enriched grenvillian lithospheric mantle as a consequence of long lived subduction beneath laurentia geochemical and nd isotopic data from mafic and newly discovered ultramafic rocks in the adirondack lowlands suggest widespread enrichment of the lithospheric mantle under the grenville province incompatible element abundances and previously published hf t dm zircon depleted mantle model age and nd t dm ages from rocks of the anorthosite mangerite charnockite granite suite in the adirondack highlands document similar enrichment in the lower crust and its strong influence on subsequent magmatic events throughout the ontario quebec adirondack segment of the grenville province likely the consequence of long lived ca 1 4 1 2 ga northwest directed subduction along the southeast edge of laurentia previously proposed andean margin this enrichment is similar to that associated with the vast 240 000 km 2 ultrapotassic province of the western churchill province enrichment of the lithospheric mantle beneath orogenic belts is a predictable and important differentiation process that has operated on earth for at least the past 3 b y

the petrology and geochemistry of the ocate volcanic field north central new mexico the ocate volcanic field in north central new mexico is on the eastern flank of the sangre de cristo mountains it is a suite of pliocene pleistocene basaltic and intermediate lavas which were erupted in five pulses over a period of 7 m y 8 3 0 8 m y b p during rejuvenation of the rio grande rift geochemical and petrographic criteria are used to define five major rock types alkali olivine basalt aob transitional olivine basalt tob xenocrystic basaltic andesite xba olivine andesites oa and dacite the timing and eruptive volumes of these five lava types define a complex temporal sequence in which the transitional basalts and intermediate rocks are closely associated during the maximum in basaltic activity 4 5 2 0 m y b p the alkali olivine basalts have higher incompatible and compatible trace element abundances and higher normative nepheline 2 than do the tob lavas the two basalt types apparently were derived from physically discrete or heterogeneous source regions the aob lavas could be the products of smaller degrees of melting from similar sources source heterogeneity and or melting at greater depth the intermediate lavas were generated from basaltic parent magmas by varying degrees of fractionation assimilation of crustal material and mixing with silicic melts derived by crustal anatexis the xba lavas and dacites are characterized by disequilibrium phase assemblages which include olivine coexisting with biotite and quartz melt inclusions of k and si rich glass and a bimodal distribution of plagioclase phenocryst compositions with modes at an60 and an25 the high concentration of mg ni and cr in the intermediate lavas is inconsistent with simple fractionation of basaltic parent magmas the linear trends defined on variation diagrams are consistent with mixing mafic and silicic end members the lack of continuity between two hybrid types may be a reflection of contrasting subvolcanic environments in which the hybridization took place the olivine andesites show little petrographic evidence for mixing chemical variations within this group define highly nonlinear trends more consistent with extensive crystal fractionation calculated liquid lines of descent from available mafic parent magmas assuming perfect fractional crystallization do not satisfactorily produce the observed intermediate compositions if assimilation of a silicic crystal component is paired with crystal fractionation such that the mass of crustal material is approximately equal to 50 of the fractionated solids the major and trace element characteristics of the olivine andesites can be successfully modeled

ion microprobe evidence for the mechanisms of stable isotope retrogression in high grade metamorphic rocks retrograde interdiffusion is widely proposed as the dominant factor in producing the stable isotopic fractionation among minerals in slowly cooled igneous and metamorphic rocks mineral zonation consistent with interdiffusion of stable isotopes has never been directly observed however leaving doubt as to the mechanism responsible for the bulk mineral isotopic compositions commonly measured ion microprobe analyses of oxygen isotope ratios in magnetite were combined with conventional bulk mineral analyses and diffusion modeling to document the relationship between mineral zonation and the mechanism of retrogression inferred from bulk mineral data two samples of magnetitebearing quartzo feldspathic lyon mountain gneiss from the adirondack mountains n y were studied in detail conventional stable isotope analysis of both samples indicates that isotope thermometers are discordant and were reset by as much as 200 c from the estimated peak temperature of 750 c the relative order of apparent temperatures recorded by various thermometers differs between the two samples however with t qtz fsp t mt qtz and t mt fsp in one sample and t qtz fsp t mt qtz and t mt fsp in the other diffusion modeling using the fast grain boundary model shows that the former pattern of apparent temperatures is consistent with closed system interdiffusion during cooling whereas the latter is not the modeling predicts that 0 5 mm diameter magnetite grains common to this rock type will contain isotopic zonation of 1 rims lower in 18 o than cores and that the cores of smaller 0 1 mm grains will be similarly lower than to the cores of large 0 5 mm grains ion microprobe analysis reveals that the zoning patterns of magnetite grains from the first sample contain clear core to rim zonation in multiple grains core rim 1 1 0 4 and predicted grain size vs core composition variations consistent with diffusion controlled resetting of bulk mineral fractionations in contrast the second sample shows irregular inter and intra granular variations over an 8 range consistent with open system alteration these results provide direct documentation of the importance of interdiffusion in affecting stable isotope distributions in slowly cooled rocks the correlations of bulk mineral resetting with zonation show that bulk mineral data when interpreted with detailed modeling can be used to determinate what processes controlling retrogression

characterisation of archaeological mortars and plasters from kyme turkey abstract the present study focuses on the compositional characterisation of roman proto byzantine and medieval mortars sampled in the archaeological site of kyme turkey an aeolian city founded in the middle of the xi century bc by greek populations the mortars were studied through elemental sem eds xrf mineralogical xrpd de astis calcimeter and petrographic polarised optical microscopy analysis the comparison between the samples allowed us to emphasise compositional differences within mortars of the same historical period in particular we detected the prevalent use of cocciopesto which was used as a pozzolanic additive to create hydraulic mortars in the roman and byzantine period the cocciopesto was used to improve the technological properties of an initially slaked lime furthermore compositional similarities allowed us to attribute an unknown sample to the roman period our study also pointed out the fact that the lower quality mortars belonged to the medieval period as a matter of fact the most friable sample belonging to the middle ages turned out to be a mixture of clay lime and volcanic sand

geochemistry and geochronology of the sierra de gomez limestone hosted u deposit chihuahua implications for distribution of rio grande rift mineral deposits in northern mexico abstract uranium deposits form in a variety of settings they are partially controlled by the secular evolution of earth processes including deposits in extension related settings such as the intra cratonic rio grande rift plio quaternary volcanism mineral deposits and hydrothermal spots occur along the chihuahua central graben the age of the sierra de gomez u deposit is 1 8 ma based on la mc icp ms dating on a uranophane monocrystal which is contemporaneous with the late mineralization event of the pena blanca u deposit as well as rio grande rift rgr type deposits in chihuahua and intraplate volcanism studies of fluid inclusions in fluorite and late calcite indicate the presence of hydrocarbons and ch4 rich brine homogenization temperatures range from 87 to 112 c and the mean composition 2 0 mol nacl and 0 3 mol cacl with ch4 is comparable to mineralizing brines in mvt deposits and carbonated hydrocarbon reservoirs evolution of c and o stable isotopic values for the calcite cement in the sierra de gomez limestone hosted u deposit illustrates that two separate calcite precipitation events occurred 1 travertine filling karst structures in the presence of meteoric water and 2 u mineralization during deep hydrothermal fluid circulation that included interactions with a heat source and basement leaching in a regional context a metallogenic model suggests that the chihuahua trough area is deep enough to generate fluid migration by hydrothermal and or compaction processes through rgr extensional faults until a favorable trapping horizon is reached this causes uranium precipitation because water rock interaction processes generate a local redox barrier

