eruptive processes and caldera formation in a nested downsagcollapse caldera cerro panizos central andes mountains abstract the cerro panizos ignimbrite center in the central andes mountains produced two ignimbrite sheets and many lava flows the ignimbrite of quebrada cienago was erupted at 7 9 ma and effusive eruptions continued until the two cooling units of the crystal rich dacitic cerro panizos ignimbrite were emplaced at 6 7 ma the lower unit has no laterally continuous flow breaks and was erupted from a single vent or small cluster of vents with limited fluctuation in discharge lithic fragments reach significant concentrations 5 only in the uppermost two meters of this cooling unit where they document vent wall collapse or the opening of a new vent the upper cooling unit contains many flow units with variations in welding thicknesses and lithic fragment concentrations implying an unsteady eruption column the opening of many vents and probable caldera collapse triangulation of anisotropy of magnetic susceptibility flow direction measurements locate a single vent for the lower cooling unit whereas the upper cooling unit had many vents within the present dome cluster a 15 km diameter topographic depression marked by inward dips of 4 8 at the cooling unit contact is centered on the vent area of the lower cooling unit the depression is interpreted as a downsag caldera formed during emplacement of the lower cooling unit collapse began late in the eruption of the lower cooling unit and continued through the emplacement of the upper cooling unit resurgent magmatism occurred as lava flows that mundated the caldera area a ring of dacite domes erupted until at least 6 1 ma in the northern half of the downsag caldera traces the margin of a collapse caldera associated with the upper cooling unit maximum caldera subsidence 353 km 3 is not enough to account for the erupted volume 652 km 3 dre minimum the ash poor crystal rich nature up to 50 crystals in the pumice 75 in the matrix of the cerro panizos pyroclastic flows resulted in poor retention of gases and there is little evidence of fluidization the high crystal content however favored modified grain flow processes in which several particle support mechanisms combine with grain dispersive forces it is postulated that the flows were initially partially fluidized but rapidly lost their gases and grain flow processes grew in relative importance grain flow processes destroyed evidence of fluidization before deposition

partitioning of sr ba rb y and lree between plagioclase and peraluminous silicic magma trace element partition coefficients between plagioclase and coexisting glass matrix have been determined for twenty nine rhyodacite rhyolite samples from nine volcanic centers strontium partition coefficients form two clear positive trends when plotted against the an content of plagioclase one trend has a steep slope with d sr between 5 6 and 15 8 t1 whereas the other trend has a gentle slope with d sr between 1 2 and 7 6 t2 barium partition coefficients show similar patterns a steeply sloped trend is formed by samples with d ba between 1 6 and 8 8 and a gently sloped trend by samples with d ba 1 d sr and d ba in plagioclase correlate positively and both d sr and d ba correlate positively with temperature samples with large d sr and d ba are all from rocks with 1 wt cao these rocks are peraluminous and also have low total sr and ba partition coefficients for sr and ba are influenced by whole rock cao in high al rhyolite systems where these systems have low cao the opportunity increases for other divalent cations to enter the plagioclase structure strontium and ba will then enter the m position of plagioclase to balance the charge deficiency caused by the substitution of al for si because of the potent influence of whole rock composition on sr and ba partitioning for low cao rocks values of partition coefficients in petrogenetic modeling needs to be selected carefully

vertical distribution of trace elements in laterite soil suriname abstract the vertical distribution of 20 elements in a 12 5 m thick laterite profile from suriname south america has been investigated using neutron activation analysis atomic absorption spectrometry and x ray fluorescence the mineralogical composition was quantitatively determined from calculations based on results from x ray diffraction thermal gravimetric methods and from the data obtained for major elements using x ray fluorescence the most characteristic change observed in the mineralogy with depth is the increase in kaolinite relative to gibbsite a considerable fractionation with depth is found for different trace elements the enrichment of the lanthanoids la and ce at 3 4 m below surface and of sm and eu at 10 12 5 m depth demonstrates that the distribution of elements despite the similarity in chemical properties is seriously affected by weathering processes the laterite soil samples were also separated in al and fe rich fractions using naoh these separation experiments demonstrated that the trace elements are mainly associated with the fe rich red mud fraction of the laterite soil

estimation of magma depth for resurgent domes an experimental approach post collapse resurgence is a phenomenon affecting many calderas attributed to a renewed magma rise the process is still poorly understood and the associated source parameters remain poorly constrained a set of experiments has been conducted to gain insight into the structural evolution of caldera resurgent domes a sand plaster mixture was chosen as an analogue for the brittle pile of volcanic rocks and silicone putty simulates the ductile behavior of the intruding magma resurgence is driven by the vertical uplift of the silicone with variable shape and depth similarity conditions are achieved through eight dimensionless numbers which are of the same order of magnitude in both nature and experiments results show that extension due to doming is in many cases accommodated by one axial graben opposite master faults of this graben intersect at depth at the junction with the rising viscous silicone the simplicity of the geometry of the whole analogue system provides equations which allow the estimation of the silicone depth from surface parameters these equations are then used in some field examples to assess the magma depth beneath natural resurgent domes

conductive heat flux in vc 1 and the thermal regime of valles caldera jemez mountains new mexico over 5 of heat in the western united states is lost through quaternary silicic volcanic centers including the valles caldera in north central new mexico these centers are the sites of major hydrothermal activity and upper crustal metamorphism metasomatism and mineralization producing associated geothermal resources we present new heat flow data from valles caldera core hole 1 vc 1 drilled in the southwestern margin of the valles caldera thermal conductivities were measured on 55 segments of core from vc 1 waxed and wrapped to preserve fluids these values were combined with temperature gradient data to calculate heat flow above 335 m which is probably unsaturated heat flow is 247 16 mw m 2 the only deep temperature information available is from an uncalibrated commercial log made 19 months after drilling gradients derived from uncalibrated temperature logs and conductivities are inversely correlated between 335 and 737 m indicating a conductive thermal regime and component heat fluxes over three depth intervals 335 539 m 549 628 m and 628 737 m are in excellent agreement with each other with an average of 504 15 mw m 2 temperature logs to 518 m depth with well calibrated temperature sensors result in a revised heat flow of 463 15 mw m we use shallow thermal gradient data from 75 other sites in and around the caldera to interpret the thermal regime at the vc 1 site a critical review of published thermal conductivity data from the valles caldera yields an average thermal conductivity of 1 w m 1 k 1 for the near surface tuffaceous material and we assume that shallow gradient values c km 1 are approximately numerically equal to heat flow mw m 2 heat loss from the caldera is asymmetrically distributed with higher values 400 mw m 2 or higher concentrated in the west southwestern quadrant of the caldera this quadrant also contains the main drainage from the caldera and the youngest volcanism associated with the caldera we interpret the shallow thermal gradient data and the thermal regime at vc 1 to indicate a long lived hydrothermal and magmatic system in the southwestern valles caldera that has been maintained through the generation of shallow magma bodies during the long postcollapse history of the caldera high heat flow at the vc 1 site is interpreted to result from hot water circulating below the base of the core hole and we attribute the lower heat flow in the unsaturated zone to hydrologic recharge

initial results from vc 1 first continental scientific drilling program core hole in valles caldera new mexico valles caldera 1 vc 1 is the first continental scientific drilling program csdp core hole drilled in the valles caldera and the first continuously cored well in the caldera region the objectives of vc 1 were to penetrate a hydrothermal outflow plume near its source to obtain structural and stratigraphie information near the intersection of the ring fracture zone and the precaldera jemez fault zone arid to core the youngest volcanic unit inside the caldera banco bonito obsidian coring of the 856 m well took only 35 days to finish during which all objectives were attained and core recovery exceeded 95 vc 1 penetrates 298 m of moat volcanics and caldera fill ignimbrites 35 m of precaldera volcaniclastic breccia and 523 m of paleozoic carbonates sandstones and shales a previously unknown obsidian flow was encountered at 160 m depth underlying the battleship rock tuff in the caldera moat zone hydrothermal alteration is concentrated in sheared brecciated and fractured zones from the volcaniclastic breccia to total depth with both the intensity and rank of alterations increasing with depth alteration assemblages consist primarily of clays calcite pyrite quartz and chlorite but chalcopyrite and sphalerite have been identified as high as 450 m and molybdenite has been identified in a fractured zone at 847 m carbon 13 and oxygen 18 analyses of core show that the most intense zones of hydrothermal alteration occur in the madera limestone above 550 m and in the madera and sandia formations below 700 m this corresponds with zones of most intense calcite and quartz veining thermal aquifers were penetrated at the 480 540 and 845 m intervals although these intervals are associated with alteration brecciation and veining they are also intervals where clastic layers occur in the paleozoic sedimentary rocks

tourmaline in a sub cambrian palaeosol on the proterozoic lewisian rocks of nw scotland tourmaline not hitherto recorded in lewisian rocks of sutherland nor reported as a secondary mineral in precambrian palaeosols occurs in a late proterozoic sub cambrian palaeosol in nw scotland from field and petrological criteria the tourmaline is secondary and peculiarly associated with the alteration profile hence it formed either during weathering or later in the profile s history a minor constituent of the palaeoregolith immediately below the unconformity the tourmaline mostly occurs as sub millimetre radiating acicular aggregates and is schorl in composition black tourmaline quartz veinlets cross cut the profile stable isotope and fluid inclusion data suggest that the tourmaline formed from surface derived fluids at c 110oc it is concluded that boron was adsorbed onto precursor illitic clays of the soil profile during the cambrian marine transgression thus the tourmaline formed subsequently after burial under the sedimentary pile probably during the caledonian orogeny when temperatures were high enough to allow its formation at suitable sites of nucleation e g adjacent to biotite its peculiarity as a palaeosol phase is due to the combination of a marine transgression over an exposed permeable illitic soil and underlying regolith followed by increase in temperature suffcient to allow the tourmaline to form

conflicting sm nd whole rock and u pb zircon ages for archean lavas from newton township abitibi belt ontario abstract archean komatiitic and tholeiitic lavas from newton township ontario have a sufficient range of sm nd ratios to define a well constrained line on the normal 143 nd 144 nd vs 147 sm 144 nd isochron plot the data give an isochron age of 2826 64ma and an initial e nd of 2 65 0 26 however u pb analysis of zircons from a dacitic volcaniclastic that underlies the komatiite tholeiite suite give an age of 2697 1 1ma there is strong evident that the zircon age is the eruption age suggesting that the older sm nd age is incorrect and probably results from mixing between isotopically distinct mantle sources at the time of eruption the sources had e nd values from about 4 2 to 1 6 indicating that the archean mantle in this area was markedly heterogeneous and not uniformly depleted

