ga pluton that intrudes 1 7 to 1 6 ga gneisses and schists of the needle mountains in southwestern colorado paleomagnetic samples were collected from the main phases of the gabbro diabase dikes granite and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton gabbro diabase and some melt zone samples have a single polarity characteristic magnetization of northeast declination d and moderate negative inclination i demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite after correction for the minor west dip of overlying paleozoic strata we obtain a mean direction of d 32 1 i 41 9 k 94 95 3 3 n 21 sites and a paleomagnetic pole at 21 1 s 221 1 e k 89 a95 3 4 this pole is similar to poles from the middle proterozoic belt supergroup but is located at a higher southerly latitude than poles from other 1 47 1 44 ga plutons from north america most of which plot at equatorial latitudes the reason for this discrepancy is not clear but may result from a combination of factors including unrecognized tilting of the gabbro the failure of this relatively small pluton to fully average paleosecular variation and uncertainties in the overall reliability of other 1 5 1 4 ga poles of the north american apparent polar wander path

interaction between coeval mafic and felsic melts in the st francois terrane of missouri usa abstract in the eastern st francois mountains of missouri a 1 48 ga subvolcanic ring pluton known as the silvermine granite hosts swarms of dark pillow shaped enclaves exhibiting classical features of mafic melt globules trapped in granitic magma the pillows formed by turbulent injection of near liquidus mafic melt into a resident visco plastic granitic magma exchange of crystals fluids and ions between the two melts produced a hybrid tonalite granite enclave series and modified host granite composition several lines of evidence suggest interaction between granitic melt that produced the granophyric cap of the pluton and basaltic melt that formed dikes in the area rapid cooling of the pillows re melted the host creating back veins in many enclaves mineral chemistry indicates the hybrid rocks have undergone extensive subsolidus re equilibration

multiple mantle metasomatism in central southern italy geochemical effects timing and geodynamic implications the regional petrological and geochemical variations observed in the recent magmatism along the italian peninsula are interpreted to testify to the coexistence of distinct sectors of upper mantle these are suggested to result from at least three compositionally and temporally distinct metasomatic events that affected petrologically different premetasomatic mantle sources geological and geochemical evidence suggests that metasomatism in the northern sector occurred during the alpine subduction by introduction of a composition similar to the deeply subducted dora maira metagranites into a residual lithospheric mantle of upper crustal material metasomatic events beneath the roman and neapolitan areas are younger these are related to the addition of melts and fluids into a fertile asthenospheric mantle during the latest stage of the west directed apennine subduction of the adria plate and during the current northwest subduction of the ionian sea floor

the 1420 ma indiavai mafic intrusion sw amazonian craton paleomagnetic results and implications for the columbia supercontinent abstract the configuration and the timing of assembly and break up of columbia are still matter of debate in order to improve our knowledge about the mesoproterozoic evolution of columbia a paleomagnetic study was carried out on the 1420 ma indiavai mafic intrusive rocks that crosscut the polycyclic proterozoic basement of the sw amazonian craton in southwestern mato grosso state brazil alternating field and thermal demagnetization revealed south southwest chrm directions with downward inclinations for sixteen analyzed sites these directions are probably carried by sd psd magnetite with high coercivities and high unblocking temperatures as indicated by additional rock magnetic tests including thermomagnetic data hysteresis data and the progressive acquisition of isothermal remanent magnetization different stable magnetization components isolated in host rocks from the basement 10 km nw away to the indiavai intrusion further support the primary origin of the chrm a mean of the site mean directions was calculated at dm 209 8 im 50 7 95 8 0 k 22 1 which yielded a paleomagnetic pole located at 249 7 e 57 0 s a95 8 6 the similarity of this pole with the recently published 1420 ma pole from the nova guarita dykes in northern mato grosso state suggests a similar tectonic framework for these two sites located 600 km apart implying the bulk rigidity of the rondonian san ignacio crust at that time furthermore these data provide new insights on the tectonic significance of the 1100 1000 ma nova brasil ndia belt a major ew feature that cuts across the basement rocks of this province which can now be interpreted as intracratonic in contrast to previous interpretation from a global perspective a new mesoproterozoic paleogeography of columbia has been proposed based on comparison of these 1420 ma poles and a 1780 ma pole from amazonia with other paleomagnetic poles of similar age from baltica and laurentia a reconstruction in agreement with geological correlations

paleomagnetism of the middle proterozoic laramie anorthosite complex and sherman granite southern laramie range wyoming and colorado we present the results of a combined paleomagnetic and 40ar 39ar geochronologic investigation of the middle proterozoic laramie anorthosite complex and sherman granite in the southern laramie range of wyoming and colorado anorthosites and monzosyenites of the laramie anorthosite complex yield a well defined characteristic magnetization of northeast declination d and moderate negative inclination i although antipodal normal and reverse polarity magnetizations are present at three sites a grand mean direction from 29 of 35 sites in the complex is d 44 6 i 48 7 k 77 4 95 3 1 alternating field af and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low ti titanomagnetite of single or pseudo single domain character that occurs as elongate to rod shaped inclusions in plagioclase and potassium feldspar the sherman granite contains a dual polarity magnetization that is less well defined than that of the laramie anorthosite complex but similar in declination and inclination d 53 1 i 48 1 k 46 5 95 7 6 n 8 14 sites rock magnetic data indicate the primary carrier of remanence in sherman granite is magnetite the 40ar 39ar geochronologic data from sherman granite hornblende biotite and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 ma microcline data indicate that the laramie anorthosite complex and sherman granite have probably not been thermally remagnetized paleomagnetic poles from the laramie anorthosite complex and sherman granite are indistinguishable at the 95 confidence level and individual virtual geomagnetic poles vgps from both units are combined to provide a mean pole at 215 0 e 6 7 s k 46 9 a95 3 5 n 37 vgps the location of this pole is similar to paleomagnetic poles derived from 1480 ma to 1450 ma intrusions elsewhere in north america but it plots significantly north of those from middle proterozoic sedimentary strata of the belt supergroup and sibley group in addition to the characteristic magnetization samples from some sites in sherman granite contain a remanence of southeast declination and shallow negative inclination d 154 9 i 16 0 k 90 6 95 9 7 n 4 sites this secondary magnetization was probably acquired during late paleozoic time

new paleomagnetic results from the ca 1 68 1 63 ga mafic dyke swarms in western shandong province eastern china implications for the reconstruction of the columbia supercontinent abstract the existence of paleo mesoproterozoic supercontinent columbia aka nuna was established decades ago but the position of north china craton within columbia is still highly debated due to the paucity of available high quality paleomagnetic and reliable geological constraints precise geochronological dating of extensive mesoproterozoic mafic dyke swarms in western shandong province also named the luxi area china reveals two phases of dyke emplacement at 1 68 ga and 1 63 ga in this paper we report new paleomagnetic and rock magnetic results obtained from approximate 160 samples 16 sites collected from these two phases of mafic dyke swarms in the luxi area with the aim of pinpointing the location of north china craton within columbia supercontinent in this time interval rock magnetic experiments confirm that either magnetite or titanomagnetite is the main magnetic carrier in these dykes stepwise thermal demagnetization revealed two paleomagnetic poles for the 1 63 ga dykes normal and reversed high temperature remanent magnetization directions yield a mean direction d i of 86 1 53 5 43 3 95 7 9 n 9 these directions pass a reversal test and are interpreted as primary remanences the corresponding paleomagnetic pole is calculated at 20 8 n 182 5 e 28 3 a95 8 3 n 9 this pole passes the examination of secular variation of geomagnetic field deenen et al 2011 deenen et al 2014 it fulfills a van der voo 1990 value q 6 and is therefore suggested to be a key pole demarcating the precambrian north china craton for the 1 68 ga dykes only the normal directions are isolated with a mean direction d i of 89 1 47 1 35 0 95 13 1 n 5 with a corresponding paleomagnetic pole of 17 8 n 184 9 e 29 6 a95 14 3 n 5 this pole passes the examination of secular variation of geomagnetic field finally we select the 1 63 ga high quantity paleomagnetic pole in order to depict a more detailed apparent pole wander path apwp to compare with the other major precambrian cratons combined with other geological evidence our reconstruction scenario supports the spatio temporal connection between the baltica north australian craton and north china craton

geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy abstract serri g innocenti f and manetti p 1993 geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 117 147 the neogene quaternary magmatism of the northern apenninic arc took place in four phases separated in space and time which become progressively younger from west to east phase i 14 ma phase ii 7 3 6 0 ma phase iii 5 1 2 2 ma phase iv 1 3 0 1 ma this magmatism is the result of the activation of three physically separate sources 1 the adriatic continental crust extracted from the mantle in the late proterozoic 2 a strongly refractory recently k enriched harzburgitic mantle located in the mechanical boundary layer mbl of the lithosphere and 3 a recently metasomatized cpx rich mantle compositionally variable from iherzolite to wehrlite clinopyroxenite interpreted as an ephemerally k enriched asthenosphere the adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the tuscan region the acid magmatism is mostly found inside an ellipsoidal area about 150 300 km centred on giglio island here defined as the tuscan crustal dome within this area mantle derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found pure crustal magmas are rare but are represented for example by some of the san vincenzo and roccastrada rhyolites virtually all the tuscan acid centres show evidence of mixing with potassic mantle derived magmas major and trace elements as well as 87 sr 86 sr and 143 nd 144 nd data on primitive rocks mg 65 reveal two groups of mantle derived magmas these define two distinct mantle enrichment trends both essentially due to the additions of k rich components which metasomatized separate compositionally diverse upper mantle sectors in both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and or melts with the mantle the rocks of group i ol hy and q normative lamproites ultrapotassic high mg latites ultrapotassic shoshonites and shoshonites saturated trend are considered to be derived by partial melting at low pressure 87 sr 86 sr 0 717 ce sr 0 3 and k 2 o na 2 o 6 7 and low ratios of 143 nd 144 nd 0 5121 0 5120 and ba la 87 sr 86 sr 0 712 and k 2 o na 2 o 8 9 values and low 143 nd 144 nd ba la 0 10 ratios these constraints do not allow to exclude a complete identity between the k rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas the geochemical and isotopic features of the components that metasomatized the mantle sources of the northern apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle we propose that the delamination and subduction of the adriatic continental lithosphere related to the still ongoing northern apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central italy the subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle adriatic mbl and crustal units previously imbricated in the apennine chain to the heating advected by the upwelling of a recently and ephemerally k enriched asthenospheric mantle wedge and by the underplating of magmas derived from it we consider that the diapiric uprising of a hot crustally contaminated asthenosphere occurs in the wake left above the sinking of the adriatic delaminated subducting continental lithosphere the delamination subduction process of the adriatic lithosphere has probably started in the early middle miocene but earlier than 15 14 ma ago as indicated by the age and petrologic characteristics of the first magmatic episode sisco lamproite of the northern apennine orogenesis

structural evolution of the pleistocene cimini trachytic volcanic complex central italy structural geomorphological geophysical and volcanological data have been processed for the implementation of a dedicated gis through which the structural evolution of the pleistocene trachytic cimini volcano central italy has been reconstructed the evolution of the cimini complex includes three main close in time phases 1 intrusion of a shallow laccolith rising along nw and ne trending faults and stagnating at the contact between the mesozoic cenozoic and the pliocene pleistocene sedimentary units constituting the bedrock of the volcano 2 emplacement of lava domes along radial and tangential fractures formed by the swelling induced by the laccolith growth 3 ignimbrite eruptions and final effusion of olivine latitic lavas domes are both of pelean and low lava dome type and their morphology was controlled by the location on the inclined surface of the swelled area some domes show to have uplifted upper pliocene thermally metamorphosed clay sediments suggesting a cryptodome like growth comparison of the top of the mesozoic cenozoic units with the top of the upper pliocene pleistocene sedimentary complex suggests that the laccolith emplaced in a graben of the mesozoic cenozoic sedimentary complex filled by the pliocene pleistocene sediments uplifted by the shallow intrusion stress patterns acting on the cimini area have been deduced analysing the drainage network and the morphotectonic lineaments rose diagrams show a large dispersion of the lineaments reflecting the local presence of radial and tangential fractures the most frequent extensional nw and ne trending lineaments have regional significance and controlled the magma uprise leading to the laccolith emplacement

paleomagnetic and rock magnetic study of the mistastin lake impact structure labrador canada implications for geomagnetic perturbation and shock effects abstract we carried out an integrated rock magnetic and paleomagnetic study of the 36 ma mistastin lake labrador canada meteorite impact structure in order to investigate whether energy from the collision influenced the geodynamo and to assess the effects of shock on the magnetic properties of the target basement rocks stepwise demagnetization of 114 specimens isolates a well defined magnetization component throughout the crater whose overall mean deviates slightly from the expected direction for north america at the time of impact paleointensity results from seven samples meeting stringent selection criteria show no significant difference with a global compilation from 40 to 30 ma the combined results including those from a 80 m thick profile of an impact melt unit discovery hill lend no support that the impact caused an aberration of the geodynamo within a few centuries of a bolide collision that created the 28 km diameter crater both titanium rich and titanium poor titanomagnetite carry the magnetic remanence in the impact melt rocks their relative proportions compositions and domain states are cooling rate dependent magnetic hysteresis parameters of the magnetite bearing anorthositic basement rocks reveal systematic changes as a function of distance from the crater s center with an increasing prevalence of single domain like grains toward the center changes with radial distance are also found in the character of the verwey transition in magnetite basement rocks were thermally overprinted when lying less than a meter from the impact melt rocks mesoproterozoic basement rocks more than a meter below the impact melt rocks hold similar magnetization directions to those expected from a 1500 ma result for laurentia no evidence exists that shock heating of the basement rocks exceeded 200 c at distances of 6 7 km from the crater s center