tephrological implications of beam size sample size effects in electron microprobe analysis of glass shards this paper concerns the potential consequences of varying procedures for the determination of tephra geochemistry by electron microprobe application of electron probe microanalysis to tephrostratigraphical methods has increasingly facilitated the resolution and refinement of quaternary chronology associated with records of proxy environmental or proxy climatic change the geographical range over which tephras are recovered has expanded significantly with the identification and analysis of crypto or hidden tephras in areas far removed from tephra sources these tephras are dominated by glass shards which in many distal environments may be either small in size m or may be highly pumiceous with low glass void ratios and thin 10 m shard walls we demonstrate that reducing the size of the electron beam used to analyse shard geochemistry cannot be used reliably to permit analysis of thin glass walls this approach distorts the geochemical data creating analytical differences that may generate inappropriate tephrogeochemical fingerprints additional distortion of the geochemical fingerprint in the form of hybrid analyses may be encountered in glass fragments containing micron sized crystalline phases such as feldspar copyright 2001 john wiley sons ltd

trondhjemitic 1 35 1 31 ga gneisses of the mount holly complex of vermont evidence for an elzevirian event in the grenville basement of the united states appalachians a newly recognized suite of trondhjemite tonalite and dacitic gneiss forms a 10 km wide belt of rocks within the mount holly complex in the central part of the green mountain massif of vermont field relationships and chemistry indicate that these gneisses are calc alkaline volcanic and hypabyssal plutonic rocks older than the middle proterozoic regional deformation that affected the mount holly complex u pb zircon dates indicate ages as great as 1 35 ga for crystallization of the volcanic protoliths and for intrusion of crosscutting trondhjemite tonalitic plutonism continued until 1 31 ga map scale contacts between the trondhjemitic tonalitic dacitic gneisses and the paragneiss sequence of the mount holly complex are sharp suggesting that the volcanic rocks of the trondhjemite tonalite suite underlie the paragneiss units and do not intrude them these relationships suggest that the trondhjemite tonalite suite is either considerably older than and unconformable beneath the paragneiss cover rocks or

the real southern basin and range mid to late cenozoic extension in mexico much of northern and central mexico underwent east northeast extension in the mid to late cenozoic this area constitutes a major but little recognized part of the basin and range province the extended region is bounded on the east by the laramide thrust front of the sierra madre oriental on the west the relatively unfaulted sierra madre occidental separates the extended area in central mexico from that around the gulf of california extension occurred as far south as what is now the trans mexican volcanic belt and in oaxaca south of the belt the basin and range province in mexico constitutes approximately half of the 19 x 10 5 km 2 of western north america that underwent mid to late cenozoic extension north northwest orientations of numerous epithermal vein systems indicate that east northeast extension began as early as 30 ma in areas north of the volcanic belt major episodes of faulting began at about 23 to 24 ma and 12 to 13 ma both in mexico and in the southwestern united states faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting widespread quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the trans mexican volcanic belt in contrast the tectonics in and south of the belt are now probably related to subduction of the rivera plate contemporaneity of the 12 ma episode with early extension around the gulf of california attributed to pacific north american plate boundary reorganization suggests that extension is related predominantly to plate boundary effects the beginning of extension at 30 ma may be related to initial encroachment of the east pacific rise upon the trench that lay off western north america

stratigraphy and provenance of strata along the san marcos fault central coahuila mexico stratigraphic and petrologic studies supported by geologic mapping north of the san marcos fault central coahuila indicate that as much as 3 000 m of coarse upper jurassic and lower cretaceous detritus was derived from south of the fault two coarse jurassic submarine conglomerate units las palomas and sierra el granizo beds of the la casita formation reflect separate episodes of displacement younger jurassic eolianite and marine sandstone colorado and tanque cuatro palmas beds of the la casita formation record subsequent tithonian fault inactivity neocomian rejuvenation of the fault produced alluvial deposits san marcos formation finer grained paralic or marine strata that are distal and suprajacent and manifest the same faulting are the la mula formation interruption of neocomian displacement is indicated by a tongue of carbonate padilla and barril viejo formations that divides the san marcos into upper and lower members the principal source of this detritus was the wildflysch of the late paleozoic arcproximal las delicias basin and the plutonic rocks that cut it rb sr ages on clasts and from outcrops of granitic rocks are triassic and probably reflect late stage magmatism within the las delicias arc although this arc supplied most of the detritus to the las delicias basin devonian ages on some clasts reveal that an unknown source area lay somewhere nearby during the late part of the paleozoic era the san marcos fault may have served as part of the system of transform faults that connected sea floor spreading ridges of the atlantic with those of the pacific if so then the jurassic units record periods of jurassic activity and inactivity of that system neocomian displacment may have been caused by subsequent isostatic adjustment

synvolcanic and syntectonic sedimentation of the mixed volcaniclastic epiclastic succession in the miocene janggi basin se korea understanding the variability and complex interplay of volcanism tectonism and sedimentation is crucial for unraveling the evolution of sedimentary basins and volcanotectonic depressions in many settings the miocene janggi basin in se korea formed in association with backarc opening of the east sea sea of japan is filled by abundant volcanic deposits intercalated with fluvio lacustrine sedimentary deposits four lithostratigraphic units units i to iv in ascending order are identified in the southern part of the basin including extensive and voluminous dacitic lapilli tuffs units ii and iv and mixed dacitic volcaniclastic and epiclastic sedimentary deposits units i and iii these units suggest repetitive ignimbrite emplacement onto a braided stream to lacustrine environment paleoflow measurements show that the epiclastic conglomerates were deposited by a north flowing fluvial system that debouched from a drainage area to the south of the basin whereas the ignimbrites were introduced from the east because of the separate source areas the epiclastic and volcaniclastic depositional processes did not interfere with each other resulting in discrete alternations of epiclastic and mostly primary volcaniclastic deposits the studied succession also lacks an aggradation degradation cycle which results from the alternation of periods of rapid volcaniclastic sedimentation and relatively longer periods of fluvial incision the volcaniclastic and epiclastic deposits are therefore interpreted to have accumulated in rapid succession in association with rapid basin subsidence and contemporaneous ignimbrite forming eruptions the studied succession can thus be regarded as a syntectonic sequence in which the stratal geometry was significantly modulated by tectonostratigraphic controls with overprints of syntectonic contemporaneous volcanism

seismic attenuation in northeastern sonora mexico we studied the attenuation of local and regional earthquake body waves recorded by the seismic network of northeastern sonora mexico red sismica del noreste de sonora resnes the source station paths of the selected data set cover the epicentral region of the may 1887 m w 7 5 sonora earthquake we used the best located earthquakes recorded by resnes between 2003 and 2004 to analyze the spectral amplitude decay of body waves with hypocentral distance the database consists of 44 earthquakes with magnitudes between 1 8 and 3 2 and hypocentral distances between 25 and 150 km we calculated fourier spectral amplitudes from more than 400 records and defined attenuation functions with a nonparametric model by inverting the observed spectral amplitudes at 23 individual frequencies between 0 4 and 63 1 hz we determined regional attenuation curves based on events located more than 100 km northwest of the center of the network and local curves based on earthquakes located near the fault zone that ruptured in the 1887 earthquake the obtained attenuation curves indicate that the spectral amplitudes decay faster in the area of the 1887 rupture than in the surrounding region to analyze the possible sources of attenuation we parameterized the attenuation functions considering three terms one that accounts for geometrical spreading another one for anelastic attenuation and a third for near surface attenuation we found the s wave quality factor q to be similar in both the general region and near the 1887 rupture however in the region of the 1887 rupture the near surface attenuation exceeds the regional average finally in the region of the 1887 rupture the nonparametric attenuation functions and the estimates of q indicate that at between 0 6 and 63 1 hz p waves attenuate more than s waves q p 20 8 f 1 1 whereas q s 83 8 f 0 9