stratigraphic relations and lithologic variations in the jemez volcanic field new mexico over 100 radiometric dates and recent detailed geologic mapping allow some refinements of the stratigraphic relations of major units and generalization of temporal lithologic variations in the jemez volcanic field volcanism had begun in the area by about 16 5 ma with episodic eruptions of alkaline basalts by 13 ma alkaline volcanism had been replaced with eruptions of more voluminous olivine tholeiite high silica rhyolite derived from melts of lower crust also was erupting by about 13 ma basalt and high silica rhyolite continued to be erupted until about 7 and 6 ma respectively but effusions of dominantly andesitic differentiates of basalt that began as early as about 12 ma volumetrically overshadowed all other eruptive products between 10 and 7 ma from 7 to 3 ma the dominant erupted lithology was dacite which appears to have been generated by mixing of magmas whose compositions are approximated by earlier andesites and high silica rhyolites less than 4 3 ma volcanism was dominated by eruption of rhyolitic tuffs field relations geochemistry and dates specifically indicate the following with regards to stratigraphie relations 1 distinctions among basalt of chamisa mesa paliza canyon formation basalts and lobato basalt for other than geographic reasons are artificial basaltic volcanism was continuous in volcanic field from 13 to 7 ma 2 canovas canyon and bearhead rhyolites form a continuum of high silica rhyolite volcanism from 13 to 6 ma 3 hypabyssal and volcanic rocks of the cochiti mining district probably represent the exhumed interior of a keres group volcano s 4 temporal overlaps exist among the major stratigraphie groups which may imply some genetic relations and 5 the tewa group formation cerro rubio quartz latite may more appropriately be considered part of the tschicoma formation of the polvadera group preliminary analysis of hydrothermal alteration in the context of the volcanic stratigraphy suggests at least three distinct hydrothermal events have occurred in the volcanic field s history

u th he dating of volcanic phenocrysts with high u th inclusions jemez volcanic field new mexico we measured u th he ages of fayalite and hedenbergite phenocrysts with britholite and chevkinite inclusions from the 1 6 ma otowi lower member of the bandelier tuff jemez volcanic field u and th the major parent nuclides of radiogenic 4 he are concentrated in britholite and chevkinite inclusions and are essentially absent from the host grains we analyzed 36 fayalite 21 non abraded 15 abraded and 13 hedenbergite 7 non abraded 6 abraded phenocrysts and applied alpha ejection corrections assuming uniform intracrystalline distributions of inclusions and parent nuclides in the phenocrysts a subset of phenocrysts was subjected to abrasion to remove the alpha ejection affected regions the alpha ejection correction factors ft of the non abraded samples range between 0 90 and 0 96 whereas those of the abraded samples are virtually unity the fayalite and hedenbergite crystals yield mean ages of 1 60f0 10 ma s d and 1 69f0 13 ma respectively the overall mean age of 1 62f0 11 ma s d is indistinguishable from previously published 40 ar 39 ar ages of 1 61f0 01 ma 1j our results suggest that other inclusion

hydrothermal brecciation in the jemez fault zone valles caldera new mexico results from continental scientific drilling program core hole vc 1 an unusual breccia sequence penetrated in the lower 30 m of continental scientific drilling program core hole vc 1 total depth 856 m records a complex hydrothermal history culminating in hydraulic rock rupture and associated alteration at the edge of the quaternary valles caldera the breccias both tectonic and hydrothermal in origin were formed in the jemez fault zone near the intersection of this major regional structure with the caldera s ring fracture margin tectonic breccias in the sequence are contorted crushed and sheared coexisting hydrothermal breccias lack such frictional textures but display matrix flow foliation and prominent clast rounding features characteristic of fluidization these hydrothermal breccias were intensely altered during at least five major stages to quartz illite phengite pyrite aggregates traces of molybdenite occur locally this assemblage indicates interaction with hydrothermal fluid at temperatures in excess of 200 c the extrapolated present maximum temperature of 184 c in the breccia zone therefore represents considerable cooling since these phases were formed fluid inclusions in the breccias also preserve evidence of the prior passage of hotter fluids the inclusions are principally two phase liquid rich secondary in origin and concentrated in hydrothermal quartz older high salinity inclusions unrelated to brecciation homogenize in the temperature range 189 246 c younger inclusions in part of interbreccia origin are low salinity and homogenize in the temperature range 230 283 c locally coexisting liquid and vapor rich inclusions document periodic boiling of the dilute fluids these fluid inclusion data along with the probable age of the hydrothermal breccias 1 5 ma the assumed depth at which they developed about 515 m and the contemporaneous state of stress extensional can be combined to model hydrothermal brecciation at the vc 1 site the minimum fluid pressure pfr required to hydrofracture these rocks was probably about 7 5 mpa 0 0146 mpa m a boiling point versus depth curve based on these pfr values graphically defines the physical conditions prevailing when the breccias were formed when fluid pressure at the assumed depth of brecciation exceeded that curve in response to rapid release of confining pressure possibly augmented by renewed heating flashing hydrothermal fluid fractured the enclosing rock large overpressures most likely induced by sudden seismic cracking of a hydrothermally sealed portion of the jemez fault zone led to local fluidization of the resulting breccias late quartz veining hydrothermal alteration and molybdenite mineralization were probably produced by the fluids responsible for brecciation

precambrian paleosols in canada although at least 38 precambrian paleosols have been reported in canada few have received the attention afforded most other geological phenomena the most intensively studied of these are widely distributed early proterozoic paleosols associated with the sub huronian and matonabbee new name unconformities criteria that have been most useful in identifying precambrian paleosols in canada include macrostructures ascending protolith disruption saprolith and corestones micromorphology sepic texture and peds mineralogical changes dissolution transformation geochemical signatures ascending depletion of some elements characteristic weathering indices and distinctive colour contrast diagenetic mechanical and geochemical overprinting of the paleosols evident in many canadian examples includes paleosol compaction veins cutting pedogenic features and widespread alkali and alkaline earth element metasomatism metamorphism and deformation also tend to mask pedogenic features as attested by th

hot dry rock geothermal energy in the jemez volcanic field new mexico abstract large young calderas possess immense geothermal potential due to the size of shallow magma bodies that underlie them through the example of the valles and toledo calderas new mexico and older more deeply eroded and exposed calderas it is possible to reconstruct a general view of geothermal environments associated with such magmatic systems although a zone of anomalous heat flow extends well beyond caldera margins high to moderate temperature hydrothermal systems appear to be restricted to zones along intracaldera faults and may account for only about five percent of the thermal energy available from the magmatic system much of the remaining thermal energy resides within caldera fill and basement rocks under and along the flanks of the caldera and can only be extracted by manmade geothermal systems depending upon the age and cooling history considerable thermal energy may still reside in crystallizing magma

volcanism and sedimentation along the western margin of the rio grande rift between caldera forming eruptions of the jemez mountains volcanic field north central new mexico usa abstract the cerro toledo formation ctf a series of intracaldera rhyolitic dome complexes and their associated extracaldera tephras and epiclastic sedimentary deposits records the dynamic interplay between volcanic tectonic and geomorphic processes that were occurring along the western margin of the rio grande rift between major caldera forming eruptions of the bandelier tuff 1 65 1 26 ma the alamo canyon and pueblo canyon members differ significantly despite deposition within a few kilometers of each other on the pajarito plateau these differences highlight spatial distinctions in vent sources eruptive styles and depositional environments along the eastern side of the jemez mountains volcanic field during this ca 400 000 year interval intercalated pyroclastic fall deposits and sandstones of the pueblo canyon member reflect deposition with a basin thick alamo canyon member deposits of block and ash flow tuff and pyroclastic fall deposits fill a paleovalley carved into coarse grained sedimentary units reflecting deposition along the mountain front chemistry and ages of glass from fall deposits together with clast lithologies of sedimentary units allow correlation of outcrops subsurface units and sources dates on pyroclastic fall deposits from alamo canyon record deep incision into the underlying otowi member in the southern part of the pajarito plateau within 100 k y of the toledo caldera forming eruption reconstruction of the ctf surface shows that this period of rapid incision was followed by aggradation where sediments largely filled pre existing paleocanyons complex sequences within the upper portion of the otowi member in outcrop and in the subsurface record changes in the style of eruptive activity during the waning stages of the toledo caldera forming eruption

lithic breccia and ignimbrite erupted during the collapse of crater lake caldera oregon abstract the climactic eruption of mount mazama 6845 y b p vented a total of 50 km3 of compositionally zoned rhyodacitic to basaltic magma from a a single vent as a plinian pumice fall deposit and the overlying wineglass welded tuff and b ring vents as ignimbrite and coignimbrite lithic breccia accompanying the collapse of crater lake caldera new field and grain size data for the ring vent products are presented in this report the coarse grained poorly bedded clast supported lithic breccia extends as far as 18 km from the caldera center like the associated ignimbrite the breccia is compositionally zoned both radially and vertically and silicic mixed and mafic types can be recognized based on the proportion of rhyodacitic pumice matrix fractions in silicic breccias are depleted of fines and are lithic and crystal enriched relative to silicic ignimbrite due to vigorous gas sorting during emplacement ignimbrite occurs as a proximal veneer deposit overlying the breccia a medial 8 to 25 km from the caldera center compositionally zoned valley fill as much as 110 m thick and an unzoned distal 20 km facies which extends as far as 55 km from the caldera breccia within 9 km of the caldera center is interpreted as a coignimbrite lag breccia formed within the deflation zone of the collapsing ring vent eruption columns expanded pyroclastic flows of the deflation zone were probably vertically graded in both size and concentration of blocks as recently postulated for some turbidity currents an inflection in the rate of falloff of lithic clast size within the lithic breccia at 9 km may mark the outer edge of the deflation zone or may be an artifact of incomplete exposure the onset of ring vent activity at mt mazama was accompanied by a marked increase in eruptive discharge pyroclastic flows were emplaced as a semicontinuous stream as few ignimbrite flow unit boundaries are evident as eruption from the ring vents progressed flow runout distance and the extent of breccia deposition decreased due to a greater internal flow friction and b decreasing eruption column heights effect b probably resulted from a progressive decrease in magmatic gas content and discharge rate waning discharge may have been promoted by the tapping of more viscous crystal rich magma collapse of conduit walls and declining caldera collapse rate

tectonic and geologic evolution of the espanola basin rio grande rift structure rate of extension and relation to the state of stress in the western united states abstract the espanola basin of the rio grande rift began as a broad crustal downwarp in latest oligocene time most of the basin is 2 3 km deep but localized faulting allowed accumulation of up to 5 km of sedimentary fill in a central sub basin the localized early faulting ended before filling of the central espanola basin was completed about 10 m y ago movement on faults that define the present western margin of the espanola basin began 10 m y ago jemez mountain volcanism in the western espanola basin also began at about this same time west tilting of up to 30 occurred due to movement along pervasive n trending intrabasin faults about 7 5 m y ago in conjunction with continued movement along the western border faults volcanism continued after this tilting forming many of the large volcanic constructs of the jemez mountains regional uplift of the entire northern rio grande rift began 7 m y ago movement on the pajarito fault zone began about 5 m y ago and continues to the present this fault zone defines the western margin of the velarde graben a narrow central sub basin where recent movement has been concentrated some volcanism also has occurred within the southern velarde graben total extension across the espanola basin since 26 m y ago is estimated to have been 5 5 km roughly 10 or between 3 5 and 8 km assuming high angle planar faulting the 0 2 mm yr averaged long term rate of extension has been separated into three periods of activity 1 1 0 14 mm yr from 26 to 10 m y ago 2 2 0 5 mm yr from 10 to 5 m y ago 3 3 0 14 mm yr from 5 m y ago to present a change in least principal stress direction from wsw ene to wnw ese that occurred throughout the western united states about 10 m y ago coincides with a roughly 3 5 times increase in the rate of extension preferential development and movement of n to ne trending normal faults and a few degrees of clockwise rotation of rocks in the western espanola basin similar to the espanola basin initial basins of the southern rio grande rift were broad downwarps and rifting was greatly accelerated after 10 m y ago accelerated uplift of the northern rio grande rift also occurred at about this time indicating that activity in the entire rio grande rift was modulated by this change in extension direction 10 m y ago that appears related to pacific north american plate interactions this modulation coupled with major faulting 10 m y ago preceding uplift 7 m y ago in the espanola basin suggest a passive rifting process for the rio grande rift whereby stresses due to plate interactions elsewhere cause faulting in the lithosphere which leads to the development of a passive asthenospheric uplift furthermore the roughly 20 m y pre uplift history of sediment accumulation in basins of the central and southern rift and the inherited character trend and geometry of the rio grande rift as a whole are also more consistent with a passive rifting process

paleoclimate deduced from some early jurassic basalt derived paleosols from northern israel abstract earthy red layers intercalated amid basalt flows from an early jurassic 2500 m thick volcanic sequence in northern israel were examined mineralogically micromorphologically and chemically distinct pedological features such as wavy illuviation argillans and iron oxide nodules observed by micromorphological examinations identified the clayrich layers as basalt derived paleosols the clay by comparison with the mineralogy and micromorphology of modern basalt derived vertisols a moderately wet mediterranean type climate with pronounced seasonality was inferred for the formation time of the paleosols a similar lower jurassic paleoclimate had previously been inferred from a paleosol in the negev