palaeomagnetism of middle proterozoic basic intrusives in central sweden and the fennoscandian apparent polar wander path abstract palaeomagnetic results are presented from three sets of post svecokarelian intrusions in central sweden the first is a system of e w trending doleritic dykes including the large breven and hallefors dykes and with a probable emplacement age of c 1530 ma their original nrm is disturbed by later events but two magnetic directions group ew1 with decl 359 5 incl 6 2 95 8 2 n 35 and group ew 2 with decl 11 1 incl 51 3 95 10 1 n 14 have been isolated and give palaeopoles at 27 7 n 196 8 e and 61 7 n 176 1 e respectively the group ew1 direction is considered to represent the original emplacement nrm this is based on comparison with contemporary palaeomagnetic data from elsewhere in fennoscandia the second set comprises dolerites and quartzporphyries the tuna dykes age 1371 ma they present a stable magnetization with decl 14 8 incl 18 1 95 9 6 and a palaeopole at 21 1 n 180 4 e the third set studied are dolerites belonging to the dala and jamtland complexes of the central scandinavian dolerite group csdg age c 1250 1200 ma post jotnian they present well defined magnetic directions that give palaeopoles close to the equator around 155 e in agreement with other published palaeomagnetic csdg data however there are deviating results obtain from dolerites belonging to the dala complex of the csdg those deviating have a mean decl of 12 9 incl 1 4 95 18 1 n 5 which gives a palaeopole at 27 0 n 179 5 e situated within the area of the 1600 1500 ma poles results from three probably sweconorwegian c 1000 850 ma doleritic dykes are also presented the results obtained are compared with other contemporary palaeomagnetic poles from fennoscandia and a revised apparent polar wander path is presented and discussed

the butler hill caldera a mid proterozoic ignimbrite granite complex abstract uplift along rift related faults in the eastern st francois igneous terrane of southeastern missouri exposes a high level anorogenic igneous complex consisting of 1 5 km of intracaldera ignimbrites a zoned central pluton and late stage ring plutons a collapse structure named the butler hill caldera bhc formed in response to eruption of comenditic ignimbrites of the lake killarney formation which attains an intracaldera thickness of 400 m continued peralkaline volcanism resulted in an additional 1000 m of grassy mountain ignimbrite within the caldera and a thin but widespread outflow sheet following degassing of the magma chamber the central pluton forcefully intruded the intracaldera volcanic sequence producing arcuate folds concentric to the caldera axis remnants of the original magma chamber roof are not preserved the final igneous event related to the bhc involved intrusion of amphibole granitoids in an annular pattern within and concentric to the caldera walls the latter bodies probably merge with depth into a single ring pluton the petrographic zoning exhibited by the mildly peraluminous central pluton of the bhc consists of 1 a roof zone of fine grained brick red hypersolvus granophyre with abundant miarolitic cavities which is nearly devoid of hydrous minerals 2 a transitional zone of pink fine to medium grained seriate granite in which granophyric intergrowths are common and plagioclase and biotite are present in trace amounts only and 3 a border zone of medium to coarse grained pink to grey subsolvus biotite granite with rapakivi texture the paradoxical association of peralkaline and peraluminous suites with hypersolvus and subsolvus petrographic traits at the same structural level is recognized in other high level anorogenic complexes the common factor in these terranes regardless of their age appears to be that initial magmas were hot relatively dry low in cao and high in f these compositional features favor ascent of hypersolvus magmas to subvolcanic levels where saturation is accomplished by influx of meteoric water field chemical isotopic and petrographic evidence indicate that bhc rocks have experienced at least two periods of subsolidus metasomatism the resulting metasomatic features are typical of anorogenic ignimbrites and high level granitoids elsewhere and must be quantified before existing chemical data can be applied to questions of igneous petrogenesis

can field data constrain rock viscosities abstract in the 1960s it looked as though the ratio of wavelength to thickness of folds along shortened competent single layers might allow constraint of the viscosity ratios between the layers and their hosts when they deformed together in the 1970s the possibility arose that simple field measurements of boudins and mullions might also constrain rock viscosity ratios and thereby distinguish deformation facies and map rock viscosities in pressure temperature time space even more potential tools for constraining rock viscosities appeared in the 1980s but since then progress appears to have stagnated in a welter of problems an attempt is made to refocus attention on direct retrospective measurements of rock rheologies during natural deformations by reviewing the potential field tools for constraining rock viscosities discussing some of their problems and by a crude application of the most developed approach further advances are likely to come from iteration between modellers and structural geologists working in a variety of tectonic settings as well as constraining the pressure temperature time paths of our rocks we should also be attempting to measure their viscosities

tectonic implications of the 1419 ma nova guarita mafic intrusives paleomagnetic pole amazonian craton on the longevity of nuna abstract the nuna supercontinent was probably assembled in the paleoproterozoic but its paleogeography and the timing for its demise are still a matter of debate a paleomagnetic and geochronological study carried out on the mesoproterozoic nova guarita dyke swarm northern mato grosso state sw amazonian craton provides additional constraints on the duration of this supercontinent paleomagnetic af and thermal treatment revealed south southwest northeast magnetic directions with downward upward inclinations for 19 analyzed sites these directions are carried by psd magnetite with high unblocking temperatures as indicated by additional magnetic tests including thermomagnetic curves hysteresis loops and the progressive acquisition of isothermal remanence in selected samples a positive contact test with the host granite in one of the studied dykes further attests to the primary origin of the characteristic magnetic component a mean site direction was calculated at d m 220 5 i m 45 9 95 6 5 k 27 7 which yielded a paleomagnetic pole located at 245 9 e 47 9 s a 95 7 0 40 ar 39 ar dating carried out on biotites from four analyzed dykes yielded well defined plateau ages with a mean of 1418 5 3 5 ma the nova guarita pole precludes a long lived nuna configuration in which laurentia baltica north china and amazonia formed a long and continuous block as previously proposed for the paleoproterozoic it is nevertheless fully compatible with a samba amazonia baltica link at mesoproterozoic times

fluid geochemistry and geothermometry in the western sector of the sabatini volcanic district and the tolfa mountains central italy abstract a geochemical survey of 197 fluid discharges cold and thermal waters and bubbling pools and 15 gas emissions from the western sector of the sabatini volcanic district and the tolfa mountains latium central italy was carried out in 2007 2008 the chemical and isotopic compositions of the fluid discharges indicate the occurrence of two main sources 1 relatively shallow aquifers with ca na k hco 3 and ca mg hco 3 compositions when trapped in volcanic and sedimentary formations respectively and 2 a deep reservoir which is hosted in the mesozoic carbonate sequence rich in co 2 and having a ca so 4 hco 3 composition dissolution of a co 2 rich gas phase into the shallow aquifers produces high tds and high pco 2 cold waters while oxidation of deep derived h 2 s to so 4 2 generates low ph the 13 c co 2 values for gas emissions from 2 8 to 2 7 vs vpdb suggest that the origin of co 2 associated with the deep fluids is mainly related to thermo metamorphic reactions within the carbonate reservoir although significant mantle contribution may also occur however r r a values 0 37 0 62 indicate that he is mainly produced by a crustal source with a minor component from a crust contaminated mantle on the basis of the 13 c ch 4 and d ch 4 values from 25 7 to 19 5 vs vpdb and from 152 to 93 4 vs vsmow respectively ch 4 production is associated with thermogenic processes possibly related to abiogenic co 2 reduction within the carbonate reservoir the 34 s h 2 s values from 9 3 to 10 4 vs vcdt are consistent with the hypothesis of a sedimentary source of sulfur from thermogenic reduction of triassic sulfates geothermometric evaluations based on chemical equilibria co 2 ch 4 and separately h 2 s suggest that the reservoir equilibrium temperature is up to 300 c the d and 18 o data indicate that water recharging both the shallow and deep aquifers has a meteoric origin fluid geochemistry coupled with gravimetric data and tectonic lineaments supports the idea that significant contributions from a deep seated geothermal brine are present in the stigliano thermal fluid discharges exploration surveys investigated this area during 70 s 90 s for geothermal purposes nevertheless presently the area is still under exploited the presence of thermal waters and anomalous heat flow together with the demographic growth of the last years makes this site a suitable location for direct applications of the geothermal resource