resetting of oxybarometers and oxygen isotope ratios in granulite facies orthogneisses during cooling and shearing adirondack mountains new york the petrography petrology and oxygenisotope geochemistry of granulite facies granitic and syenitic orthogneisses of the diana and stark complexes adirondack mountains new york show that the extent and nature of resetting of isotopic and mineralogic systems is highly variable there is a strong correlation between retrogression and shearing and the rocks may be divided texturally into 1 unsheared lithologies that preserve little retrogressed pyroxene or hornblendebearing peak metamorphic mineralogies and 2 sheared rocks that underwent retrogression marked by the growth of late biotite in centimetre to metre wide shear zones after the peak of metamorphism oxygen fugacities in the unsheared lithologies were estimated for reintegrated mineral compositions from magnetiteilmenite mt ilm and ferrosilite magnetic quartz fs mt qtz equilibria mt ilm yields logfo2mt ilm values of 15 9 to 17 6 0 6 to 1 3 log units below the fayalite magnetite quartz buffer fmq and temperatures of 670 745 c that agree with those from other geothermometry and phase equilibria studies these data suggest that aside from oxyexsolution of ilmenite from magnetite the fe ti system underwent only minor resetting during cooling and the fe ti oxides yield good estimates of peak metamorphic temperatures and fo2 in unsheared ilmenite magnetite orthopyroxene quartz assemblages values of logfo2mt ilm are lower than logfo2fs mt qtz by an average of 0 6 when the orthopyroxene activity model of sack and ghiorso is used minor resetting of the fe ti oxides analytical errors and errors in the placement of end member reactions probably account for this relatively small difference in fo2 values whole rock 18o values of unsheared diana and stark lithologies range from 4 0 to 10 3 reflecting pre regional metamorphic oxygen isotope ratios peak metamorphic minerals preserve high temperature oxygen isotope fractionations and in many samples the effective diffusion of oxygen in minerals ceased at higher temperatures than predicted from wet experimental diffusion data these data suggest that the rocks did not contain an aqueous fluid phase during cooling the combination of petrologic isotopic and textural data also permits a detailed study of shearing and retrogression ilmenites in the sheared lithologies underwent greater degrees of hematite loss than in the unsheared rocks resulting in logfo2mt ilm values as low as 24 1 3 1 log units below fmq and mt ilm temperatures that are up to 175 c below regional estimates sheared rocks also have higher 18o values up to 13 3 during shearing 18o values of biotite k feldspar and magnetite reset readily while the degree of isotopic resetting of quartz correlates with the intensity for recrystallization

new metallogenic model of telescoped eocene miocene au u epithermal mineral deposit in the placer de guadalupe district chihuahua mexico abstract the chronological repartition of mineral deposits in mexico and the southern usa coincides with the distribution of the three main volcanic pulses of the sierra madre occidental these pulses are eocene to early miocene in age suggesting that a broad migration occurred from northwest to southeast where the last ignimbrite flare up occurred the most fertile event occurred from 36 to 28 ma and produced all of the largest ag au mining districts the placer de guadalupe au u district contain a unique association of metals with high variations in their u au ratios that are not common worldwide the uraninite population dated by wells 1930 yielded an age of ca 36 ma which is comparable to that of the volcanic intrusions and dykes dated in the district yielding ages of ca 36 ma altered uraninite cemented by gold sheets provide an age of 21 ma which is interpreted to be the age of the gold epithermal event studies of fluid inclusions in the quartz and calcite cements formed in the gold event indicate the presence of immiscible magmatic brines and co2 homogenization temperatures by halite melting range from 268 c to 305 c and 266 6 c to 283 c with salinities ranging from 35 9 to 38 9 and 36 0 to 37 5 wt eq nacl in the quartz and calcite stages respectively c o isotopic compositions of u and au calcite cements suggest a trend of low temperature alteration caused by hydrothermal fluids these c o isotopes suggest that during the gold event water rock interaction processes occurred between magmatic fluids and the carbonate host rock ree enrichment in the u calcite cement suggests that la ce enriched carbonatite volcanic formations participated in the mineralizing process the u mineralization event was controlled by 1 a metal and heat source in this case late eocene volcanism and in particular a carbonatite volcanic event 2 an efficient transport mechanism for the large scale input of meteoric water which was channeled heated and u enriched into reactivated crustal eocene faults from a deep u source to a shallow sedimentary chemical trap 3 an effective trap which is represented in this study by the presence of an organic matter rich sedimentary formation the gold mineralization event was characterized by the miocene extensional peak coupled with asthenospheric upwelling which facilitated the melting metamorphism of the lower crust and the expulsion and transport of large amounts of co2 bearing fluids to a shallow crustal level through the reactivated crustal laramide thrust faults the fluids interactions with the entire stratigraphic column from the grenvillian to the late triassic basement favored the leaching of gold and the enrichment of co2 bearing fluids gold deposition appears to be related to boiling but was ultimately favored by water rock interaction with u mineralization and the upper plomosas shale study of the au u placer de guadalupe deposit reveals evidence of a new miocene metallogenic event in central and northern mexico its genetic relationship with the basin and range bnr and rio gande rift rgr in usa and its transition from post orogenic to miocene intraplate extension in mexico provides a new perspective regarding the exploration of miocene gold epithermal deposits in northern mexico and the southern usa

marco gonzalez ambergris caye belize a geoarchaeological record of ground raising associated with surface soil formation and the presence of a dark earth marco gonzalez on the south west end of the island of ambergris caye belize has well preserved maya archaeological stratigraphy dating from preclassic times ca 300 b c to the late classic period ca a d 550 600 to 700 760 although later occupations are recorded by house platforms and inhumations terminal classic to early postclassic and use of the site continued until the 16th century a d intact stratigraphy is rare in these cases owing to a greater degree of disturbance nonetheless understanding site formation entails accounting for all processes including disturbance the site s depositional sequence as revealed through soil micromorphology and chemistry and detailed here has yielded critical information in two spheres of research as regards archaeology and the elucidation of maya activities on the caye over time soil micromorphology has contributed beyond measure to what we have been able to distinguish as material remains of cultural activity detailed descriptions of the nature of the material remains has in turn helped us to clarify or alter interpretations based on artefacts that have been identified or sediments characterised according to traditional recovery techniques the other major sphere in which soil micromorphology and chemistry play a critical role is in assessment of the environmental impact of human activity which enables us to construct and test hypotheses concerning how the site formed over time what materials and elements contributed to the character of the sediments especially in the formation of a specific maya dark earth type that is developed from carbonate rich deposits and how the modern surface soils acquired the appearance of a dark earth but essentially differ from them in terms of agricultural soil sustainability the marco gonzalez surface soil is neo formed by a woodland vegetation drawing upon the nutrients and constituents present in both the dark earth and underlying better preserved stratified deposits