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

k ar dates of hydrothermal clays from core hole vc 2b valles caldera new mexico and their relation to alteration in a large hydrothermal system abstract seventeen k ar dates were obtained on illitic clays within valles caldera 1 13 ma to investigate the impact of hydrothermal alteration on quaternary to precambrian intracaldera and pre caldera rocks in a large long lived hydrothermal system 1 0 ma to present clay samples came from scientific core hole vc 2b 295 c at 1762 m which was spudded in the sulphur springs thermal area and drilled into the boundary between the central resurgent dome and the western ring fracture zone six illitic clays within quaternary caldera fill debris flow tuffaceous sediment and ash flow tuff 48 to 587 m depth yield ages from 0 35 to 1 09 ma illite from miocene pre caldera sandstone 765 m gives an age of 6 74 ma two dates on illite from sandstones in permian red beds 1008 and 1187 m are 4 33 and 4 07 ma respectively surprisingly three dates on illites from altered andesite pebbles within the red beds 1010 1014 m are 0 95 to 1 06 ma four illite dates on variably altered precambrian quartz monzonite 1615 1762 m range from 2 90 to 276 ma post valles age illite is not correlated with alteration style argillic to propylitic rather post valles ages are uniformly obtained from illites in highly fractured intensely altered caldera fill rocks and the permian volcanic clasts generally finer clay fractions from identical samples yield younger ages plots of 40 ar 36 ar versus 40 k 36 ar and 40 ar versus 40 k for the illites in caldera fill rocks lie close to a 1 ma isochron most illite dates older than valles caldera are difficult to interpret because they correspond to the ages of pre valles volcanic and hydrothermal episodes in the jemez volcanic field 13 ma in addition older dates may be caused by co mingling of different illites during sample preparation or by inherited argon or lost argon in illites from rocks with potentially complex hydrothermal histories however the range of ages obtained from illites in permian sands and pebbles and from precambrian crystalline rocks indicates that valles hydrothermal activity is overwhelming illite produced by earlier geologic events

geochemistry of fine grained clastic sediments in the archean abitibi greenstone belt canada implications for provenance and tectonic setting abstract archean sandstone and shale samples from turbiditic tf and alluvial facies af in the southern volcanic zone of the abitibi greenstone belt canada have been analyzed for major and trace elements the sandstones from tf are mainly greywackes whereas those from af are lithic arenite sio 2 ranges from 51 to 68 geochemically the clastic sediments cluster into mafic element mep and low mafic element lmep populations the mep sediments which include turbiditic facies sediment samples from the larder lake group in the kirkland lake area are characterized by elevated abundances of mgo 5 ni 200 ppm cr 400 ppm sc 22 ppm and feo 7 5 abundances lower contents of incompatible elements such as th u ta hf zr and total rees n ratios are 4 5 with eu eu 1 the lmep sediments which include samples from the beatty dome and three nations lake formations in timmins the kewagama group noranda area and timiskaming group in the kirkland lake area however show reverse abundance distributions the geochemical gaps shown by the major and trace elements define two populations of sediments suggesting contrasting provenances for them the mep sediments require a dominantly mafic and ultramafic 90 source with 10 felsic contribution whereas the lmep sediments require a mixture of felsic 75 and mafic 25 rocks this compositional distribution of the sediments is closely related to their tectonic environments the mep was formed in basins proximal to or within an active oceanic rift consequently having ultramafic and mafic rocks as the major source for the lmep calc alkaline volcanic sequences formed in an island arc like setting were the major source these two contrasting volcanic plutonic terranes were tectonically juxtaposed during late archean collisional tectonics

saprolite formation beneath coastal plain sediments near washington d c crystalline rocks buried beneath atlantic coastal plain sediments in the washington d c area commonly have weathering profiles that resemble exposed saprolite of the piedmont cuts along the washington metropolitan area transit authority wmata route in northern virginia show a bedrock weathering profile beneath cretaceous and younger sediments three lines of evidence indicate that this buried weathering profile saprolite has formed in the subsurface and is post miocene in age the thickness of the profile ranging from 2 to 15 m is thinner beneath clays and thicker beneath sands apparently a function of the permeability of the overlying material the buried profile shows no evidence of soil formation at its upper surface and therefore no evidence of ever having been sub aerially exposed consolidation tests on the cohesive clayey silts in the samples of the weathering profile indicate mechanical equilibrium with the present overburden whereas consolidation tests on superjacent cretaceous clays indicate over consolidation resulting from a thicker pre upper miocene overburden the post miocene age for the subsurface saprolite profile shows that not all saprolite beneath cretaceous sediment is pre cretaceous in age these observations have important implications for the interpretation of age of the exposed piedmont saprolite

properties and genesis of a soil and the underlying gibbsite bearing saprolite cape breton island canada brownish yellow saprolite derived from granite gneiss is exposed beneath 1 2 m of reddish brown till along a roadcut in the cape breton highlands the strongly acid soil is similar to other podzolic soils of the region except for the presence of gibbsite in the silt and clay fractions in the saprolite feldspar and biotite have weathered to poorly crystalline kaolinite gibbsite and a minor component of iron oxide the poorly crystalline kaolinite does not occur in the overlying till the evidence presented is consistent with the view that the area was covered either by cold based or by stagnant warm based glaciers that eroded the pre quaternary surface only slightly and deposited a thin discontinuous veneer of till derived mainly from local bedrock and saprolite weathering products from the saprolite are probably incorporated commonly in tills of the area during the formation of soils in these tills the poorly crystalline kaolinite and most of the gibbsite are either transformed to other minerals

distribution of ree in the eastern belt supergroup montana u s a implications for stratigraphic correlations and basin evolution abstract shales of the mid proterozoic newland formation belt basin contain a geochemical marker horizon that is characterized by the appearance of negative eu anomalies in shale ree patterns ree pattern changes appear to be related to changes in weathering intensity and tectonic activity in the hinterland stratigraphic and sedimentologic considerations suggest that this ree marker horizon defines an approximate time line within the sequence comparison of ree patterns of the newland formation and the coarse clastic lahood formation that was deposited to the south shows that the lower portions of the newland formation were deposited prior to lahood sedimentation such a correlation implies that early belt sediments may have covered a much larger area than delineated by the outline of the present day belt basin and that contrary to earlier views of basin evolution the half graben configuration of the eastern belt basin was established at some later point of basin history

tectonics of the jemez lineament in the jemez mountains and rio grande rift the jemez lineament is a ne trending crustal flaw that controlled volcanism and tectonism in the jemez mountains and the rio grande rift zone the fault system associated with the lineament in the rift zone includes from west to east the jemez fault zone southwest of the valles toledo caldera complex a series of ne trending faults on the resurgent dome in the valles caldera a structural discontinuity with a high fracture intensity in the ne jemez mountains and the embudo fault zone in the espanola basin the active western boundary faulting of the espanola basin may have been restricted to the south side of the lineament since the mid miocene the faulting apparently began on the sierrita fault on the east side of the nacimiento mountains in the late oligocene and stepped eastward in the early miocene to the canada de cochiti fault zone at the end of the miocene about 5 ma the active boundary faulting again stepped eastward to the pajarito fault zone on the east side of the jemez mountains the north end of the pajarito fault terminates against the jemez lineament at a point where it changes from a structural discontinuity zone of high fracture intensity on the west to the embudo fault zone on the east major transcurrent movement occurred on the embudo fault zone during the pliocene and has continued at a much slower rate since then the relative sense of displacement changes from right slip on the western part of the fault zone to left slip on the east the kinematics of this faulting probably reflect the combined effects of faster spreading in the espanola basin than the area north of the lineament abiquiu embayment and san luis basin the right step in the rift that juxtaposes the san luis basin against the picuris mountains and counterclockwise rotation of various crustal blocks within the rift zone no strike slip displacements have occurred on the lineament in the central and eastern jemez mountains since at least the mid miocene although movements on the still active jemez fault zone in the western jemez mountains may have a significant strike slip component basaltic volcanism was occurring in the jemez mountains at four discrete vent areas on the lineament between about 15 ma and 10 ma and possibly as late as 7 ma indicating that it was being extended during that time

tectonics of nisyros island greece by field and offshore data and analogue modelling abstract we use new on land and offshore structural data and scaled analogue models to analyse the fault tectonic pattern at nisyros island greece which has an active caldera that shows a complicated network of faults fractures and volcano tectonic structures we measured 157 faults that show dominant dip slip normal motions along planes mainly striking ne sw nne ssw and nnw sse inside the caldera dykes necks and morphometric parameters of volcanic domes explosion craters and fumarole pits indicate the control by ne striking discontinuities on magma and gas paths the nnw and ne striking faults bound a major block that underwent repeated downthrow and uplift movements during the late pleistocene holocene experiments with scaled models of caldera resurgence and two magma chambers indicate the formation of an hourglass shaped fault pattern as seen in plan view with an asymmetric increase in the fault offset and a widening of the fault divergence towards the volcano flank all these data suggest that regional fault tectonics and stress state strongly guided magma upwelling and the emplacement of volcanic centres whereas periodical bulging due to the overpressure of a second magma chamber located northwest of the caldera combined with faulting due to tectonic stresses can account for the overall deformation field