extension and compression related basins in central italy during the messinian lago mare event abstract during the messinian lago mare event in the central mediterranean area compression and extension related basins developed as a consequence of the post tortonian evolution of the tyrrhenian apennines system all these syn tectonic sedimentary basins were characterized by different subsidence rates and sedimentary processes several sections located in different tectonic domains from the tyrrhenian to the adriatic margin of the apennine chain have been investigated s valentino in abruzzo citeriore roccamorice penscosansonesco roccacaramanico and le vicenne sections are located in compression related basins foreland basin system while the tolfa blera sections and the sedimentary succession of mondragone 1 well drilled in the garigliano coastal plain are located in extension related basins hinterland or back arc basins the recognition of ostracod assemblages with paratethyan affinities allows them to be referred to the messinian lago mare event loxoconcha djaffarovi zone this result allows us to consider the tyrrhenian rifting the accretion of the apennine thrust belt and the increase in subsidence rate during the flexuration of the foreland domain as synchronous processes due to the same geodynamical process taking into account the thicknesses of the analysed sedimentary basins different total subsidence rates have been computed for both the compression and extension related basins the syn rift basins that developed on the eastern margin of the northern tyrrhenian basin volterra val di fine and tolfa blera basins recorded a mean total subsidence rate of 1 25 mm a whereas higher subsidence rates affected the eastern margin of the central tyrrhenian basin 5 mm a garigliano coastal plain the differences in horizontal stretching and total subsidence rates that affected the tyrrhenian extensional area correspond to different migration rates of the apennine orogenic system in the northern 10 mm a and central apennines 40 mm a this difference in the total syn rift subsidence rate fits in well with the segmentation model of the tyrrhenian apennines system along a nne ssw lithosphere discontinuity which crosses the system from ancona northern apennines to the r selli lineament in the tyrrhenian basin

palaeomagnetism of granitic intrusives from the precambrian basement under eastern kansas orienting drill cores using secondary magnetization components summary the precambrian basement under east central kansas was drilled at two circular aeromagnetic positives one at osawattamie and one at big springs the core retrieved from these sites is a coarse to medium grained granite which has been dated by u pb to be 1350 ma old the palaeomagnetism of these azimuthally unoriented cores was studied to see if a technique which uses low coercivity low temperature magnetization components to orient the cores would allow an independent confirmation of the core s mid proterozoic age orthogonal projection plots of the alternating field af and thermal demagnetization data show that the magnetization of these cores is relatively simple having only two components a low temperature low coercivity magnetization with steep positive inclinations and a shallow negative inclination characteristic magnetization for the osawattamie core or a positive moderate inclination characteristic magnetization for the big springs core if the declination of the low temperature low coercivity component is aligned parallel to the present field declination the characteristic directions may be azimuthally oriented this allows the calculation of palaeomagnetic poles for the big springs core lat 4 5 s long 29 9 e and the osawattamie core lat 20 2 n long 39 3 e which are consistent with irving s apparent polar wander path for laurentia at about 1300 1400 ma comparison of anhysteretic remanent magnetization arm viscous remanent magnetization vrm and isothermal remanent magnetization af demagnetization curves with a natural remanent magnetization nrm demagnetization curve suggests that the osawattamie core probably acquired a piezoremanent magnetization prm parallel to the core axis during drilling

paleomagnetism of ca 1 35ga sills in northern north china craton and implications for paleogeographic reconstruction of the mesoproterozoic supercontinent abstract we report paleomagnetic data on precisely dated mesoproterozoic sills in zircon and baddeleyite 207pb 206pb ages of 1345 12 and 1353 14 ma respectively intruding the xiamaling and wumishan formations in north china with an aim of evaluating how the north china craton ncc was involved in the evolution of the columbia supercontinent from ca 1 78 to 1 35 ga after systematic thermal demagnetization we isolate a high temperature characteristic remanence from 18 sites with a tilt corrected site mean direction of d 294 4 i 31 7 and 95 4 3 corresponding to a mean paleomagnetic pole at 5 9 n 359 6 e n 18 with angular standard deviation of 10 0 and a95 4 3 rock magnetic experiments light microscopy and scanning electron microscopy all indicate that the main magnetic minerals in the sampled sill rocks are medium sized titanomagnetites the positive fold test and fresh titanomagnetite grains suggest that the characteristic remanence is likely to be a primary acquired at ca 1 35 ga the preliminary paleogeography reconstruction based upon the well matched apparent polar wander paths between the laurentia baltica siberia united block and the ncc supports the interpretation that the ncc may have drifted together with this united block before ca 1 35 ga during the late history of the columbia supercontinent we argue that the ncc was located in low latitudes and kept tight connections with the siberian and indian cratons during the interval between ca 1 8 and 1 35 ga and that the ca 1 35 ga sills widely developed in the mesoproterozoic xiamaling and wumishan formations as well as the ca 1 3 1 2 ga magma events well developed in siberia and northern margin of the ncc may represent the complete fragmentation of the ncc with the neighboring cratons

palaeomagnetic geochronological and geochemical study of mesoproterozoic lakhna dykes in the bastar craton india implications for the mesoproterozoic supercontinent abstract palaeomagnetic analysis of the lakhna dykes bastar craton india yields a palaeopole at 36 6 n 132 8 e dp 12 4 dm 15 9 and the u pb zircon age obtained from one of the rhyolitic dykes is 1466 4 2 6 ma mswd 0 21 concordia age based on two analyses with identical pb u ages similar to previously published u pb ages major and trace element analyses of the lakhna dykes show shoshonitic and high k calc alkaline affinities consistent with a subduction related characteristics suggesting an active continental margin setting this is in keeping with the palaeo to mesoproterozoic tectonic environments in the eastern indian margin the new 1460 ma indian palaeopole was used to test possible palaeopositions of india within the mesoproterozoic supercontinent columbia of the four palaeomagnetically permissible reconstructions juxtaposing western india against south west baltica is geologically the most reliably constrained and best fitting model our preferred reconstruction implies a long palaeo to mesoproterozoic accretionary orogen stretching from south eastern laurentia through south western baltica to south eastern india breakup of india and baltica probably occurred in the late mesoproterozoic but additional constraints are needed