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

variation of cenozoic extension and volcanism across the southern sierra madre occidental volcanic province mexico the middle to late cenozoic tectonic magmatic evolution of the sierra madre occidental volcanic province south of the tropic of cancer is summarized and analyzed for the first time based on new geologic and structural work and published information in the eastern part of the study region mesa central physiographic province silicic volcanism occurred in a short lived episode culminating at ca 30 ma and was followed by crustal scale extension between 30 and 27 ma in the western part of the study area sierra madre occidental physiographic province a voluminous episode of ignimbrite volcanism at 24 21 ma was succeeded by east west extension that produced regularly spaced grabens affecting only the upper crust in the westernmost part of the study region an andesitic to rhyolitic arc formed between 17 and 12 ma was affected by crustal scale north northwest trending extensional faulting leading to the formation of the gulf of california in the mesa central the maximum extension was oriented approximately east west and amounted to 20 in the eastern sierra madre occidental physiographic province extension was only 8 and oriented approximately east west we observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late oligocene early miocene and late miocene time comparison with the offshore tectonics indicates that the first two tectonic magmatic pulses coincide with periods of fast spreading at the pacific farallon boundary south of the shirley fracture zone we propose that increases in the spreading rate are related to periods of high subduction rate which in turn correspond to episodes of retreating subduction a retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate boundary conditions i e plate tectonics ultimately determined timing magnitude and orientation of extension whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism

high precision lu and hf isotope analyses of both spiked and unspiked samples a new approach 1 the functional form of instrumentally produced mass fractionation associated with mc icp ms analysis is not accurately known and therefore cannot be fully corrected by traditional approaches of internal normalization using power linear or exponential mass bias laws we present a method for robust correction of instrumentally produced mass fractionation of both spiked and unspiked samples that can be applied to mass analysis of hf as well as nd sr os etc correction of 176hf 177hf for unspiked samples follows a traditional approach of internal normalization using an exponential law followed by normalization to a standard of known composition such as jmc 475 for spiked samples standards are used to characterize a linear instrumental mass bias coefficient the mass bias coefficient is defined by the slope of a tie line between measured and true values of a standard this approximation results in identical precision and accuracy of measurements for spiked and unspiked samples 0 005 2 external reproducibility the effects of the spike on the 176hf 177hf ratio and calculation of the molar spike sample ratio is determined by a closed form solution modified from the double spike approach used for fe isotope analysis by tims johnson and beard 1999 the measured 176lu 175lu ratios are corrected by doping the sample with er and using the 167er 166er ratio to externally normalize the 176lu 175lu ratio using an exponential law finally spike sample equilibration is confirmed for our sample dissolution protocol through analysis of varying physical mixtures of 1 ga garnet and hornblende where all the data lie on a mixing line within error on a 176lu 177hf 176hf 177hf diagram precision of 176lu 177hf ratios is determined to be 0 2 2 for standards and for physical mixtures of garnet and hornblende

structural configuration of the otates fault southern basin and range province and its rupture in the 3 may 1887 mw 7 5 sonora mexico earthquake abstract during the great 3 may 1887 sonoran earthquake surface rupture end to end length 101 8 km m w 7 5 0 3 an array of three north south striking basin and range province faults from north to south pitaycachi teras and otates slipped along the western margin of the sierra madre occidental plateau of northern mexico this detailed field survey of the previously undocumented 1887 earthquake rupture zone along the otates fault includes mapping the rupture scarp and measurements of surface deformation at 25 sites the surface rupture coincides with the mapped trace of the otates fault dips 68 w and has an end point to end point length of 18 2 km the lateral rupture terminations are controlled by cross faults striation measurements and the style of rupture indicate extensional dip slip without significant lateral displacement the average shortening p and extension t kinematic axes inferred from the striations are similar to published composite focal mechanisms of the microseismicity recorded in the epicentral region of the 1887 earthquake based on surface deformation maximum slip along the otates fault in the 1887 earthquake was 270 cm and the average slip was 189 cm a quadratic best fit to the along rupture surface offsets indicates an asymmetric distribution with the maximum offset near the northern end of the rupture segment which can be explained by the mechanical interaction of the teras and otates surface rupture segments through their stress fields the average recurrence interval of 1887 sized earthquakes on the otates fault is 30 42 kyr based on the long term geologic dip slip rate of the fault 0 06 mm yr and its amount of slip in the 1887 earthquake

sm nd and u pb isotopic evidence of juvenile crust in the adirondack lowlands and implications for the evolution of the adirondack mts sm nd investigations of hyde school gneiss yield t dm ages only slightly older than u pb zircon emplacement ages thus indicating substantial addition of juvenile calcalkaline crust to the adirondack lowlands at ca 1300 ma juvenile crust of similar age has been recognized within the adirondack highlands the elzevir terrane further to the northwest in canada and in the green mts of vermont suggesting the presence of an extensive magmatic arc at that time while the precise nature of the arc remains uncertain an indonesian analogue is favored u pb zircon ages of ca 1150 1160 ma are reported for lowlands samples of syenitic and granitic rocks belonging to the anorthosite mangerite charnockite granite amcg suite that also occurs in the highlands as well as in the canadian sector of the frontenac terrane the regional occurrence of these anorogenic intrusions indicates that the adirondack highlands and frontenac terrane including the adirondack lowlands constituted a single contiguous entity

primary versus secondary and subaerial versus submarine hydrovolcanic deposits in the subsurface of jeju island korea jeju island is a quaternary shield volcano built upon the yellow sea continental shelf off the korean peninsula decades of borehole drilling reveals that the shield forming lavas of the island are underlain by extensive hydrovolcanic deposits the seoguipo formation which are about 100 m thick and show diverse depositional features this study provides criteria for distinguishing between hydrovolcanic deposits formed by primary pyroclastic and secondary resedimentation processes in subaerial and submarine settings based on the observations of several selected cores from the formation five facies associations are identified including i primary hydrovolcanic deposits formed by pyroclastic surges and co surge fallouts in tuff rings facies association phtr ii primary hydrovolcanic deposits formed by surtseyan fallout and related pyroclastic transport processes in tuff cones facies association phtc iii secondary hydrovolcanic deposits formed by debris flows hyperconcentrated flood flows sheet floods and rill flows in subaerial settings facies association rhae iv secondary hydrovolcanic deposits formed in submarine settings under the influence of waves tides and occasional mass flows facies association rhmar and v non volcaniclastic and fine grained deposits formed in nearshore to offshore settings facies association nvmar the primary hydrovolcanic facies associations phtr and phtc are distinguished from one another on the basis of distinct lithofacies characteristics and vertical sequence profiles these facies differ from the secondary hydrovolcanic and non volcaniclastic facies associations rhae rhmar and nvmar because of their distinctive sedimentary structures textures and compositions the depositional processes and settings of some massive and crudely stratified volcaniclastic deposits which occur in many facies associations could not be discriminated unambiguously even with microscopic observations nevertheless these facies associations could generally be distinguished because they occur typically in packets or sequences several metres to tens of metres thick and bounded by distinct stratigraphic discontinuities and comprise generally distinct sets of lithofacies the overall characteristics of the seoguipo formation suggest that it is composed of numerous superposed phreatomagmatic volcanoes intercalated with marine or non marine volcaniclastic or non volcaniclastic deposits widespread and continual hydrovolcanic activity together with volcaniclastic sedimentation is inferred to have persisted for more than a million years in jeju island under the influence of fluctuating quaternary sea levels before effusion of the shield forming lavas extensive distribution of hydrovolcanic deposits in the subsurface of jeju island highlights that there can be significant differences in the eruption style growth history and internal structure between shelfal shield volcanoes and oceanic island volcanoes