volcanic ash beds recorders of upper cenozoic silicic pyroclastic volcanism in the western united states stratigraphic relationships and tephrochronologic information for volcanic ash beds 4 0 0 1 m y of the western interior allow the recognition of at least 68 different ash falls the average reoccurrence rate based on the succession of the isotopically dated ash beds is near one eruption every 57 000 years the ash beds were grouped into two chemical types rhyolitic and dacitic the main property that distinguishes dacitic from rhyolitic ashes is the larger ca and mg and smaller k content in dacitic ashes dacitic volcanic ashes were defined to have more than 0 55 wt ca rhyolitic ashes were subdivided further into two types w and g type based on the presence of biotite in the w type and its absence in g type w type rhyolitic ashes are chalky white have less than 0 55 wt fe have colorless glass shards of pumiceous habit g type rhyolitic ashes are light to medium gray have from 0 55 to 2 0 wt fe in the glass shards lack biotite have colorless platy bubble wall and bubble junction glass shards dacitic ashes are light gray to grayish brown have more than 0 55 wt fe and ca and have a complex mixture of colorless to light brown shards including pumiceous fibrous chunky and bubble wall and bubble junction types w type rhyolitic ashes possibly formed from magmas that were cooler 695 to 830 c and thus more viscous than the hotter 870 to 1000 c magma progenitors of g type ashes based on fe ti oxide mineral compositions primary microphenocrysts in the ash beds comprise assemblages of the following quartz sanidine plagioclase biotite amphibole clinopyroxene orthopyroxene fayalite zircon apatite allanite chevkinite sphene magnetite and ilmentite fayalite and chevkinite only have been found in g type biotite and sphene only have been found in w type rhyolitic ashes are highly evolved and contain 74 79 wt sio2 dacitic ashes vary from 67 77 wt sio2 al ranges from 10 5 to 14 5 wt al2o3 mg ranges from 0 06 to 1 0 wt mgo and fe as fe3o4 ranges from 0 50 to 4 1 wt dacitic ash beds have the most 0 80 3 8 wt as fe3o4 and rhyolitic ashes have the least 0 50 1 9 wt as fe3o4 ca varies from an unusually low amount 0 25 wt cao in a few rhyolitic ashes to as much as 2 2 in some dacitic ashes rhyolitic glasses show larger eu deficiencies compared to dacitic glasses on rare earth element diagrams w type rhyolitic glasses have moderately large la to sm ratios and have la contents from about 30 to 100 times chondrite values g type rhyolitic glasses also have moderately large la to sm ratios and contain somewhat more la than do the w type rhyolitic ashes generally from 100 to 300 times chondrites amounts

sediment and basalt hosted regoliths in the huronian supergroup role of parent lithology in middle precambrian weathering profiles weathering profiles developed side by side on sandstone and a mafic dike provide an unusual opportunity to examine the role of parent rock bulk composition in the chemical evolution of middle precambrian regoliths because the profiles are adjacent to one another differences in topography can be eliminated in accounting for differences in the chemical evolution of the two profiles both profiles show upward increases in al ti k and rb and decreases in mg ca and na in addition the mafic regolith increases upward in zr and nb and decreases in zn and ni total fe decreases upward in both profiles but the sandstone profile retains significantly more of its initial fe than does the mafic dike this difference in fe loss is consistent with weathering in a low oxygen atmosphere of rock types with very different initial fe contents and therefore different atmospheric requirements for complete oxidation of the fe present the fe in the sandstone was mostly oxidized and retained within the profile whereas

geochemistry of archean shales from the witwatersrand supergroup south africa source area weathering and provenance with a few exceptions shales from the archean witwatersrand supergroup 2800 ma in south africa are depleted in na ca lile ree and hfse compared to phanerozoic shales cr co and ni are enriched in all witwatersrand shales and fe and mg are high in shales from the west rand groups wrg and lower central rand group crg shales from the crg and uppermost wrg are enriched in na al lile ree hfse and transition metals relative to shales from the lower wrg chondrite normalized ree patterns for all witwatersrand shales are enriched in light ree and exhibit small to moderate negative eu anomalies a positive correlation of ree and al2o3 contents in the shales suggests that ree are contained principally in clay minerals relative to shales from the crg shales from the wrg exhibit depletions of na ca and sr a feature probably reflecting intense chemical weathering of their source rocks cia indices in witwatersrand shales are variable chiefly 70 98 even within the same shale unit such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism compared to present day upper continental crust all but the orange grove roodepoort and k8 shales appear to have been derived from continental sources depleted in lile ree and hfse and enriched in transition metals computer mixing models based on six relatively immobile elements th hf yb la sc co and four source rocks indicate that the relative proportions of granite basalt and komatiite increased with time in sediment source areas at the expense of tonalite the contributions of basalt and komatiite appear to reach a maximum during deposition of the booysens shale and granite during deposition of the k8 shales and possibly during deposition of the orange grove shales

flow directions in ash flow tuffs a comparison of geological and magnetic susceptibility measurements tshirege member upper bandelier tuff valles caldera new mexico usa this study concludes that the elongation axis k 1 of the ellipsoid of anisotropic magnetic susceptibility ams is a suitable proxy for flow axis in ashflow tuffs 153 oriented samples 176 specimens were studied from 18 sites in the 1 1 ma tshirege member of the bandelier tuff these sites are distributed around the valles caldera at distances of 5 25 km outside of the rim k 1 axes correlate well with postulated radial flow axes at 13 sites k 1 also agrees with measured geological flow indicators mainly imbricated larger clasts at 7 sites at 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredk 1 axes thek 1 axes correspond well with geological flow indicators indicating that the divergence of flow from the predicted radial flow pattern is real two major topographic buttresses are suggested as the cause of flow divergence for the tshirege ash flows the san pedro buttress northwest of the caldera and the san miguel buttress in the southeast in situk 1 axes plunge about 7 toward the source at two thirds of the sites therefore the plunge ofk 1 is a plausible in situ indicator for thedirection of flow multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash flow emplacement these observations raisre the question of how best to represent mean flow directions in ash flow sheets by eigenvector methods by vector sum methods or by modes a method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined imbrication determined in this way at one third of the sites dips toward the source i e up flow the minimum k 3 axis of the ams ellipsoid correlates with the flow foliation rather than with the larger clast imbrication the flow axes of ash flows correspond with thek 1 axes not with the declination ofk 3 axes as suggested by some authors initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction which is reversed with a mean ofd 173 5 i 38 4 95 3 4 n 18 this mean is not different at the 95 confidence level from that of earlier workers the mean pole at 098 0 e 74 8 n a 95 3 3 n 18 is about 15 far sided relative to the expected time averaged geomagnetic pole suggesting a history of emplacement too short to adequately average secular variation

explosive rhyolitic volcanism in the jemez mountains vent locations caldera development and relation to regional structure the jemez volcanic field straddles the western margin of the rio grande rift where the rift is intersected by the jemez lineament in north central new mexico the field has a record of volcanism extending back to before 13 ma initial basaltic activity was related to active rifting with minor rhyolitic eruptions occurring along n s rift bounding faults between 10 and 7 ma voluminous andesitic volcanism took place in the central jemez mountains area overwhelming contemporaneous basaltic and rhyolitic magmatism an apparent tectonic lull took place from 7 to 4 ma accompanied by lower eruption rates during this interval dacitic magmas were erupted to form the tschicoma volcanic center but mafic and rhyolitic volcanism virtually ceased since 4 ma accompanying resumption of rifting a growing silicic magma system has been present under the central part of the jemez mountains ultimately evolving to the magma body that produced the voluminous rhyolitic bandelier tuffs explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 ma early eruptions 3 6 2 8 ma produced high silica rhyolite ignimbrites restricted to the southwest part of the jemez mountains formation of these units may have been accompanied by caldera collapse these events were followed by the two caldera forming bandelier tuff ignimbrite eruptions 1 45 and 1 12 ma post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both bandelier events vent and caldera locations for the rhyolitic eruptions during 3 4 ma have been inferred from grain size characteristics dispersal patterns and facies variations in the plinian deposits and ignimbrites lithic breccia zones of the pre bandelier ignimbrites indicate possible caldera sources in the southwest part of the present valles caldera during the eruption of both bandelier tuffs initial plinian falls and early pyroclastic flows emanated from vents centrally located in the jemez mountains in the lower bandelier tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows but there is no strong evidence to suggest such a transition occurred during the upper bandelier eruption calderas associated with the lower and upper bandelier tuffs toledo and valles respectively are almost identical in location as are the plinian vent sites for these two large eruptions the toledo embayment northeast of the valles caldera contains lava domes from up to 3 6 ma and may be a caldera or crater associated with early explosive dacitic volcanism in the tschicoma volcanic center post 1 4 ma lava domes also fill this depression the main volcanic features of the jemez mountains field including the valles caldera complex eruption vents and the apical graben of the post valles redondo resurgent block appear to be aligned along the ne sw trending jemez fault zone this zone the local expression of a precambrian basement feature the jemez lineament has exerted strong control on the location and style of eruptions from the bandelier rhyolitic magma system

premetamorphic lateritisation in proterozoic metabasites of rogaland sw norway heterogeneous layers of granulite facies metamorphic basites of the proterozoic supracrustal faurefjell formation in rogaland s w norway display an extreme chemical variation within a single layer the bulk chemical composition gradually changes from approximately basaltic in basic granulites to alumina iron rich in granofelses component ratios and composition volume relations indicate open system chemical reactions mainly involving the extraction of silica apparent enrichment in fe ti p al zr ni co zn y nb hf and ree and variations in resulting metamorphic mineral assemblages are related to premetamorphic progressive lateritisation of a basaltic protolith the weathering generated a continuous chemical suite from sio2 48 wt fe2o3 10 wt and al2o3 19 wt in the basic granulites to 14 wt 40 wt and 25 wt in the fe al granofelses metasomatism during diagenesis and during very high grade metamorphism 1200 900 ma further perturbed the concentrations of relatively mobile elements ca mg k rb sr ba na and li in the laterites without affecting the transition metal ratios in particular the ree did not fractionate differentially during the supracrustal and metasomatic alteration

asymmetric multiple block collapse at rotorua caldera taupo volcanic zone new zealand calderas worldwide have been classified according to their dominant collapse styles although there is a good deal of speculation about the processes involved recent laboratory experiments have tried to constrain these processes by modelling magma withdrawal and observing the effects on overlying materials however many other factors also contribute to final caldera morphology rotorua caldera formed during the eruption of the mamaku ignimbrite collapse structure and evolution of rotorua caldera is interpreted based its geophysical response geology and geomorphology and the stratigraphy of the mamaku ignimbrite rotorua caldera is situated at the edge of the extensional taupo volcanic zone in which major faults strike ne sw a second less dominant fault set strikes nw se these two fault sets have a strong influence on the morphology of rotorua caldera no one style of collapse can be applied to rotorua caldera it was formed during a single eruption but subsided as many blocks and shows features of trapdoor piecemeal and downsag types of collapse here rotorua caldera is described according to its composition activity and geometry as a rhyolitic single event asymmetric multiple block single locus collapse structure the mamaku ignimbrite is the only ignimbrite to have erupted from rotorua caldera extracaldera thickness of the mamaku ignimbrite is up to 145 m whereas inside the caldera it may be greater than 1 km thick the mamaku ignimbrite can be separated into a basal tephra sequence and main ignimbrite sequence the main ignimbrite sequence contains no observable flow unit boundaries but can be split into lower middle and upper parts lmi mmi umi respectively based on crystal content welding jointing devitrification and vapour phase alteration juvenile clasts within the ignimbrite comprise three consanguineous silicic pumice types and andesitic fragments only the most evolved pumice type occurs in the basal tephra sequence all three pumice types occur together throughout the main ignimbrite sequence whereas the andesitic fragments are only present in umi lithic lag breccias in umi indicate a late stage of caldera collapse concentration of lithic fragments increases towards the middle of the ignimbrite and may also reflect increased subsidence rate during an earlier stage collapse of rotorua caldera is thought to have occurred throughout the eruption of the main ignimbrite sequence of the mamaku ignimbrite allowing simultaneous eruption of all the different pumice types and causing the abrupt transition from deposition of the basal tephra sequence to the main ignimbrite sequence