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

a revised magnetostratigraphy for the mid proterozoic gardar lava succession south greenland abstract new palaeomagnetic results from the ca 1300 ma gardar lava succession within the eriksfjord formation of south greenland are presented this study incorporates stepwise thermal demagnetisation of 256 samples from 66 lava flows from all three lava groups in this formation accompanied by principal component analysis the results identify two complete reversals n r n in the upper lava group and two r n and n r in the lower group with a possible palaeoexcursion in the lower group a minimum of two further reversals are predicted in the time interval represented by the sandstone units intercalated with the lava flows both complete reversals in the upper group exhibit inclination asymmetries of 30 this degree of asymmetry is satisfied by a geomagnetic field model equivalent to a dipole axially offset by 570 km however a more complex source is not precluded by the present evidence four new pole positions are calculated for the gardar lavas two for the upper lava group and one each for the middle and lower groups the middle lava group shows an anomalous mean direction and pole position which is unrepresentative of the remainder of the lava succession and is thought to record a virtual geomagnetic pole representative of a transitional field direction the presence of a non axial geocentric dipole field means that the use of the laurentian apparent polar wander path for dating the new poles is not valid

the long life of samba connection in columbia a paleomagnetic study of the 1535 ma mucajai complex northern amazonian craton brazil abstract in recent years there has been a significant increase in the paleomagnetic data of the amazonian craton with important geodynamic and paleogeographic implications for the paleo mesoproterozoic columbia supercontinent a k a nuna hudsoland despite recent increase of paleomagnetic data for several other cratons in columbia its longevity and the geodynamic processes that resulted in its formation are not well known a paleomagnetic study was performed on rocks from the 1535 ma amg anorthosite mangerite rapakivi granite mucajai complex located in the roraima state brazil in the northern portion of the amazonian craton the guiana shield thermal and af treatments revealed northwestern southeastern directions with upward downward inclinations for samples from twelve sites this characteristic remanent magnetization is mainly carried by ti poor magnetite and in a lesser amount by hematite site mean directions were combined with previous results obtained for three other sites from the mucajai complex producing the dual polarity mean direction dm 132 2 im 35 4 n 15 95 12 7 k 10 0 and a paleomagnetic pole located at 0 1 e 38 2 s a95 12 6 k 10 2 the mucajai pole favours the samba south america baltica link in a configuration formed by amazonia and baltica in columbia also there is geological and paleomagnetic evidence that the juxtaposition of baltica and laurentia at 1 76 1 26 ga forms the core of columbia the present paleomagnetic data predict a long life 1 78 1 43 ga samba connection forming part of the core of the supercontinent

meandering flow of a pyroclastic density current documented by the anisotropy of magnetic susceptibility ams in the quartz latite ignimbrite of the pleistocene monte cimino volcanic centre central italy abstract a pattern of shifting current direction is revealed by the anisotropy of magnetic susceptibility ams in the quartz latite ignimbrite associated with the pleistocene cimini dome complex in central italy high field ams documents that both paramagnetic and ferrimagnetic minerals contribute to the magnetic fabric of the ignimbrite the results of low field ams studies suggest that the pyroclastic density current changed both direction and character during deposition of each flow unit that makes up the ignimbrite the meandering behaviour of the currents can be observed on the scale of 15 cm increments in the fine grained basal layer 2a of one flow unit in an area lacking significant palaeotopography ams data from 1 2 and 4 metre increments through the flow units also show complex changes in flow direction throughout the deposition of each unit studied the most closely aligned ams fabrics occur near the bases of flow units and dispersion increases upward this trend may represent decreased shear between the flow and the substrate or increased turbulence in the upper depositional zone of the pyroclastic density current or both suggesting a progressively waning current in terms of capacity or particle concentration the phenomena observed in this study have broad implications for the general methodology of ams studies detailed vertical profiles made up of several individual sample sites within 1 metre thickness intervals can reveal small scale current flow variations that may not be evident if sampling is done more sparsely

the role of extensional tectonics at different crustal levels on granite ascent and emplacement an example from tuscany italy abstract the role of regional extension on the rise and emplacement of granites in the crust is still debated pluton ascent and emplacement widely occurred in tuscany italy since late miocene during the post orogenic collapse of the inner apennines and are presently occurring in the geothermal areas of amiata and larderello tuscany offers a preferred test site to study the role of regional extension on pluton ascent and emplacement at different crustal levels ductile extension enhanced the segregation and ascent of granitic melts in the lower crust controlling pluton emplacement in correspondence with the brittle ductile transition in the brittle crust magma ascent occurred through subvertical faults and fractures compatible with the regional extension direction pluton emplacement mainly occurred by means of roof lifting the case of tuscany suggests that the extensional structures enhance melt segregation and ascent in the ductile crust but are not efficient alone to provide a pathway for the ascent of granitic magmas in the brittle extending crust the estimated magmatic strain rates due to pluton emplacement in the geothermal areas are much larger than the regional tectonic strain rates this suggests that regional tectonics did not control magma emplacement in the brittle crust and explains why nontectonic processes roof lifting accommodated the space required for pluton emplacement

a mesoproterozoic paleomagnetic pole from the yangzhuang formation north china and its tectonics implications abstract a new paleomagnetic study has been carried out on mesoproterozoic sediments in the jixian area north china detailed stepwise thermal demagnetization allows us to isolate three components from 141 oriented drill core samples 14 sites a low temperature component component a identified from most of samples falls close to the present local earth field direction a middle temperature component component b is found only in 21 samples from four sites the component b directions after bedding correction yield a paleopole position at 336 0 e 10 2 n with a 95 17 1 which is close to the lower cambrian pole of the north china block characteristic remanent directions with dual polarities obtained from high temperature component c pass fold test the directions yield a pole position at 190 4 e 2 4 n with a 95 11 9 this result indicates that the north china block was located in low latitude at ca 1 35 ga recently several studies suggested that laurentia baltica and siberia were also situated at low latitudes from well dated paleopoles between ca 1 5 and 1 25 ga this would imply that the north china block laurentia and baltica might drift together or close in the mesoproterozoic

spatiotemporal variations of the slow slip event between 2008 and 2013 in the southcentral alaska subduction zone we apply a kalman filter based time dependent slip inversion method to model a long term slow slip event sse in the southcentral alaska subduction zone from 2008 to 2013 this event occurred downdip of the asperity that ruptured in the 1964 earthquake the same part of plate interface that slipped during a previous sse between 1998 and 2001 most of the slip deficit that accumulated during the steady period between 2001 and 2008 8 years total in the sse source region was released by this sse our results indicate both lateral and downdip propagation during this event the sse started at the end of 2008 at the upper section of the slip patch and gradually propagated to the east and to the deeper part of the interface our results indicate no connection between this sse in upper cook inlet and another sse in lower cook inlet that started in 2010 analysis of the earthquake catalog in the southcentral alaska subduction zone shows a clear increase in seismicity associated with the 2008 2013 sse with the data from a newly available continuous gps site we now can better constrain the start time of the 1998 2001 sse as 1998 58

on the emplacement of tabular granites granites in both crystalline terranes and continental magmatic arcs tend to be circular to elliptical in map view and vary in width from about 3 to 100 km available gravity and structural data suggests that many of these plutons are tabular in shape with an average thickness of about 3 km ductile structures observed around mesozonal granites indicate that space is created by a combination of lateral and vertical displacements of wall rocks whereas contact relationships of epizonal plutons imply that only vertical displacements are involved during emplacement in both settings magma arrives at the emplacement site via one or more vertical feeder zones and flows laterally with the exception of very high level epizonal plutons structural studies suggest that space for many tabular intrusions must be provided mainly by floor depression lopolith emplacement rather than roof lifting laccolith emplacement an emplacement model for this type of tabular granite is proposed which involves progressive depression of the floor of an initially horizontal chamber as it is filled by one or more vertical conduits a crustal scale balance in the rates of melt extraction magma ascent and pluton filling is required by the model and transfer of material from the source to the pluton is accommodated by broadly distributed deformation of low strain magnitude the process is evaluated with end member cantilever and piston sinking mechanisms the models predict that large 10 100 km wide tabular plutons 3 km thick can be emplaced quickly 100 a to 1 ma with floor depression and related wall rock strain rates similar those expected during tectonic deformation 10 10 to 10 15 s 1 bulk strains in the intervening crustal column rarely exceed a strain ratio of 1 5 which is likely to remain undetected in the geological record unless the required deformation is accommodated on discrete structures such as normal faults or shear zones at the base of the pluton