volcanic ash in ancient maya ceramics of the limestone lowlands implications for prehistoric volcanic activity in the guatemala highlands abstract in the spirit of collaborative research glicken and ford embarked on the problem of identifying the source of volcanic ash used as temper in prehistoric maya ceramics verification of the presence of glass shards and associated volcanic mineralogy in thin sections of maya ceramics was straightforward and pointed to the guatemala highland volcanic chain considering seasonal wind rose patterns target volcanoes include those from the area west of and including guatemala city joint field research conducted in 1983 by glicken and ford in the limestone lowlands of belize and neighboring guatemala 300 km north of the volcanic zone and 150 km from the nearest identified ash deposits was unsuccessful in discovering local volcanic ash deposits the abundance of the ash in common maya ceramic vessels coupled with the difficulties of long distance procurement without draft animals lead glicken to suggest that ashfall into the lowlands would most parsimoniously explain prehistoric procurement it literally dropped into their hands a major archaeological problem with this explanation is that the use of volcanic ash occurring over several centuries of the late classic period ca 600 900 ad to accept the ashfall hypothesis for ancient maya volcanic ash procurement one would have to demonstrate a long span of consistent volcanic activity in the guatemala highlands for the last half of the first millennium ad should this be documented through careful petrographic microprobe and tephrachronological studies a number of related archaeological phenomena would be explained in addition the proposed model of volcanic activity has implications for understanding volcanism and potential volcanic hazards in central america over a significantly longer time span than the historic period these avenues are explored and a call for further collaborative research of this interdisciplinary problem is extended in this paper

weathering of carbonate materials in ancient maya constructions rio bec and dzibanche limestone and stucco deterioration patterns carbonate materials limestone and lime stuccos were widely used in ancient maya construction deterioration of these materials presents a major problem for the conservation of maya architectural heritage however the mechanisms of deterioration and their interaction with the diverse inner structures of these materials are still insufficiently understood we performed micromorphological and mineralogical research of the limestone alteration in the buildings of rio bec and stuccos in dzibanche two late classic maya sites in the southern yucatan peninsula dissolution and recrystallization of calcite were found to be universal and the most important weathering process in both materials dissolution is selective affecting mostly micrite larger crystals especially rhombohedral calcitic pseudomorphs developed after dolomite are more stable selective dissolution of micritic cement of limestones and stuccoes enhances physical breakdown by producing numerous pores and loosening bounds between coarser inclusions this process clearly controls the deterioration grade of different limestones those with more intensive fragmentation and powdering have higher contents of micrite secondary carbonates are represented mostly by acicular calcite filling fissures and forming coatings on the outer surface clear spatial relation to fungi and roots and their similarity to well known soil neoformations indicate biogenic origin of these neoformations in most cases they contribute to macroscopic powdering being loose however in the cracks of some limestones they form dense infillings which could plug the fractures and prevent further advance of weathering to the core of the stone neoformed sulfates and iron oxides are fewer but also participate in the deterioration

the use of pozzolanic materials in maya mortars new evidence from rio bec campeche mexico the use of pozzolanas to improve the performance of lime mortar is usually regarded as a major contribution of the greco roman civilizations to mortar technology while this remains unquestioned recent investigations have shown that this practice was also known in mesoamerica before the spanish conquest the mortars produced in the maya lowlands in particular have been shown to contain various compounds similar to those formed as a result of the reaction between lime and pozzolanas such as volcanic ash or other siliceous and or aluminous materials however until recently petrographic studies had failed to present direct evidence that the formation of these compounds was due to the addition of pozzolanas rather than to the use of a natural hydraulic lime the microscopic analysis of mortar samples from the archaeological site of rio bec in southeastern campeche provides this evidence the identification of pyroclastic material in the form of unaltered glass shards confirms for the first time the use of pozzolanic materials in ancient maya mortars furthermore the analysis suggests the use of other materials that potentially acted as pozzolanas such as phytoliths and argillaceous inclusions the purpose of this paper is to present these new data and to discuss their significance for the understanding of maya mortar technology

migration of volcanism in the san francisco volcanic field arizona the remanent magnetization of volcanic rocks has been determined at 650 sites in the san francisco volcanic field in the southern part of the colorado plateau the polarity of remanent magnetization combined with k ar age determinations spatial and petrographic associations stratigraphic relations and state of preservation of the cinder cones provides a basis for assignment to known magnetic polarity epochs of 610 mafic vents and 100 intermediate to silicic flows flow sequences and vents the age assignments for basaltic rocks include 243 brunhes 0 73 ma vents 220 matuyama 0 73 to 2 48 ma vents and 147 pre matuyama 2 48 to about 5 0 ma vents basaltic volcanism migrated northeastward before matuyama time at a rate of 1 2 cm yr and eastward s87 5 e over the past 2 5 m y at a rate of 2 9 0 3 cm yr concomitant acceleration in total magma production from 75 to 1 400 10 6 km3 yr and frequency of basaltic eruptions from 1 per 17 000 yr to 1 per 3 000 yr occurred between 5 and 0 25 ma for the past 0 25 m y magma production 180 10 6 km3 yr and perhaps eruption frequency have decreased this evolutionary sequence coupled with the lead and strontium isotopic composition of the rocks can be explained by magmatism caused by shear heating at the base of the lithosphere we propose that this eastward drift of volcanic activity represents absolute westward motion of the north american plate our model is in agreement with a model in which the african plate is fixed to the deep mantle

history of recurrent activity along a major fault in northeastern mexico mesozoic and tertiary faulting within a 280 km long west northwest trending zone in coahuila mexico is inferred from stratigraphic and structural evidence the fault forms the northern boundary of the late jurassic early cretaceous coahuila island uplift on the south during the jurassic and early cretaceous is indicated by a large detrital lithosome that formed on the northern side of the fault clast size within the early cretaceous part diminishes northward where the unit is divided into upper and lower parts by a limestone tongue these two early cretaceous detrital units plus two jurassic conglomerates at valle san marcos suggest episodic pre tertiary movement deformation of 242 2 m y old granites at valle san marcos also reflects several periods of pre tertiary movement uplift on the north along the same fault zone during the tertiary laramide raised permian and jurassic rocks into juxtaposition with cretaceous limestones extensive left lateral movement is neither proven nor precluded nevertheless because of the size orientation general location and times of movement of the fault we suggest that it may either be part of the megashear postulated by several authors for northern mexico or a splay of that megashear

plate interactions control middle late miocene proto gulf and basin and range extension in the southern basin and range middle late miocene proto gulf 12 6 ma extension around the gulf of california gulf extensional province is commonly interpreted as resulting from partitioning of oblique pacific north american plate motion into strike slip displacement along the margin and east northeast extension perpendicular to the margin within the north american plate we propose that this mechanism also applies to kinematically similar predominantly east northeast extension that occurred at the same time throughout the southern basin and range province from southern arizona and new mexico to the trans mexican volcanic belt new field and 40ar 39ar data in sinaloa and durango confirm that this episode of extension occurred on the mainland side of the gulf and in the basin and range east of the sierra madre occidental which is generally considered the eastern margin of the gulf extensional province published data indicate the middle late miocene episode also occurred across the northern and southern ends of the sierra madre where the gulf extensional province connects with the basin and range 1 from central sonora into southern arizona and new mexico and 2 from nayarit into central mexico north of the trans mexican volcanic belt this episode appears to have avected an area that continues to the eastern edge of the basin and range province in texas and san luis potosi recognition that this episode of extension avected the entire southern basin and range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the gulf extensional province alone published plate reconstructions require 160 to 110 km of east northeast extension between 12 and 6 ma if taken up solely within the gulf extensional province this would have generated 66 to 78 extension which is much greater than observed spread across the entire southern basin and range it requires only 20 total extension which is more consistent with observations of cumulative extension between 12 and 6 ma extension was partitioned into the gulf extensional province because 1 it lies between two stable batholith belts mesozoic peninsular ranges on the west and mid tertiary sierra madre occidental on the east that resisted extension and 2 the gulf was thermally weakened by immediately preceding arc magmatism extension in the main basin and range province in part probably avoided the relatively strong batholithic crust of the sierra madre occidental 2000 elsevier science b v all rights reserved