chemistry and mineralogy of precambrian paleosols at the base of the dominion and pongola groups transvaal south africa abstract we have studied alteration zones found between sedimentary rocks of the lowest proterozoic dominion and archean pongola groups and underlying granitic rocks transvaal south africa the alteration zones have a transitional lower boundary primary igneous minerals are gradually destroyed upward toward the contact with overlying sedimentary rocks ca mg na p and mn are progressively lost upwards in the zone the chemical and mineralogical variations and available stratigraphic evidence are all consistent with the interpretation that these zones are paleosols formed by precambrian weathering 2 8 3 0 ga ago after formation the paleosols underwent metasomatism and metamorphism sericite now present in the paleosols was probably formed by low temperature potassium metasomatism of original clays andalusite in the pre pongola paleosol was probably formed by metamorphism of original clays in impermeable zones which prevented groundwater flow and potassium metasomatism the dominion paleosol was investigated at three localities the effects of weathering were similar at the three sites except for the behavior of iron one site lost about 20 of iron whereas the other sites lost 75 and 80 of initial iron content the differences in iron loss may be due to variation in soil ventilation and diffusion of atmospheric gases as a function of grain size and topography variations in iron loss may be used to place some constraints on the composition of the precambrian atmosphere given constraints imposed by dominion paleosols overlying uraniferous conglomerates requirements for chemical weathering and the greenhouse effect and possible presence of non diffusing photochemical oxidants the partial pressures of oxygen and carbon dioxide gases probably are in the ranges po1 0 02 to 0 5 pal present atmospheric level and pco2 5 to 30 pal

petrography and geochemistry of a 2000 2200 ma old hematitic paleo alteration profile on ongeluk basalt of the transvaal supergroup griqualand west south africa abstract a 4 m thick paleo alteration profile thought to represent one of the oldest known ferruginized hematitic paleoweathering zones is described from a depth of 720 m in a drill core on the farm drakenstein 263 some 20 km northwest of hotazel in griqualand west south africa the paleo alteration profile is developed on 2240 57 ma ongeluk basalt along the angular unconformity that separates transvaal strata from 2070 95 ma basal red beds of the olifantshoek group basaltic ongeluk lava was subjected to greenschist facies metamorphism represented by a mineral assemblage of fe pumpellyite chlorite albite and quartz before paleosol development the subsequent paleo alteration is characterized by concentration of fe2o3 in the form of hematite iron rich chlorite and abundant sericite the abundance of sericite results in a large excess of k2o relative to the amount of al2o3 that could have been derived from weathering of the parent basalt the origin of the excess potassium is problematical and could have been derived from basin water groundwater or metamorphic fluids field relationships favour introduction from basin water during deposition of overlying olifantshoek red beds elements such as sodium calcium and magnesium were leached from the alteration profile whereas ree became concentrated within it the most interesting aspect of the ree distribution is the preferred concentration of the lighter lanthanides la and nd in the middle part of the alteration profile relative to heavy ree and the development of a marked negative ce anomaly in the paleo weathered material hydrothermal vein alteration took place after deposition of the overlying olifantshoek red beds

a vertic paleosol at the archean proterozoic contact from the singhbhum orissa craton eastern india abstract we report on an inferred paleoproterozoic soil horizon from the singhbhum orissa craton near keonjhargarh orissa eastern india which preserves vertic vertisol like features the 2 10 m thick white shale consists of quartz muscovite pyrophyllite interpreted as a paleosol overlies 3 32 ga granite and is covered by pre 2 0 ga sandstones kolhan group deposited in marginal marine environments the paleosol passes downwards through a partly altered granite saprolith zone to fresh granite abundant pedogenic slickensides ped structures gilgai topography reddish stain on ped surfaces and sepic plasmic microfabric in the paleosol were probably related to shrink and swell processes driven by marked seasonal precipitation in a well drained landscape an observed decrease of alkali elements and an increase of silica alumina and cia values from granite to paleosol together with relatively constant ti al ratios suggest in situ weathering of granite protolith however a paleo vertisol necessitates a mafic parent rock either directly igneous or reworked through alluvium the c 2 25 ga malangtoli jagannathpur basaltic rocks are relatively widespread and close to the inferred paleosol occurrences and provide a logical solution geochemistry of the highly depleted paleosol in contrast particularly when applying the isochon method strongly supports a granitic parent the apparent conundrum can possibly be solved through repeated alluvial reworking of both granitic and mafic detritus and accumulation of weathered material with varying proportions of these two components yet enough smectitic clays 30 to form a paleo vertisol this pre 2 0 ga weathering event in the singhbhum craton is consistent with a global scenario of paleoproterozoic weathering which followed an extended global glaciation and thus has the potential to contribute to understanding of pre 2 0 ga amalgamation of regional and global cratonic blocks

the role of pre existing tectonic structures and magma chamber shape on the geometry of resurgent blocks analogue models abstract a set of analogue models has been carried out to understand the role of an asymmetric magma chamber on the resurgence related deformation of a previously deformed crustal sector the results are then compared with those of similar experiments previously performed using a symmetric magma chamber two lines of experiments were performed to simulate resurgence in an area with a simple graben like structure and resurgence in a caldera that collapsed within the previously generated graben like structure on the basis of commonly accepted scaling laws we used dry quartz sand to simulate the brittle behaviour of the crust and newtonian silicone to simulate the ductile behaviour of the intruding magma an asymmetric shape of the magma chamber was simulated by moulding the upper surface of the silicone the resulting empty space was then filled with sand the results of the asymmetric resurgence experiments are similar to those obtained with symmetrically shaped silicone in the sample with a simple graben like structure resurgence occurs through the formation of a discrete number of differentially displaced blocks the most uplifted portion of the deformed depression floor is affected by newly formed high angle inward dipping reverse ring faults the least uplifted portion of the caldera is affected by normal faults with similar orientation either newly formed or resulting from reactivation of the pre existing graben faults this asymmetric block resurgence is also observed in experiments performed with a previous caldera collapse in this case the caldera collapse related reverse ring fault is completely erased along the shortened side and enhances the effect of the extensional faults on the opposite side so facilitating the intrusion of the silicone the most uplifted sector due to an asymmetrically shaped intrusion is always in correspondence of the thickest overburden these results suggest that the stress field induced by resurgence is likely dictated by the geometry of the intruding magma body and the related deformation is partially controlled by pre existing tectonic and or volcano tectonic structures

autochthonous development of superior province greenstone belts abstract models for superior province greenstone belt development have changed over the last 30 years from ensimatic generally autochthonous models to suspect allochthonous terrane models based on the plate tectonic paradigm models intermediate between these end members such as autochthonous terrane models have not been prominent within the superior province there is a secular variation from early ca 3 ga platformal assemblages with overlying rift related komatiite tholeiite units representing breakup of ca 3 ga continental fragments to late ca 2 7 ga pull apart basins autochthonous terranes should demonstrate some sort of logical order such as that described above whereas allochthonous terranes will be largely oceanic in character and should be essentially random in their occurrence rather than exhibiting secular order in the face of this situation this paper reviews evidence from throughout the superior province that suggests many but not all greenstone assemblages developed autochthonously or parautochthonously the earliest greenstone assemblages in the superior province are generally quartzite and carbonate bearing platform sequences in the north caribou terrane and the marmion terrane deposited unconformably upon granitoid or greenstone basement the evidence for autochthonous parautochthonous development includes the presence of unconformities between keewatin type greenstone units such as 1 basal unconformities between ca 3 ga granitoid or volcanic basement and slightly younger platformal assemblages 2 between ca 2 9 ga platformal assemblages and younger platformal and andean arc assemblages stratigraphic patterns such as adjacent greenstone belt assemblages representing sharply differing geodynamic settings with some indication of palinspastic order or progression in the northern central and southern superior province are considered suggestive of orderly autochthonous progression from platforms through rifting of continental fragments and late assembly during the kenoran orogeny petrogenetic data indicating involvement of older granitoid crust in genesis of greenstone assemblages of a variety of ages throughout the superior province suggests the presence of older basement implying autochthonous or parautochthonous development of many assemblages younger greenstone assemblages in the abitibi wawa subprovince and the north caribou terrane contain xenocrystic zircons similar in age to stratigraphically underlying greenstone assemblages this pattern has been interpreted to represent autochthonous greenstone development the current above enumerated evidence for autochthonous development of greenstone assemblages is generally indirect few actual unconformities between keewatin type greenstone assemblages have been recognized in the superior province to aid in future recognition of unconformities in keewatin type greenstone assemblages examples of unconformities of varying degrees of subtlety in archean sequences are reviewed archean unconformities ranging from sharp angular discordances in the northern superior province to subtle stratigraphic patterns to weathering related geochemical mobilization along inter formational contacts are described these styles of unconformity in archean sequences suggest previously unrecognized major disconformities and unconformities may well exist between keewatin type assemblages in the superior province generally and the abitibi wawa subprovince specifically the most important disconformity suggested in the abitibi wawa subprovince is between the ca 2726 ma deloro and the ca 2708 ma tisdale assemblages the presence of unconformities and disconformities coupled with the indirect lines of evidence cited above suggest an autochthonous origin for many superior province greenstone belts this autochthonous origin involves either autochthonous or parautochthonous evolution of assemblages at least in part in place that is development of ca 3 ga platform sequences in selected locales followed by in place evolution of a succession of plume oceanic and arc assemblages culminating in late pull apart basin sequences implications of the autochthonous model for greenstone belts of the superior province particularly the abitibi wawa subprovince include the future ability to recognize unconformity bounded sequences potentially improving regional scale correlation

propagation linkage and interaction of caldera ring faults comparison between analogue experiments and caldera collapse at miyakejima japan in 2000 the formation of ring faults yields important implications for understanding the structural and dynamic evolution of collapse calderas and potentially associated ash flow eruptions caldera collapse occurred in 2000 at miyakejima island japan in response to a lateral intrusion based on geophysical data it is inferred that a set of caldera ring faults was propagating upward to understand the kinematics of ring fault propagation linkage and interac tion we describe new laboratory sand box experiments that were analyzed through digital image correlation dic and post processed using 2d strain analysis the results help us gain a better understanding of the processes occurring during caldera subsidence at miyakejima we show that magma chamber evacuation induces strain localization at the lateral chamber margin in the form of a set of reverse faults that sequentially develops and propagates upwards then a set of normal faults initiates from tension fractures at the surface propagating downwards to link with the reverse faults at depth with increasing amounts of subsidence interaction between the reverse and normal fault segments results in a deactivation of the reverse faults while displacement becomes focused on the outer normal faults modeling results show that the area of faulting and collapse migrates successively outward as peak displace ment transfers from the inner ring faults to later developed outer ring faults the final structural architecture of the faults bounding the subsiding piston like block is hence a consequence of the amount of subsidence in agreement with other caldera structures observed in nature the experimental simulations provide an analogy to the obser vations and seismic records of caldera collapse at miyake jima volcano but are also applicable to caldera collapse in general