revisiting the paleomagnetism of the 1 476 ga st francois mountains igneous province missouri 1 a paleomagnetic investigation of the st francois mountains igneous province in southeastern missouri provides a key 1476 16 ma paleomagnetic pole for laurentia the pole 13 2 s 219 0 e dp 4 7 dm 8 0 is considered primary on the basis of positive conglomerate inverse baked contact and fold tests an analysis of 1470 1430 ma poles from laurentia highlights key differences between poles obtained from the belt supergroup electra lake gabbro and cratonic north america paleolatitudes based on the lower belt supergroup poles are enigmatic as two previous studies yielded a difference of 10 our new pole combined with an analysis of previous results favors the higher latitude interpretation for the lower belt supergroup paleolatitudes from the younger belt rocks indicate lower latitudes than coeval rocks from elsewhere in laurentia for which there has been no adequate explanation a comparison of the st francois mountain pole with similar age poles from baltica siberia and australia allow first order tests of proposed continental configurations paleomagnetic data from australia are compatible with proposed rodinia reconstructions whereas paleomagnetic data from baltica are not we are unable to rigorously test the alternative suggestion that places siberia against the western margin of laurentia due in part to large errors associated with siberian paleomagnetic data

paleomagnetism of mesoproterozoic margins of the anabar shield a hypothesized billion year partnership of siberia and northern laurentia abstract siberia and laurentia have been suggested as near neighbors in proterozoic supercontinents nuna and rodinia but paleomagnetic evidence has been sparse and ambiguous here we present four new paleomagnetic poles from undeformed paleo mesoproterozoic lower riphean sedimentary rocks and mafic intrusions of the northwestern anabar uplift in northern siberia combining these results with other proterozoic data from siberia and laurentia we propose a tight juxtaposition of those two blocks euler parameters 77 098 137 for anabar to north america spanning the interval 1 7 0 7 ga constituting a long lived connection that outlasted both the nuna and rodinia supercontinental assemblages

refined proterozoic evolution of the gawler craton south australia through u pb zircon geochronology abstract through the application of both conventional u pb zircon analyses and small sample u pb isotopic analyses the nature and timing of tectonic events leading to the formation of the gawler craton have been defined more precisely constraints on deposition of early proterozoic iron formation bearing sediments have been narrowed down to the period 1960 1847 ma deformed acid volcanics including the economically important moonta porphyry have zircon ages of 1790 and 1740 ma the voluminous acid gawler range volcanics and correlatives to the east were erupted over a short interval at 1592 2 ma and were intruded by anorogenic granites at 1575 ma small sample zircon analyses proved to be an extremely valuable adjunct to conventional analyses generally yielding more concordant data which forced a curved discordia through an upper intercept slightly younger than from a conventional straight line discordia

paleomagnetism of proterozoic mafic dikes from the tobacco root mountains southwest montana abstract paleomagnetic data from proterozoic mafic dikes in southwestern montana provides evidence for two distinct episodes of subparallel dike emplacement at ca 1450 and 780 ma published geochemical data from dikes in the southern tobacco root mountains has identified three distinct compositional groups termed groups a b and c geochronological data from the group a dikes yielded a sm nd age of 1448 49 ma emplacement of these dikes is thought to reflect mafic magmatism associated with extension accompanying development of the adjacent mesoproterozoic belt basin paleomagnetic results from these dikes and a group c dike yield antipodal magnetizations with a group mean direction of d 225 0 i 61 8 k 27 9 95 7 7 n 14 independent means 24 sites the average paleomagnetic pole 8 7 n 216 1 e a95 10 3 is considered to be primary on the basis of positive baked contact tests and similarity to poles of ca 1 45 1 4 ga from intrusions elsewhere in north america but is discordant with respect to poles from age equivalent sedimentary rocks of the meosoproterozoic belt supergroup 40ar 39ar dates from geochemical group b dikes are consistent with published u pb dates that demonstrate dike emplacement at 780 ma as part of the regional gunbarrel magmatic event hornblende concentrates from the group b dikes yield 40ar 39ar apparent ages of 778 772 ma whereas biotite from a baked contact zone yielded a plateau date of 788 ma paleomagnetic results from the group b dikes yield a mean direction of d 301 5 i 17 1 k 65 7 95 4 0 n 12 independent means 23 sites with a paleomagnetic pole at 14 6 n 127 0 e a95 3 2 the combination of geochronologic data results of a baked contact test and spatial agreement of the paleomagnetic poles with poles of similar age elsewhere in north america indicates that this is also a primary magnetization associated with dike emplacement paleomagnetic data from some of the tobacco root mountains dikes provide evidence that they were partially to completely remagnetized during latest cretaceous to early tertiary time perhaps due to thermal affects associated with emplacement of the late cretaceous tobacco root batholith the overall agreement of paleomagnetic poles from the proterozoic dikes with those of age equivalent rocks elsewhere in north america and agreement of the secondary magnetization with expected directions for the latest cretaceous early tertiary indicate that the rocks of the tobacco root mountains have not experienced significant tilting or vertical axis rotation since the mesoproterozoic the new paleomagnetic poles from this study thus provide key data for refining meso and neoproterozoic parts of the north american apw path

geologic volcanologic and tectonic setting of the vico cimino area italy abstract although the cimino and vico volcanic complexes are located close to each other in a structural low they belong to two different provinces cimino is connected to the tuscan roman anatectic magmatic province and vico to the roman campanian potassic alkaline province barberi et al 1971 marinelli 1975 the two volcanic centers lie on a basement formed by clays and sands of a miocene pliocene aged sedimentary cycle the mount cimino complex forms a central volcano composed of a series of rhyodacitic domes it was emplaced along a nw se trending fracture zone which can be followed for about 16 km cimino volcano was active from 1 4 m y to 0 95 m y ago eruption of rhyodacitic ignimbrites were preceded and followed by the emplacement of domes of the same composition fluid lavas of latitic and olivine latitic composition were emitted from the central volcano during the final phase of activity vico is a central volcano mainly composed of lava with a composite summit caldera it was emplaced along a ne sw trending fracture system the vico stratovolcano is formed by lavas of various compositions including phonolitic tephrites tephritic phonolites and slightly undersaturated trachytes the construction of the stratovolcano the most ancient exposed products of which have an age of 0 4 m y was followed by the eruption of four pyroclastic flows totalling several cubic kilometers which are dated at 0 3 0 15 m y the collapse of the caldera is most likely connected to the eruption of these pyroclastic flows the ring fractures of the caldera are associated with the emplacement of a pyroclastic unit most of which erupted along the inner northeast and north rim of the caldera post caldera activity is characterized by the emission of tephritic phonolitic and tephritic products that form the intra caldera stratovolcano named mount venere the vico cimino area northern latium located between the apennine chain and the tyrrhenian sea lies along the circum tyrrhenian belt that extends from northern tuscany to campania this zone has experienced post orogenic tensional movements since the middle miocene the teconic evolution of the tyrrhenian basin is closely associated with acid anatectic and potassic alkaline volcanism along the tyrrhenian margin of the apennines the age composition and geographic position of the volcanoes are controlled by different magnitudes of tensional tectonics and crustal thinning processes of the western margin of the apennine chain