a model for calculating eruptive volumes for monogenetic volcanoes implication for the quaternary auckland volcanic field new zealand abstract monogenetic basaltic volcanism is characterised by a complex array of behaviours in the spatial distribution of magma output and also temporal variability in magma flux and eruptive frequency investigating this in detail is hindered by the difficulty in evaluating ages of volcanic events as well as volumes erupted in each volcano eruptive volumes are an important input parameter for volcanic hazard assessment and may control eruptive scenarios especially transitions between explosive and effusive behaviour and the length of eruptions erosion superposition and lack of exposure limit the accuracy of volume determination even for very young volcanoes in this study a systematic volume estimation model is developed and applied to the auckland volcanic field in new zealand in this model a basaltic monogenetic volcano is categorised in six parts subsurface portions of volcanoes such as diatremes beneath phreatomagmatic volcanoes or crater infills are approximated by geometrical considerations based on exposed analogue volcanoes positive volcanic landforms such as scoria spatter cones tephras rings and lava flow were defined by using a light detection and ranging lidar survey based digital surface model dsm finally the distal tephra associated with explosive eruptions was approximated using published relationships that relate original crater size to ejecta volumes considering only those parts with high reliability the overall magma output converted to dense rock equivalent for the post 250 ka active auckland volcanic field in new zealand is a minimum of 1 704 km3 this is made up of 1 329 km3 in lava flows 0 067 km3 in phreatomagmatic crater lava infills 0 090 km3 within tephra tuff rings 0 112 km3 inside crater lava infills and 0 104 km3 within scoria cones using the minimum eruptive volumes the spatial and temporal magma fluxes are estimated at 0 005 km3 km2 and 0 007 km3 ka the temporal volumetric evolution of auckland is characterised by an increasing magma flux in the last 40 ky which is inferred to be triggered by plate tectonics processes e g increased asthenospheric shearing and backarc spreading of underneath the auckland region

the use of volcanic materials for the manufacture of pozzolanic plasters in the maya lowlands a preliminary report this paper focuses on the description and characterization of the nature of isotropic and silicon rich particles observed in plaster samples from the maya archaeological sites of calakmul and lamanai based on the composition and morphology of these particles it is proposed that volcanic ash and glass were incorporated into some of the ancient maya plasters in order to produce hydraulic reactions from a technological point of view the intent on the part of the maya to produce such reactions is relevant because it would reflect resource planning and complex knowledge of materials results also confirm previous reports of the presence of volcanic ash layers at the site of calakmul and suggest that there were considerable ash falls in the maya lowlands during prehispanic times which may have made the material available in many areas of the central lowlands if as we propose here volcanic ash was employed in lowland maya plasters then the use of volcanic materials by the lowland maya was more intensive than has been recognized our paper also reviews the literature about the use of volcanic materials in lowland maya materials and suggests future lines of research

the 40ar 39ar and k ar dating of lavas from the hilo 1 km core hole hawaii scientific drilling project mauna kea lava flows cored in the hilo hole range in age from 200 ka to about 400 ka based on 40ar 39ar incremental heating and k ar analyses of 16 groundmass samples and one coexisting plagioclase the lavas all subaerially deposited include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic transitional tholeiitic and tholeiitic basalts the lower section has yielded predominantly complex discordant 40ar 39ar age spectra that result from mobility of 40ar and perhaps k the presence of excess 40ar and redistribution of 39ar by recoil comparison of k ar ages with 40ar 39ar integrated ages indicates that some of these samples have also lost 39ar nevertheless two plateau ages of 391 40 and 400 26 ka from deep in the hole combined with data from the upper section show that the tholeiitic section accumulated at an average rate of about 7 to 8 m kyr and has a mean recurrence interval of 0 5 kyr flow unit samples from the upper section yield relatively precise 40ar 39ar plateau and isotope correlation ages of 326 23 241 5 232 4 and 199 9 ka for depths of 415 7 m to 299 2 m within their uncertainty these ages define a linear relationship with depth with an average accumulation rate of 0 9 m kyr and an average recurrence interval of 4 8 kyr flow unit the top of the mauna kea sequence at 280 m must be older than the plateau age of 132 32 ka obtained for the basal mauna loa flow in the corehole the upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to mauna kea as it rode the pacific plate away from its magma source the hawaiian mantle plume the age depth relation in the core hole may be used to test and refine models that relate the growth of mauna kea to the thermal and compositional structure of the mantle plume

u pb zircon geochronology of the adirondack mountains and implications for their geologic evolution u pb zircon studies of metamorphosed igneous rocks in the adirondack mountains have yielded preliminary ages within the range 1420 990 ma several geochronologically and geochemically distinct episodes of igneous intrusion and at least one pre granulite facies dynamothermal metamorphic event are documented this information is consistent with recent field and geochronological studies throughout the grenville province and suggests that a complex sequence of events occurred in the adirondack mountains prior to the widespread deformation and metamorphism commonly attributed to the approximately 1100 1000 ma ottawan phase of the grenvillian orogenic cycle

upb geochronology of the sri lankan basement abstract u pb ages of zircons and monazites from ortho and paragneisses representing the major crystalline basement units of sri lanka establish a new geochronological framework which is consistent with previously published work on sm nd model ages rb sr whole rock ages and u pb mineral ages in the highland complex three newly dated orthogneisses one previously dated by baur et al 1991 and one dated by kroner and williams 1992 all intruded 1 85 to 1 9 ga ago this widespread intrusion event occurred soon after the end of sediment deposition which is estimated at 1 9 to 2 0 ga using upper intercept ages of zircons from metapelites in the highland complex within the boundaries of this complex established by sm nd model age mapping milisenda et al 1994 only one orthogneiss has been found which is substantially younger than 1 8 ga this is the metatonalite from polonnaruwa which intruded 670 ma ago baur et al 1991 three orthogneisses from the vijayan complex which lies to the east of the highland complex yield similarly consistent crystallization ages of 1023 8 1016 28 and 1033 121 ma in the wanni complex which lies to the west of the highland complex a sillimanite bearing gneiss which is apparently of metasedimentary origin but contains zircons of igneous appearance yields a primary crystallization age of 793 4 ma consistent with a previously dated orthogneiss from kurunegala of 770 ma baur et al 1991 in addition single zircon data by kroner et al 1994 indicate primary crystallization ages of 1 0 to 1 1 ga for orthogneisses from the vavuniya charnockite province which is considered to be part of the wanni complex these results confirm the age contrast between the highland complex on the one hand and the vijayan complex and wanni complex on the other as previously inferred from sm nd model ages milisenda et al 1988 1994 post tectonic granitoids and pegmatites intruded throughout all three complexes 550 ma ago the age of regional high grade metamorphism mostly granulite facies in the highland complex mostly amphibolite facies in the vijayan complex and both granulite and amphibolite facies in the wanni complex is constrained to be less than 670 ma the primary crystallization age of the polonnaruwa metatonalite it is more closely defined by lower intercept zircon ages which include one concordant age of 608 4 ma and one discordant but well defined age of 611 23 ma and by a nearly concordant monazite age of 592 2 ma this indicates that peak metamorphic conditions occurred 610 ma ago numerous other lower intercept ages between 590 and 550 ma and post tectonic intrusion ages of 550 ma set a lower age limit for high grade conditions and deformation the temperature history between 610 and 550 ma is not yet well resolved and it is not clear whether there was a separate thermal event 550 ma ago previously published whole rock rb sr and sm nd isochrons for gneisses from the highland complex yielding ages of 2 ga and greater and previously interpreted to date the time of granulite grade metamorphism must be reinterpreted in the light of the new data presented here as well as other recently published results e g burton and o nions 1990b baur et al 1991 and in view of the accumulated evidence for very limited element mobility during high grade metamorphism these whole rock ages are in fact identical or close to the ages of primary granitoid emplacement rather than the age of granulite facies metamorphism