the role of extensional structures on experimental calderas and resurgence the structure and shape of collapses and resurgences is often controlled by pre existing discontinuities such as normal faults in rift zones in order to study the role of extensional structures on collapse and resurgence we used analogue models dry sand simulated the brittle crust silicone located at the base of the sand pack simulated magma in the experiments regional extension pre dated collapse or resurgence forming normal faults in a graben like structure the graben was filled with additional sand simulating post rift deposits a piston then moved the silicone downward or upward inducing collapse or resurgence within the previously deformed sand the collapses showed an ellipticity length of minor axis length of major axis between 0 8 and 0 9 with the major axis parallel to the extension direction the partial reactivation of the pre existing normal faults was observed during the development of the caldera reverse faults which conversely to what was expected from experiments without pre existing extension became partly inward dipping resurgence showed an elongation of the uplifted part with the main axis perpendicular to the extension direction at depth pre existing normal faults were partly reactivated by the reverse faults formed during resurgence these locally became outward dipping normal faults a total reactivation of pre existing faults was also observed during resurgence the experiments suggest that the observed elongation of calderas and resurgences is the result of the reactivation of pre existing structures during differential uplift such a reactivation is mainly related to the loss in the coefficient of friction of the sand the results suggest that elliptic calderas and resurgences in nature may develop even from circular magma chambers

strain at radially fractured centers on venus 1 radially fractured centers rfcs on venus are distinctive 100 km long radiating systems of graben fissures and fractures arranged around a central topographic high we use cycle 1 and cycle 3 magellan images to determine the depths and wall dip angles of several large radial graben using two different methods at four rfcs becuma mons didilia corona h uraru corona and pavlova corona we record graben depths of 0 3 0 9 km and mean wall dip angles of about 36 which we interpret as a primary talus slope rather than a fault plane using the calculated depths and a more realistic fault dip of 60 we find extension of between 0 5 and 1 km on graben between about 5 and 10 km wide we use this extension to estimate the hoop strain at each rfc maximum measured strains on the flanks are typically between 1 and 4 and may be up to 7 toward the center we conclude that this strain is too large to be explained by current plume related uplift models and also by magma chamber inflation we conclude that dikes are probably responsible for the majority of the strain at the large radial graben

geochemistry of late archaean metagreywackes from the western dharwar craton south india implications for provenance and nature of the late archaean crust abstract late archaean metagreywackes of the ranibennur formation dharwar supergroup in the dharwar shimoga schist belt of the western dharwar craton wdc are texturally and mineralogically immature of the quartz intermediate type the sio 2 content in them ranges from 60 58 to 65 26 wt chemical index of weathering ciw values varies between 50 and 65 4 indicating a low degree of chemical alteration of the provenance rocks a high degree of correlation between k 2 o and al 2 o 3 r 0 73 and low rb sr ratios also suggest a low degree of alteration of provenance rocks abundances of transition group elements cr 118 221 n 89 154 v 89 192 and sc 11 16 ppm as well zr 132 191 ppm suggest a mixed mafic felsic provenance for the metagreywackes low hree and y content and low tb yb ratios 0 23 0 41 suggest the presence of tonalite as an important component in the provenance areas values of eu eu 0 78 and th sc 0 55 suggest that the granodioritic upper crust had evolved prior to serving as the provenance mixing calculations suggest 50 55 vol tonalite 20 25 vol granite 18 20 vol basalt and 5 vol komatiite composition for the provenance geochemical characteristics of the ranibennur metagreywackes suggest that sedimentary basin formed in the vicinity of a magmatic arc in a continental island arc setting and the detritus were shed from the arc rock

mineralogy and geochemical dispersion in lateritic soil profiles of northern nigeria abstract six lateritic soil profiles overlying metasedimentary and crystalline basement rocks of northern nigeria have been investigated in order to study the mineralogical and geochemical relationships between bedrock and weathering products a total of 96 samples have been collected including 26 from exposed quarry and road cut faces and 70 from auger holes to a depth of 8 m two selective leaching methods cold extraction with 0 1 m hci and hot extraction with aqua regia have been applied to the m fraction and up to sixteen elements determined by aas conclusions are based largely on the auger hole samples where element distributions have not been modified by subaerial exposure quartz is the dominant mineral in the upper part of the soil profile while kaolinite increases strongly with depth at the base of some profiles mica is also abundant elements are distributed through the soil profile according to their association with clays iron oxides and to a lesser extent carbonates while intercalations of lateritic crusts obscure general trends with depth dispersion and accumulation processes appear to be similar within each profile in spite of the intensive weathering and secondary accumulation processes characteristic of the savanna belt the lateritic soils within surface sampling depth appear to retain a geochemical imprint of the underlying bedrock criteria and potential problems in the application of soil geochemistry to bedrock mapping are discussed

internal geology and evolution of the redondo dome valles caldera new mexico deep geothermal drilling in the resurgent redondo dome of the valles caldera has allowed us to define a consistent intracaldera stratigraphic sequence that differs in a number of respects from the temporally equivalent sequence outside the caldera above the deeply eroded pliocene paliza canyon formation felsic ash flows and sediments in the dome area form a complex sequence of undetermined age that we call the lower tuffs an erosional interval separates these rocks from the overlying otowi member 1 4 m y of the bandelier tuff another period of erosion during which a tuffaceous sandstone was deposited separates the otowi from the overlying tshirege member 1 1 m y of the bandelier both the otowi and tshirege members with maximum thicknesses of 833 and 1155 m respectively are substantially thicker within the caldera than outside indicating simultaneous deposition and cauldron subsidence both are predominantly densely welded with distinctive interior zones of granophyric crystallization resurgent doming was initiated after emplacement of the tshirege member as evidenced by erosion of the upper portions of the cooling unit prior to deposition of overlying units an isopach map of the sandstone deposited during this erosional period shows that streams draining the uplifting dome were localized along the present redondo creek trend subsequent volcanic activity resulted in the formation of at least three additional ash flow tuff cooling units prior to deposition of caldera fill and eruption of the redondo creek member of the valles rhyolite a numerical model applied to formation of the redondo dome suggests that the top of the causative magma body is located at a depth of about 4700 m 1458 m beneath the bottom of the deepest geothermal well in the dome no wells have penetrated intrusives that could be related to this magma we suggest that the locations of faults bounding the apical graben of the dome were influenced by older faults associated with the northeast trending jemez lineament these faults were active early in the uplift history of the dome and account for many of the structural differences between hypothetical dome development and reality

an archean quartz arenite andesite association in the eastern baltic shield russia implications for assemblage types and shield history abstract shallow water sedimentary units are generally considered scarce in archean greenstone belts we describe an unusual quartz arenite subaerial andesite association within the archean hisovaara greenstone belt a fragment of the parandovo tikshozero belt within the karelian craton of the baltic shield the hisovaara greenstone belt consists of several lithotectonic assemblages 1 a komatiite tholeiite assemblage 2803 ma based on ages of cross cutting dikes 2 an andesite quartz arenite assemblage cut by similar dikes 3 an assemblage of coarse volcaniclastic rocks and 4 an upper mafic assemblage of tholeiitic basalts with minor pyroxene komatiite volcanic rocks the andesite quartz arenite assemblage 100 to ca 750 m thick has basal amygdaloidal fragmental andesites overlain by massive andesites then amygdaloidal and plagioclase phyric andesites unconformably overlying the andesite is a unit of quartz rich sandstones 6 40 m thick dominated by quartz arenite extending several km along strike at the north end the quartz arenite succession consists of basal andesite overlain by quartz arenite exhibiting hummocky cross stratification followed by aluminous coarse metasediments and sulfidic argillite and an unconformably overlying tholeiite komatiite unit at the south end the succession is basal andesite regolith cross bedded quartz arenite weathered andesites a second quartz arenite argillite and then subaerial rhyolite ree and hfse geochemistry has been obtained on the rocks of the andesite quartz arenite assemblage the quartz arenites contain low abundances and chondrite normalized patterns vary from relatively fractionated to flat with most of the variation related to grain size with pebbly units having higher abundance and more fractionated patterns combined major and trace element geochemistry indicates that a sodic felsic source with some admixture of mafic material will explain the geochemistry of the quartz arenites the andesites display moderately fractionated spidergrams with negative anomalies at ti ta and nb typical of arc volcanism the andesite quartz arenite assemblage represents accumulation of shallow water quartz rich sediments in a setting typical of the later stages of arc volcanism in which the volcanic edifice is subaerial at the southern end of the assemblage however at the north end our evidence is interpreted as indicating subaerial andesitic volcanism subsidence to a shallow marine basin which then deepens and rifts therefore the hisovaara andesite quartz arenite assemblage provides a linkage in archean greenstones between assemblages representing continental volcanism and a platform to rift setting the presence of an erosional interval in 2 8 ga greenstones suggests possible pre 2 7 ga orogeny in the baltic shield the pre 2 7 ga quartzrich sedimentation is similar in age to platformal assemblages in the pre 2 7 ga north caribou terrane of the superior province canada

intracaldera volcanic activity toledo caldera and embayment jemez mountains new mexico the toledo caldera was formed at 1 47 0 06 ma during the catastrophic eruption of the lower member bandelier tuff the caldera was obscured at 1 12 0 03 ma during eruption of the equally voluminous upper member of the bandelier tuff that led to formation of the valles caldera earlier workers interpreted a 9 km diameter embayment located ne of the valles caldera toledo embayment to be a remnant of the toledo caldera drill hole data and new k ar dates of toledo intracaldera domes redefine the position of toledo caldera nearly coincident with and of the same dimensions as the younger valles caldera the toledo embayment may be of tectonic origin or a small tschicoma volcanic center caldera this interpretation is consistent with distribution of the lower member of the bandelier tuff and with several other field and drilling related observations explosive activity associated with cerro toledo rhyolite domes is recorded in tuff deposits located between the lower and upper members of the bandelier tuff on the northeast flank of the jemez mountains recorded in the tuff deposits are seven cycles of explosive activity most cycles consist of phreatomagmatic tuffs that grade upward into plinian pumice beds a separate deposit of the same age and consisting of pyroclastic surges and flows is associated with rabbit mountain located on the southeast rim of the valles toledo caldera complex these are the surface expression of what may be a thicker more voluminous intracaldera tuff sequence the combined deposits of the lower and upper members of the bandelier tuff toledo and valles intracaldera sediments tuffs and dome lavas form what we interpret to be a wedge shaped caldera fill this sequence is confirmed by deep drill holes and gravity surveys this fill accumulated in depressions formed during precaldera rifting and episodes of caldera collapse we interpret the toledo valles caldera complex to be a pair of nearly coincident trapdoor calderas with the hinge on the west side and thick caldera fill in the east