calderas in the precambrian terrane of the st francois mountains southeastern missouri precambrian igneous rocks in the st francois mountains of southeastern missouri consist of silicic volcanic rocks mostly ash flow tuff which crop out mostly in the western and southwestern part of the region and epizonal granitic plutons which are mostly exposed in the northeastern part of the area these relations suggest that the exposures in the northeast are of a somewhat deeper part of the crust and that rocks there are now exposed because of west southwesterly tilting and beveling by erosion in the eastern st francois mountains the occurrence of a thick rhyolitic ash flow tuff grassy mountain ignimbrite and its distribution relative to a large high silica pluton butler hill granite suggest that the ash flow is the principal eruptive material of a caldera the butler hill caldera and that butler hill granite is the frozen magma chamber that was originally beneath it structural relations indicate that butler hill granite was emplaced into the volcanic rocks of the caldera during resurgent doming four small low silica plutons silvermine stono knoblick and slabtown granites are distributed peripherally to the butler hill granite and are interpreted as ring plutons emplaced along the ring fracture of the caldera or along arcuate intracaldera faults after resurgent doming and emplacement of butler hill granite more than 3 km of silicic volcanic rocks occupy a basinal structure in the western st francois mountains these rocks lie with angular unconformity upon up domed older rocks of the butler hill caldera along an arcuate zone of megabreccia and disturbed structure on the western margin of the older caldera this feature is interpreted as the topographic wall of a second caldera which lies to the west the occurrence of the thick taum sauk rhyolite stratigraphically about 3 km higher than the rocks involved in the megabreccias may indicate that there was yet another caldera formed in this western part of the st francois mountains

relatively rapid emplacement of dome forming magma inferred from strain analyses the case of the acid latian dome complexes central italy abstract we have analysed the relationship between the volcanic substratum and magma body emplacement for the acid dome complexes of latium central italy our study shows that the volcanic edifices which are mainly pleistocene cryptodomes and related explosive products were derived from mantle magmas contaminated by crustal materials the cimini tolfa and cerite manziate dome complexes of latium show the following characteristics a shallow laccolith origin emplacement in basins that have identical tectonic evolution and geological structure the same magmatic composition and density contrast between magma and host rock and geochronological data that are inconsistent with field evidence in the cimini and tolfa dome complexes the deformation induced by shallow intrusions was accompanied by 200 m uplift of the sedimentary cover the estimated pluton infilling time for the cimini and tolfa domes is 10 2 years while the strain rate required to uplift their pliocene overburden by 200 m is e m 10 9 s 1 the rapid evolution of the dome complexes is consistent with field data that show no relevant interruptions in the volcanic activity and no significant compositional changes in the volcanic products related to the extrusion of the domes for the cerite manziate dome complex the minimal input rate of magma favoured a monogenetic style of volcanism independent of the regional stress conditions

seasonal and long term vertical deformation in the nepal himalaya constrained by gps and grace measurements 1 we analyze continuous gps measurements in nepal southern side of the himalaya and compare gps results with grace observations in this area we find both gps and grace show significant seasonal variations further comparison indicates that the observed seasonal gps height variation and grace derived seasonal vertical displacement due to the changing hydrologic load exhibit very consistent results for both amplitude and phase for continuous gps stations whose observation time span are longer than 3 years the average wrms reduction is 45 when we subtract grace derived vertical displacements from gps observed time series the comparison for annual amplitudes between gps observed and grace derived seasonal displacements also shows consistent correlation the good seasonal correlation between gps and grace is due to the improved gps processing strategies and also because of the strong seasonal hydrological variations in nepal besides the seasonal signal grace also indicates a long term mass loss in the himalaya region assuming no gia effect this mass loss therefore will lead to crustal uplift since the earth behaves as an elastic body we model this effect and remove it from gps observed vertical rates with a 2d dislocation model most gps vertical rates especially in the central part of nepal can be interpreted by interseismic strain from the main himalayan thrust and several exceptions may indicate the complexity of vertical motion in this region and some potential local effects

intense interface seismicity triggered by a shallow slow slip event in the central ecuador subduction zone we document a 1 week long slow slip event sse with an equivalent moment magnitude of 6 0 6 3 which occurred in august 2010 below la plata island ecuador south of the rupture area of the 1906 mw 8 8 megathrust earthquake gps data reveal that the sse occurred at a depth of about 10 km within the downdip part of a shallow 15 km isolated locked patch along the subduction interface the availability of both broadband seismometer and continuous geodetic station located at the la plata island 10 km above the sse enables a careful analysis of the relationships between slow and rapid processes of stress release along the subduction interface during the slow slip sequence the seismic data show a sharp increase of the local seismicity with more than 650 earthquakes detected among which 50 have a moment magnitude between 1 8 and 4 1 however the cumulative moment released through earthquakes accounts at most for 0 2 of the total moment release estimated from gps displacements most of the largest earthquakes are located along or very close to the subduction interface with focal mechanism consistent with the relative plate motion while the earthquake sizes show a classical distribution gutenberg richter law with a b value close to 1 the space time occurrence presents a specific pattern first the largest earthquakes appear to occur randomly during the slow slip sequence which further evidence that the seismicity is driven by the stress fluctuations related to aseismic slip moreover the seismicity observed during the sse consists in individual events and families of repeating earthquakes these observations indicate that the stress increment induced by the episodic aseismic slip may lead both to sudden seismic moment release and to progressive rupture within small locked patches this study offers an a posteriori interpretation of the seismogenesis in the central ecuador subduction zone where intense seismic swarms have been regularly observed 1977 1998 2002 and 2005 these swarms have likely been triggered by large magnitude slow slip events

paleo position of the north china craton within the supercontinent columbia constraints from new paleomagnetic results abstract several new paleomagnetic and geological studies focused on the reconstruction of the north china craton ncc within the paleo mesoproterozoic columbia supercontinent in spite of these new data there are still widely divergent opinions regarding supercontinental reconstructions in addition to qualitative correlations of orogenic belts rift basin ages stratigraphy and distribution of igneous provinces paleomagnetic data can provide key constraints on the positioning of individual cratons on the globe in this paper we report a detailed paleomagnetic study on the coeval 1780 ma mafic dyke swarm and xiong er volcanic province which extended for more than one thousand kilometers in the central ncc rock magnetic studies including thermomagnetic curves hysteresis loops and the progressive acquisition of isothermal remanence conducted in selected samples indicate that the dominant magnetic carriers are psd magnetite stepwise thermal demagnetization isolated higher temperature components directed to nne ssw with shallow inclinations from 37 sampling sites 16 sites in yinshan area 13 sites in taihang area and 8 sites in xiaoshan area a baked contact test conducted on two yinshan dykes intruded by a younger dyke demonstrates the magnetization in the yinshan dykes pre dates 1320 ma the existence of dual polarity magnetizations in both taihang and xiong er areas support our contention that the chrm was acquired during the cooling of the magma the primary origin of the chrm is also supported by a positive fold test on the xiong er data and a coherent regional test between the results from the taihang and xiong er areas two different site mean directions were compiled from these new results along with previous publications the first direction from the taihang and xiong er areas yields declination d inclination i 12 4 3 7 20 5 95 4 3 n 57 sites the second from the yinshan area is at d 36 7 i 12 4 86 8 95 2 7 n 32 sites we argue that the difference is due to mesozoic and or cenozoic vertical axis rotation of the taihang and xiong er areas with respect to the fixed yinshan ordos basin the corresponding paleopoles for the yinshan dykes falls at 245 2 e 35 5 n a95 2 4 a comparison between the ncc laurentia siberia and baltica is consistent with possible links between these four blocks in a perhaps even larger continent the proximity of australia and india to the ncc is also evaluated