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

raman microprobe analysis of stucco samples from the buildings of maya classic copan micro raman spectroscopy has been applied to painted stucco samples from three buildings at the maya site of copan honduras the buildings clavel ad 450 550 am ad550 650 and structure 10l 22 ad730 date from three phases of the classic period ad400 820 acropolis construction the red pigment has been identified as haematite and the stucco as a mixture of calcite particles dispersed throughout a calcite based lime wash stucco the physical structure of the stucco changed through time indicating a refining of production techniques over this period by combining micro raman spectroscopy with other micro atr infrared spectroscopy and environmental scanning electron microscopy a detailed investigation of the materials and production techniques used to decorate these buildings has been made differences in the mineralogy of the red pigment used on each building indicate a different geological pigment source for each period c 2006 elsevier ltd all rights reserved

origin and formation of neck in a basin landform examples from the camargo volcanic field chihuahua mexico abstract the term neck in a basin nib landform is proposed for volcanic structures characterized by nearly circular to elliptical open basins located near the headwater of small streams or drainages which contain small volcanic necks and or erosion remnants of one or more cinder cones nib landforms are typically 400 1000 m in diameter and 30 100 m deep and are invariably surrounded by steep walls cut into one or more basaltic lava flows nib landforms lack evidence for a primary volcanogenic origin through either collapse or youthful eruptive activity in the pliocene portion 4 2 ma of the plio quaternary camargo volcanic field of chihuahua mexico they are relatively numerous and are best developed at the margins of a gently sloping 3 5 basaltic lava plateau and near major fault scarps mature nib landforms have ring like circular drainage patterns and central elevations marked by small volcanic necks and associated radial dikes intruded into basaltic scoria fall and or agglutinate deposits we interpret nib landforms to be erosional in origin they develop where a cinder cone is surrounded by one or more sheet like lava flows from one or more separate subsequent vents once eruptive activity ceases at the younger volcano es fluvial erosion gradually produces a ring like drainage pattern along the contact between the lava and the older cinder cone as a response to a marked contrast in resistance to erosion between lava flows and unconsolidated or poorly lithified pyroclastic deposits the older cinder cone is preferentially eroded in this manner a ring shaped steep sided erosional basin preformed by the scoria cone is produced eventually fluvial erosion exposes the central neck and dikes the volume relief and age of the volcanic field are key factors in the formation and preservation of a nib landform they form in volcanic fields where lava emissions are sufficiently vigorous to engulf earlier cinder cones relief and associated high rates of fluvial erosion play an important role in nib development as demonstrated by their locations in the camargo volcanic field fully developed nib landforms are not found in quaternary volcanic fields probably because erosion has not had sufficient time to generate their characteristics features nib landforms are also absent in miocene fields because erosion has proceeded too far and thus has completely removed any nib landform that may once have existed the camargo volcanic field is the only major area of pliocene intraplate eruptive activity in northern mexico and the only place where nib landforms are relatively abundant

regional ai fe mafic magmas associated with anorthosite bearing terranes proterozoic anorthosite massifs are nearly monomineralic accumulations of plagioclase feldspar which must result from unusual conditions of crystallization and emplacement1 2 their chemistry is compatible with derivation from a high alumina basalt parent of mantle origin1 and mafic magmas rich in aluminium and iron have long been suspected of having an important connection to anorthosites and their parent magmas1 4 regional examples of mafic magmas clearly compatible with an anorthosite lineage have not been generally recognized3 in part because of the paucity of candidates within the anorthosite massifs themselves samples of such magmas should also be sought in the country rocks surrounding the massifs here we report the widespread occurrence of al fe mafic magmas in the anorthosite bearing adirondack highlands of new york state they are chemically distinct from typical high ai basalt fe basalt and most common magma types identification of similar al fe basaltic magmas in other areas suggests that such magmas are characteristic of anorthosite bearing terranes

age of xenolith bearing olivine metagabbro eastern adirondack mountains new york a u pb zircon age of 1143 6 pm 7 ma has been obtained on a xenolith bearing olivine metagabbro in the eastern adirondack highlands thus fixing a minimum age for metamorphism represented by truncated foliation in the xenolith and the igneous crystallization age of the rock the igneous age predates commonly accepted metamorphic ages of 1000 1100 ma for the adirondacks and documents that more than one high grade metamorphic event affected the region

applications of single grain zircon evaporation analyses to detrital grain studies and age discrimination in igneous suites abstract 207 pb 206 pb ages determined by evaporation of single zircon grains from rocks of the new england appalachians in the usa permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenesis tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age whose u pb upper intercept zircon ages obtained through isotope dilution methods show profound post crystallization discordance demonstrate that the evaporation method is successful in recovering protolith ages uncertainties are 10 15 my for rocks of paleozoic age or older preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics 207 pb 206 pb ages of detrital zircon grains from two metasedimentary units from opposite sides of the new england appalachians reveal the distinct provenances of these rocks the missisquoi formation in southeastern vermont usa is comprised in part of detritus derived from the 1 0 1 2 ga grenvillian province of laurentia whereas this component is conspicuously absent from the plainfield formation in easternmost connecticut the detrital zircon age data suggest that the missisquoi formation accumulated peripheral to the laurentian craton during the early paleozoic whereas the plainfield formation detritus was supplied by non laurentian ossibly west african sources during the late proterozoic or early paleozoic in southern vermont the evaporation method successfully distinguishes between two groups of augen gneisses with ages of 960 ma and 1120 ma which were first recognized through isotope dilution methods collectively these results indicate that the dating of single zircon grains by the evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed rocks

frequency distribution of geologically determined slip rates for normal faults in the western united states short term and long term slip rates are contrasted for 18 well studied faults in the basin and range and rio grande rift provinces over periods ranging from 3000 to 4 000 000 yr the intrinsic variability of the 62 interval slip rates on these normal faults is assessed by normalizing the long term slip rate of each fault to 0 1 mm yr the cumulative density function of the grouped data predicts that given a mean net slip rate of 0 12 mm yr extreme slip rates with 10 and 5 probability of exceedance are 0 51 and 0 92 mm yr respectively these slip rates are considerably higher than those chosen by an expert panel for the pajarito fault new mexico which has a long term mean slip rate of ca 0 12 mm yr

provenance of glass shards in archaeological lime plasters by la icp ms implications for the ancient routes from the gulf of mexico to teotihuacan in central mexico abstract we present the chemical characteristics of samples collected from the central courtyard of teopancazco a neighbourhood centre in teotihuacan mexico the most important classic mesoamerican city the rhyolitic glass shards used as aggregates have a composition comparable to the glass material sourced from the magmatic system of altotonga located along the route to the gulf coast people travelling from the gulf coast may have transported the glass shards to teotihuacan during the xolalpan phase 350 550 ad however the reasons for using glass shards as a construction material remain unknown these new data confirm the close relationship between inhabitants of teopancazco and the gulf coast already revealed by previous archaeological and archaeometric studies