the flow dynamics of an extremely large volume pyroclastic flow the 2 08 ma cerro galan ignimbrite nw argentina and comparison with other flow types the 2 08 ma cerro galan ignimbrite cgi represents a 630 km3 dense rock equivalent vei 8 eruption from the long lived cerro galan magma system 6 ma it is a crystal rich 35 60 pumice 10 generally and lithic poor 5 generally rhyodacitic ignimbrite lacking a preceding plinian fallout deposit the cgi is preserved up to 80 km from the structural margins of the caldera but almost certainly was deposited up to 100 km from the caldera in some places only one emplacement unit is preserved in proximal to medial settings and in most distal settings suggesting constant flow conditions but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges it interacted with valley walls resulting in flow instabilities that generated multiple depositional units often separated by pyroclastic surge deposits the cgi preserves a widespread sub horizontal fabric defined by aligned elongate pumice and lithic clasts and minerals e g biotite a sub horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed the cgi is poor in both vent derived accessory lithics and locally derived lithics from the ground surface accidental lithics locally derived lithics are small 20 cm and were not transported far from source points all data suggest that the pyroclastic flow system producing the cgi was characterised throughout by high sedimentation rates resulting from high particle concentration and suppressed turbulence at the depositional boundary layer despite being a low aspect ratio ignimbrite based on these features we question whether high velocity and momentum are necessary to account for extensive flow mobility it is proposed that the cgi was deposited by a pyroclastic flow system that developed a substantial high particle concentration granular under flow which flowed with suppressed turbulence high particle concentration and fine ash content hindered gas loss and maintained flow mobility in order to explain the contemporaneous maintenance of high particle concentration high sedimentation rate at the depositional boundary layer and a high level of mobility it is also proposed that the flow s was continuously supplied at a high mass feeding rate it is also proposed that internal gas pressure within the flow directed downwards onto the substrate over which the flow was passing reduced the friction between the flow and the substrate and also enhanced its mobility the pervasive sub horizontal fabric of aligned pumice lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow consistent with the existence of a laminar shearing granular flow regime during the final stages of transport and deposition

direct evidence of late archean to early proterozoic anoxic atmosphere from a product of 2 5 ga old weathering because precambrian paleosols ancient soils formed by weathering are usually subjected to later alteration the evidence gleaned from chemical studies has provided inconclusive evidence on the atmospheric o2 evolution in a 2 6 2 45 ga paleosol developed on archean granite near pronto mine canada we found that ce rich rhabdophane formed directly during weathering replaced primary apatite and has survived for about 2 5 ga because of its low solubility and high resistance to heat our data show that la ce and nd behaved similarly in both rocks and rhabdophane i e most ce existed as ce3 in the weathering solution unlike the younger weathering profiles where ce3 oxidizes and forms cerianite ceo2 the presence of rhabdophane with ce3 throughout the pronto paleosol provides compelling evidence of an anoxic atmosphere 2 6 2 45 ga ago because apatite is a common accessory mineral in granitic rocks ce content of the replaced rhabdophane can be a useful indicator for tracing o2 evolution in the precambrian

profiles of elemental concentrations in precambrian paleosols on basaltic and granitic parent materials abstract profiles of elemental concentrations are interpreted for four precambrian paleosols two developed on basalt and two on granodiorite all four paleosols appear to be the erosional remnants of originally thick soil saprolite regoliths the granitic paleosols are in south africa where they underlie the 2 9 3 0 ga pongola supergroup and the 2 8 2 9 ga dominion reef conglomerate one of the basaltic paleosols also occurs in south africa where it caps the ventersdorp basalt and underlies the 2 3 ga black reef quartzite the other basaltic paleosol underlies sandstone of the 2 7 ga timiskaming group in the abitibi belt of the canadian shield all four paleosols exhibit pronounced upward loss of sodium but no consistent loss of the heavier alkali elements rubidium and cesium iron decreases upward above iron rich basaltic parent rocks but there is no consistent loss of iron above iron poor granitic parents the rare earth elements are less fractionated than during intensive modern weathering uranium locally has been fractionated from thorium possibly due to oxidative dissolution during precambrian weathering

the behavior of light lithophile and halogen elements in felsic magma geochemistry of the post caldera valles rhyolites jemez mountains volcanic field new mexico abstract post collapse rhyolite lava domes lava flows and pyroclastic rocks from valles caldera 1140 ka erupted from 1133 ka to approximately 520 60 ka have been sampled to study variations of light lithophile li be b and halogen f cl elements our principal objectives were 1 to examine the mobility of these elements during post eruptive devitrification and hydration and 2 to study their behavior during magma differentiation compared to fresh glassy samples devitrified rocks from the same dome are depleted in b li f and cl but not in be during devitrification be was immobile while the other elements were progressively more mobile in the order b

statistical separation of tectonic and inflation driven components of deformation on silicic reservoirs laguna del maule volcanic field chile abstract the reconstruction of the structural history of inflating silicic systems is challenging because their faulting record encompasses tectonic and inflation driven deformation and separating the two can rarely be accomplished here we present and utilize a statistical methodology to differentiate tectonic from inflation driven deformation in the laguna del maule ldm a post glacial rhyolitic volcanic field southern volcanic zone chile latitude 36 s ldm is cut by the troncoso fault a major normal fault that strikes ne and dips to the nw the troncoso hanging wall contains abundant young 19 9 4 ka where normal faults are developed on the troncoso hanging wall during shoreline based inflation

san diego canyon ignimbrites pre bandelier tuff explosive rhyolitic volcanism in the jemez mountains new mexico ignimbrites up to 80 m thick underlie the lower bandelier tuff in the southwestern part of the jemez mountains volcanic field new mexico they were formed during silicic explosive eruptions that preceded the climactic events of the bandelier tuffs and collapse of the valles i toledo and valles ii calderas source vents for these units herein called the san diego canyon ignimbrites probably underlie the southwestern portion of the valles caldera small scale caldera collapse may have been associated with the eruption of these ignimbrites but evidence of an early caldera or any related vent structure has since been obliterated this late pliocene about 2 5 ma high silica rhyolite activity was contemporaneous with eruptions from the predominantly dacitic tschicoma volcanic center in the northeastern part of the jemez mountains and suggests a significantly different history for the plio pleistocene development of the jemez mountains volcanic field than has previously been considered

fluid melt trace element partitioning behaviour between evolved melts and aqueous fluids experimental constraints on the magmatic hydrothermal transport of metals abstract the fluid melt partition coefficients of ten trace metals li mn cu zn rb sr mo w and pb along with k between hydrous evolved melts and coexisting cl bearing fluid s have been experimentally determined at 850 1050 c 0 6 2 2 kbar and o2 nno 0 5 to nno 2 log units the measured compositions of the quenched fluid phases vary from low salinity 2 wt cl to 19 wt cl with maximum starting fluid concentrations of 21 wt cl the results of 15 partitioning experiments confirm that dmetalfluid melt values depend strongly on the melt and fluid composition with the greatest metal affinities for the fluid phase shown in the experiments with high cl fluids and the most fractionated melts five experiments conducted in au96cu4 capsule alloy show dcufluid melt values ranging from 40 to 80 and a strong correlation with melt feo wt contents at equivalent fluid cl contents in contrast li shows only weak partitioning in favour of coexisting fluids across all melt compositions with dlifluid melt ranging from 0 16 in low salinity fluids to 1 17 in the highest cl fluid 19 15 wt cl a procedure was employed to redissolve and analyse the aluminosilicate precipitates that form during quenching of the experiments this procedure allows for metal contents of the final quench fluids to be constrained by both mass balance and direct measurement by solution icp ms analysis a comparison of the mass balance calculated vs directly measured fluid melt partition coefficients generally gives good agreement within an order of magnitude for most trace elements however discrepancies between these methods arise due to errors inherent in the mass balance calculations for metals with low abundances in the fluids e g sr and especially those showing affinities to alloy with the experiment capsule material e g mo these discrepancies in constraining experimental fluid trace element budgets also suggest that the precious metal capsule alloys cannot always be considered completely inert containers in experimental investigations and semi quantitative la icp ms analysis of the raw capsule alloys confirms the detectable presence of several of the trace elements investigated here integrating these new partitioning data with previous experimental and natural studies confirms that aqueous cl bearing fluids exsolved by hydrous calc alkaline melts stored at mid to upper crustal conditions are capable of efficiently sequestering and transporting some economically important trace metals e g mn cu zn however the new fluid melt li partitioning data in conjunction with evidence of very late post eruptive li modification suggests that li is moderately strongly fluid immobile at depth and suffers from significant post eruptive re equilibration this has implications for deciphering the signatures of volatile fluxing in arc magmas that are preserved in mineral and melt inclusion trace element concentrations and the development of mineralised stockwork regions in porphyry style metal deposits

a shallow attenuating anomaly inside the ring fracture of the valles caldera new mexico abstract spectral ratios of teleseismic direct and scattered p waves observed in the valles caldera new mexico show a systematic pattern of low amplitudes at sites inside the caldera relative to sites on or outside the ring fracture waveforms recorded at caldera stations are considerably more complex than those recorded outside the caldera the data used in this study were collected during a passive seismic monitoring experiment conducted in 1987 twenty four teleseismic events were recorded on two linear arrays spanning the caldera to first order the pattern of low amplitudes did not vary with source incidence angle or azimuth of approach and could not be explained by anomalous amplification at the ring fracture this observation suggests the presence of a shallow attenuating zone associated with the caldera fill material inside of the ring fracture we estimated the general features of the caldera s near surface structure for the two dimensional vertical cross section beneath the array using a modification of the aki larner discrete wavenumber method to forward model the observed amplitude variations our results indicate that the caldera fill material must be subdivided into at least two distinct zones a strongly attenuating lower zone extending to depths in excess of 4 km and a mildly attenuating surface layer to fit the data we had to assign an unrealistically low value to seismic q in the deeper attenuating anomaly we attribute this to the inability of the aki larner method to account for strong re direction of energy away from the caldera due to local heterogeneity that we could not include within the low q anomaly this interpretation is consistent with the pervasive fractured hydrothermal system that is known to exist in the caldera fill material

geology of precambrian paleosols at the base of the huronian supergroup elliot lake ontario canada abstract detailed examination of two new profiles through the denison series paleosol and one new profile through a pronto series paleosol in the elliot lake area lend fresh insight into atmospheric and hydrogeologic environmental conditions of precambrian earth these are reassessed with reference to the results of previous studies as true indigenous paleosols of polygenetic character virtually all of the criteria grandstaff et al 1986 for identification of paleosols are met the paleosols bear strong mineralogical and chemical similarities despite derivation from different parent materials and sampling at locations distant from each other there has evidently been a common reaction to weathering processes peculiar to the early proterozoic diagenetic and metamorphic events mask to a considerable extent the role played by environmental and hydrologic conditions in the development of the paleosols in the denison greenstone derived paleosol clay formed from amphibole feldspar and chlorite is altered to sericite increased quartz content is due to residual concentration and to secondary development in granodiorite derived paleosol quartz and sericite are generally more abundant quartz is relatively concentrated toward the top of the profile the paleosols are thus primarily sericite quartz and chlorite assemblages rutile and leucoxene are accessory minerals profiles of elemental chemistry show the most drastic changes in concentration at the base in transition to bedrock magnesia and lime are leached from all profiles as in modern soils although in common with most precambrian paleosols potash accumulated total fe is strongly depleted from the profiles except just above the bedrock contact with granodiorite a condition attributed to mobilization of fe from the paleosol to a position directly over relatively impermeable parent rock use of holland s method of establishing oxygenation from paleosols indicates that in terms of fe content parent material of granodiorite derived paleosol should produce a reduced fe paleosol the oxygen demand of this rock was greater than granite owing to an abundant ferromagnesian content two fossil eluvial horizons marked by high chlorite and a dramatic decrease in sericite content are recognized in the denison paleosol their existence shows that although initial stages of weathering may have occurred under water saturated conditions conditions were probably more arid than supposed by previous workers particularly during the later stages of development of the paleosol although fe enriched the eluvial horizons are drastically depleted in potash al and th removal of al 2 o 3 negates normalization of chemical analyses of paleosols to al 2 o 3 for purposes of constant volume calculations th associated with sericite in the profiles increases at the contact with the overlying matinenda formation in association with rutile and leucoxene owing to interaction with diagenetic u bearing fluids from overlying sediments rutile and leucoxene originate from parent rocks and are a late stage form of mineralization these titaniferous phases indirectly contributed to the local dominance of brannerite type orc through diagenetic processes of mineralization involving the pronto reaction paleotopographic greenstone highs are related to local anomalously high concentrations of heavy minerals in the brannerite rich orc this involves hydraulic deposition of heavy minerals in the lee of crescent shaped greenstone highs with subsequent diagenetic upgrading granodiorite derived paleosol contains 5 10 times more au than the denison paleosol and is modestly enriched above background a slight decrease in au content upward evident in all profiles is an artefact of erosion of the top of the paleosols