styles of back arc extension in the central mediterranean in the central mediterranean two back arc basins the liguro provencal lpb and the tyrrhenian basin tb opened progressively and consecutively from the late eocene oligocene to the present evolution in space and time of rifting and drifting processes along three different transects across these basins shows differences in the style of extension lpb opened with the formation of a narrow single rift while in the tb deformation and magmatism is spread over a wide area moreover at the northern end of the tb the locus of extension progressively migrated towards the east whereas in the southern tb the locus of extension and magmatism migrated inside the basin inducing continental break up and drifting of the previously formed older conjugate basins we propose that these different styles of back arc extension depend upon internal conditions such as prerift rheology linked with its geological heritage and external conditions e g the style of subduction

the specific case of the mid proterozoic rapakivi granites and associated suite within the context of the columbia supercontinent abstract rapakivi granites and the associated suite anorthosite mangerite and charnockite that developed during the mid proterozoic escape the global rules that control usual granitic magma formation about 40 points that characterise the proterozoic magmatism confirm the originality of the magmatism rapakivi granites developed without a global orogenic context within the supercontinent columbia the intrusions extend from the east european shield to the western us through fennoscandia greenland and labrador other occurrences in the amazonian shield australia or south china are less well documented they show no definite trend in age or chemistry that would explain large scale mantle plume effects a mantle upwelling of material is contradictory with the re assembly of a supercontinent because it occurred before this magmatic episode 1 9 1 8 ga at 1 7 ga the supercontinent columbia amalgamated with the collision of the yapavai and mazatzal provinces a model is suggested that takes into account the supercontinent re assembly defining a downwelling flow in the mantle that anchors the continent above it in contrast to a material flow heat is still delivered to the base of the continental lithosphere and is focused toward the juvenile suture zone the base of the crust must reach 1200 1300 c before producing anorthosite magmas under such a high temperature gradient lasting over a long time magmas are transferred toward the upper crust giving the thin 5 km and square shape 100 km that the intrusions presently have heat delivery is essentially conductive leading to long time spans for intrusions the presence of the supercontinent immobile over a descending cell poloidal mode of convection developed a tangential force toroidal mode of convection that partly split the continent through strike slip deformation due to plate rotation it developed progressively between 1 57 and 1 3 ga starting from fennoscandia and then passing to amazonia western us and labrador in a clockwise sense the associated rotation induced sinistral shear manifested by small scale shear zones and the orientation of late magmatic facies topaz bearing granites in each province the proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons the present moho still shows remnants of this process having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km

timing of proterozoic magmatism in the gardar province southern greenland the gardar province in southern greenland is believed to have experienced three episodes of rifting all associated with the extrusion of large volumes of alkalic magma a nepheline syenite of the motzfeldt center yields a u pb apatite age of 1 35 0 01 ga an augite syenite from the ilimaussaq complex yields a sm nd pyroxene alkali feldspar magnetite age of 1 13 0 05 ga on the basis of field relationships the eriksfjord formation lavas have been interpreted as the oldest manifestation of gardar magmatism sm nd plagioclase pyroxene wholerock ages of 1 17 0 03 ga and 1 20 0 03 ga for the middle member of the eriksfjord formation however indicate that these lavas may be part of the youngest magmatic activity the available chronological data indicate that the gardar province was active for at least 200 m y from 1 35 to 1 12 ga it is difficult to reconcile this protracted history with plume models for the genesis of such alkaline flood basalt provinces

geostatistical techniques application to dissolved radon hazard mapping an example from the western sector of the sabatini volcanic district and the tolfa mountains central italy abstract dissolved rn was determined in 192 samples collected from cold shallow volcanic and sedimentary aquifers deep thermal aquifers and from waters associated with bubbling gases in the western sector of the sabatini volcanic district and the tolfa mountains central italy shallow aquifers hosted in the quaternary volcanic complexes show values ranging from 1 0 to 352 bq l median value 55 and inter quartile distance 62 bq l while waters circulating within the permeable horizons of the sandy to clayey sediments of the tolfa flysch have values from 1 0 to 44 bq l median value 6 9 and inter quartile distance 8 1 bq l thermal waters are hosted in the mesozoic carbonate formations and move towards the surface along faults here dissolved rn values range from 0 to 37 bq l median value 3 0 and inter quartile distance 9 5 bq l waters associated with bubbling gases show dissolved rn contents ranging from 2 0 to 48 bq l median value 6 2 and inter quartile distance 23 bq l those results suggest that lithology is the main factor affecting the rn contents in shallow aquifers due to the high levels of rn progenitors u and ra in the volcanic rocks relative to sedimentary units the influence of other factors such as the presence of a fracture network seasonal flow variations type of discharge from well or spring was also investigated radon contents of thermal waters result from mixing with shallow waters from both volcanic and sedimentary rock aquifers and decrease of rn solubility with temperature while for bubbling pools the effects of strong degassing were also considered in terms of health hazard from direct ingestion of rn rich waters 20 8 of those circulating within the volcanic aquifer show values higher than the recommended value of 100 bq l while none of those circulating within the sedimentary aquifers exceed the threshold value geostatistical techniques were used for the elaboration of contour maps by using variogram models and kriging estimation aimed at defining the areas where a potential health hazard due to the direct ingestion of rn rich waters and to inhalation of air following degassing of rn from waters may be expected

paleomagnetism of the ulkan massif se siberian platform and the apparent polar wander path for siberia in late paleoproterozoic early mesoproterozoic times abstract paleomagnetic studies on paleoproterozoic rocks of the ulkan massif from the south eastern siberian platform yielded two ancient paleomagnetic directions and made it possible to calculate poles corresponding to them for two time intervals in the late paleoproterozoic 1732 ma for volcanogenic sedimentary rocks of the elgetey formation plat 7 1 7 1 plong 183 5 3 5 dp 10 5 dm 16 4 positive fold reversals and conglomerates tests and b less than 1720 ma for granites of the ulkan complex plat 42 1 42 1 plong 249 4 69 4 dp 3 4 dm 5 6 the analysis of the existing paleomagnetic data for siberia in late paleoproterozoic early mesoproterozoic times allowed a total of 12 new poles to be defined with five of them considered key poles table 3 based on which the apwp for siberia was generated on the basis of the data presented here one can conclude that in the context of the paleoproterozoic supercontinent model preference is given to the position of paleoproterozoic north poles of siberia in the indian ocean and australia rather than in the atlantic ocean