caldera resurgence during magma replenishment and rejuvenation at valles and lake city calderas a key question in volcanology is the driving mechanisms of resurgence at active recently active and ancient calderas valles caldera in new mexico and lake city caldera in colorado are well studied resurgent structures which provide three crucial clues for understanding the resurgence process 1 within the limits of 40ar 39ar dating techniques resurgence and hydrothermal alteration at both calderas occurred very quickly after the caldera forming eruptions tens of thousands of years or less 2 immediately before and during resurgence dacite magma was intruded and or erupted into each system this magma is chemically distinct from rhyolite magma which was resident in each system 3 at least 1 km of structural uplift occurred along regional and subsidence faults which were closely associated with shallow intrusions or lava domes of dacite magma these observations demonstrate that resurgence at these two volcanoes is temporally linked to caldera subsidence with the upward migration of dacite magma as the driver of resurgence recharge of dacite magma occurs as a response to loss of lithostatic load during the caldera forming eruption flow of dacite into the shallow magmatic system is facilitated by regional fault systems which provide pathways for magma ascent once the dacite enters the system it is able to heat remobilize and mingle with residual crystal rich rhyolite remaining in the shallow magma chamber dacite and remobilized rhyolite rise buoyantly to form laccoliths by lifting the chamber roof and producing surface resurgent uplift the resurgent deformation caused by magma ascent fractures the chamber roof increasing its structural permeability and allowing both rhyolite and dacite magmas to intrude and or erupt together this sequence of events also promotes the development of magmatic hydrothermal systems and ore deposits injection of dacite magma into the shallow rhyolite magma chamber provides a source of heat and magmatic volatiles while resurgent deformation and fracturing increase the permeability of the system these changes allow magmatic volatiles to rise and meteoric fluids to percolate downward favouring the development of hydrothermal convection cells which are driven by hot magma the end result is a vigorous hydrothermal system which is driven by magma recharge

eruption transport and deposition of ignimbrite a case study from mexico abstract the acatlan ignimbrite covers 150 km 2 around a vent situated 40 km southwest of guadalajara mexico the upper and main ignimbrite flow unit is compositionally zoned and has white or pink rhyolitic pumice in its lower part and black andesite in its upper these parts are believed to have been erupted continuously from the same magma chamber this zonation has been important in correlating an extremely coarse lithic rich deposit a co ignimbrite lag fall deposit with this ignimbrite the co ignimbrite lag fall deposit indicates that the flow unit formed by continuous collapse of an explosive eruption column and enables the source vent to be located the vertical compositional grading of the flow unit also provides a means of testing the various proposed mechanisms of ignimbrite emplacement turbulent vs high concentration flow the preservation of compositional zonation and other features in the deposit indicate that flow was not turbulent details of the grain size distribution indicate that laminar flow was important early in the flow history when collapsing material from the eruption column was superimposed in stratigraphical sequence initial high gas flow rates associated with flow segregation and formation at the site of column collapse rapidly decayed away from vent and later in its history the flow degassed to a semi rigid plug overriding a sheared basal layer

tectonic influences in piecemeal caldera collapse at glencoe volcano scotland glencoe volcano is renowned as the archetype for caldera collapse that results from piston like subsidence of a coherent crustal block on a ring fault however the caldera floor rocks and the geometry of seven intracaldera units that record large volume explosive eruptions show that the collapse involved incremental and haphazard subsidence of numerous crustal blocks before development of the ring fault some subsidence involved flexure downsag with associated development of extensional crevasses hundreds of metres deep a system of orthogonal faults and related grabens records the influence of two intersecting basement discontinuities one of which is inferred to have linked with the active great glen fault substantial extensional and or transtensional faulting with distinct related sedimentary responses occurred without eruptions the tectonic framework substantially controlled the magmatic plumbing and the locations of vents caldera depocentres and through going rivers the related sediments influenced contrasting eruption styles e g transitions between phreatomagmatic and magmatic and the shallow level emplacement of sills

topographic control on precambrian weathering in the elliot lake uranium district canada the voluminous uraniferous conglomerate beds at elliot lake canada lie directly above or within several metres of a palaeosol that developed on archaean granite and greenstone metabasalt this lower proterozoic paleosol provides evidence for the environment in which the uraniferous conglomerate accumulated previous investigators have interpreted a few analyses of the palaeosol to indicate palaeoweathering under an atmosphere devoid of free oxygen however we have independently analysed in detail three oxidized palaeosols that formed contemporaneously with reduced palaeosols the degree of oxidation correlates well with palaeoelevation

fluid inclusion evidence for rapid formation of the vapor dominated zone at sulphur springs valles caldera new mexico usa abstract microthermometric measurements were obtained for 618 fluid inclusions in hydrothermal quartz fluorite and calcite and magmatic quartz phenocrysts in intracaldera tuffs from the vc 2a core hole in order to study evolutionary processes of the sulphur springs hydrothermal system in the valles caldera relatively high t h values in samples from shallow depths indicate erosion of about 200 m of caldera fill since deposition of hydrothermal minerals at shallow depths in the sulphur springs hydrothermal system accompanied by a descent in the water table of the liquid dominated reservoir for samples collected below the current water level of the well the minimum values of homogenization temperature t h fit the present thermal profile whereas minimum t h values of samples from above the water level are several tens of degrees higher than the present thermal profile and fit a paleo thermal profile following the boiling point curve for pure water as adjusted to 92 c at 20 m below the present land surface this is attributed to development of an evolving vapor zone that formed subsequent to a sudden drop in the water table of the liquid dominated reservoir we suggest that these events were caused by the drainage of an intracaldera lake when the southwestern wall of the caldera was breached about 0 5 ma this model indicates that vapor zones above major liquid dominated geothermal reservoirs can be formed due to dramatic changes in geohydrology and not just from simple boiling

plio pleistocene pumice floods in the ancestral rio grande southern rio grande rift usa abstract at least four times during the late pliocene and early pleistocene pyroclastic eruptions in the jemez volcanic field northern rio grande rift flooded the ancestral rio grande with gravel sized pumice following as much as 400 km of fluvial transport the pumice was deposited in beds 0 2 to 2 0 m thick in the camp rice formation of the southern rio grande rift a combination of reversal magnetostratigraphy and single crystal sanidine 40 ar 39 ar dating constrains the ages of pumice clast conglomerates at 3 1 2 0 1 6 and 1 3 ma the coarsest pumice beds cobbles boulders were deposited as antidune like bedforms in a fluvial channel and as a crevasse splay sheet granule and pebble sized pumice was deposited as dune bedforms in fluvial channels and as ripple bedforms on the floodplain the abundance of pumice clasts in the gravel fraction 60 100 suggests very rapid transport downriver probably in a few days or weeks the two older pumice clast conglomerates correlate with the puye formation in the jemez volcanic field whereas the younger two are coeval to the lower bandelier tuff and cerro toledo rhyolite

lava dome growth and explosive volcanism in the jemez mountains new mexico evidence from the plio pleistocene puye alluvial fan abstract the plio pleistocene puye formation north central new mexico is a 200 km2 volcanogenic alluvial fan shed eastward from the tschicoma volcanic center part of the jemez mountains volcanic field the fan contains 15km3 of volcaniclastic material derived from closely spaced lava domes in the northeastern portion of the tschicoma center interbedded in the fan sediments are at least 25 primary pyroclastic units from explosive eruptions of dacitic and rhyolitic lava domes tephra occur mainly as pumice falls but include several pumiceous ignimbrites and two thick proximal block and ash pyroclastic flows the upper part of the fan also contains rhyolitic plinian deposits erupted from sources in the central portion of the jemez field and basaltic ash derived from the central rio grande rift fanglomeratic pyroclastic and epiclastic facies exhibit considerable lateral variation primary tephra deposits however provide a stratigraphic framework for reconstruction of the growth of individual lava domes while only volcanic domes and lava flows are exposed in the tschicoma center puye tephra layers show that vulcanian subplinian and plinian activity and block and ash pyroclastic flows accompanied many dome forming events degradation of these lava domes produced coarse grained debris flows dominated by lava and dome carapace clasts from these epiclasts it is possible to identify lithologies of lavas that are either currently buried or have been obliterated by erosion or by collapse of the valles calderas the preservation of puye deposits reflects very high rates of aggradation and gradual down faulting in the espanola basin of the central rio grande rift this record of pyroclastic and epiclastic deposition allows detailed interpretation of the evolution of a volcanic center from its pyroclastic and erosion products and is the first example of a volcanogenic alluvial fan in an intracontinental rift setting to be described in detail this study emphasizes that interpretation of any ancient volcanic terrain based solely upon data derived from mapping radiometric dating and chemical analysis of lavas or existing denuded edifices is likely to greatly underestimate both the role of explosive volcanism in the build up of volcanic centers and subsequent estimates of the volumes of material produced

archaean metasedimentary rocks from the northwestern pontiac subprovince of the canadian shield chemical characterization weathering and modelling of the source areas abstract the northwestern pontiac subprovince is dominated by a monotonous greywacke sequence metamorphosed and deformed during the late archaean kenoran orogeny limited chemical weathering of the source rocks is suggested by 1 low values of the chemical index of alteration 2 no preferential leaching of ca sr and ba 3 well defined correlations among mgo k2o and al2o3 and 4 small variations of cr ni the source was dominantly felsic as indicated by high th sc average 0 39 la co 1 06 zr cr 0 7 zr y 11 2 and sio2 mgo 19 4 the composition of these metasedimentary rocks can be modelled by mixing of three end member components 1 tonalites trondhjemites 65 in order to explain the sio2 al2o3 th u zr sr and ba abundances na k ratios strong depletion in hree and hfse relative to lree and lile low th y and k2o and fractionated la yb 19 4 2 komatiites 5 in order to explain the high cr 220 ppm and ni 66 ppm contents and 3 tholeiitic basalts 30 accounting for the observed tio2 0 62 wt v 144 ppm and sc 17 ppm concentrations the source region may be considered as a deeply eroded greenstone belt intruded by abundant tonalite trondhjemite plutons possibly now located to the south and east in the grenville province or possibly to the north underlying the dominantly volcanic abitibi subprovince