space time patterns and tectonic controls of tertiary extension and magmatism in the great basin of the western united states structural and stratigraphic relations in the great basin indicate widespread pre middle miocene crustal extension that appears to define two north trending belts most extension in these belts was oligocene age but locally it began earlier or lasted into early miocene time the eastern belt straddles the nevada utah border and includes the snake range nevada area with its southern end near 37 5 n and its western edge at the seaman butte mountains breakaway the southern boundary of the eastern belt is occupied by the 26 15 ma caliente caldera complex and approximately coincides with the present east west trending margin of the great basin north of saint george utah crust north of this boundary extended approximately east west before volcanism began at 30 32 ma but to the south extension began after about 15 ma this boundary may have been a rooted zone of left slip faults that allowed the footwall of the stampede detachment to move west relative to unextended terrain to the south the eastern margin of the eastern belt is probably located near the present eastern edge of the great basin but its northern end is poorly defined the western belt runs from the funeral and grapevine mountains california to the ruby mountains nevada and north northeast to the albion range idaho tens of kilometers of crustal extension occurred at least locally in both belts but magnitude of extension is poorly known for large areas of each tertiary volcanism in the great basin began in the north in eocene time with predominantly effusive volcanism and swept southward ending with voluminous oligocene miocene ignimbrite eruptions from calderas in an irregular discontinuous belt between marysvale utah and reno nevada a result of the south ward migration of volcanism is that the onset of extension in both belts was syn or post volcanic in the north but was pre volcanic in the south late paleogene extension and crustal magmatism coincided in both time and space only locally where south migrating magmatism overlapped active north south trending extensional belts most calderas in the southern great basin formed in previously extended belts or on their margins southward migration of ignimbrite sources was apparently blocked by unextended crust to the south in contrast volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to or during crustal extension this is consistent with observations in the southern basin and range where volcanism and crustal extension were generally synchronous and volcanism was dominantly effusive thus caldera formation may be controlled by the distribution of upper crustal extension although the physical mechanism for this control remains speculative the space time patterns of late paleogene extension in the great basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin but being driven by gravitational collapse of thick crust space time patterns of tertiary volcanism in the basin and range also appear to conform to patterns of thermal weakening or destruction of the subducted slab both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts because both predict close spatial and temporal association of extension and magmatism which is not generally observed

the volcanic section at nazas durango mexico and the possibility of widespread eocene volcanism within the sierra madre occidental descriptions of volcanic rocks in the sierra madre occidental of western mexico have thus far emphasized the widespread and nearly continuous cover of ash flow tuffs and other units attributed to the oligocene ignimbrite flare up however much less attention has been given to the pre oligocene volcanic sequence beneath this ash flow blanket at nazas durango on the eastern flank of the sierra madre occidental a well exposed section includes voluminous felsic volcanic rocks of both eocene and oligocene age the oldest igneous rocks at nazas include a small exposure of intermediate volcanic breccias and lava flows that overlie cretaceous limestones and apparently were deformed with them during laramide tectonism more commonly the limestones are overlain by tertiary volcanic rocks and continental clastic deposits the tertiary volcanic section is about 800 m thick not including intercalated fanglomerates the section records three distinct magmatic episodes from 51 to 40 ma comprising felsic ash flow tuffs and intermediate lava flows and domes a pulse at about 30 ma comprising voluminous felsic ash flow tuffs with an aggregate thickness of about 500 m and from 24 to 20 ma comprising alkalic basalts the ahuichila formation a molasse type conglomerate up to 200 m thick that underlies the tertiary volcanic section was deposited during or just after the laramide deformation the santa ines formation is a widespread fanglomerate up to 110 m thick that underlies the basalts and apparently accumulated during normal faulting eocene volcanism in the nazas area is characterized by interfingering felsic ash flow tuffs and intermediate lava flows and domes a similar sequence of eocene volcanic rocks has been mapped in central chihuahua state about 350 km to the northwest of nazas felsic tuffs interbedded with intermediate volcanic rocks older than 40 ma are also exposed at tayoltita durango about 200 km southwest of nazas these occurrences suggest that a wider spectrum of compositions and volcanic styles existed during the eocene than during the oligocene in the sierra madre occidental eocene volcanism was more typical of orogenic magmatic belts developed at continental margins whereas the oligocene activity was dominated by voluminous felsic ash flow tuffs erupted during a transition of tectonic setting from subduction along a continental margin to intraplate extension in addition to these three areas there are several other localities in western mexico having volcanic rocks with reported ages between 40 and 53 ma it is possible that the volcanic field during the eocene in mexico was comparable in extent to that of the oligocene the eocene volcanism in western mexico was in part contemporaneous with the challis volcanic episode of northwestern united states and its extension into western canada however eocene magmatism in the pacific northwest apparently developed in an extensional tectonic setting whereas in mexico no evidence for eocene extensional deformation is known

the role of magma overpressure in suppressing earthquakes and topography worldwide examples in an extending terrane basaltic magma supplied at a pressure greater than the least principal stress overpressure may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting as vertical dikes intrude they press against their walls in the direction opposite the least principal stress and increase its magnitude the emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses this process tends to equalize the stresses and thus diminishes the deviatoric stress difference between maximum and minimum stresses that creates faults and causes earthquakes observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks

geophysical surveys of the joya honda maar mexico and surroundings volcanic implications abstract joya honda jh is a quaternary maar excavated in mesozoic limestone it is located in central mexico and belongs to the ventura volcanic field vvf which is composed by cinder cones and maars made of intraplate type mafic alkalic rocks volcanoes in the region form n20w lineaments roughly parallel to a regional set of normal faults but there is no obvious relation between these faults and vent distribution in the exposed geology around the maar the volcanic rock volume is small in the vvf and most volcanoes and their products are scattered in a region where outcrops are dominated by limestone the near vent tephra associated to the jh maar lies north of the crater this relation suggests that the crater was formed by directed hydromagmatic explosions and may indicate an inclined volcanic conduit near the surface the tephra stratigraphy suggests that the initial explosions were relatively dry and the amount of water increased during the maar forming eruption therefore the existing model of the maar diatreme formation may not be applicable to joya honda as it requires the formation of a cone of depression in the aquifer and deepening of the focii of the explosions as the crater and underlying diatreme grew thus it is unlikely that there is a diatreme below joya honda aeromagnetic data shows a boundary between two regional magnetic domains near the elongated volcanic cluster of the vvf the boundary is straight with a distinct kink from ne to nw trend near jh the limit between the domains is interpreted as fault contacts between mid tertiary volcanic rocks and marine mesozoic sedimentary rocks hence magma ascent in the area may have been facilitated by fractures near the surface magnetic and gravimetric ground surveys show that the anomalies associated with the maar are not centered in the crater which could be consistent with an inclined volcanic conduit a magnetic profile measured on exposed limestone across the volcanic lineament failed to show an anomaly such as that caused by a connecting dike between the volcanoes therefore either the dike does not exist or it is so deep or so thin that it is beyond the limit of detection of the method and or equipment used thus the volcanic conduit immediately below joya honda can be reasonably modeled in the shape of a plug a 2 d model of the crater anomaly is consistent with a roughly tabular deposit formed by fall back pyroclasts and slump deposits near the surface based on this result we propose an alternative model for the formation of maar type volcanoes excavated in hard rock where there is no evidence of a gradual decrease of the water magma ratio as the eruption advanced

timing of basaltic volcanism along the mesa butte fault in the san francisco volcanic field arizona from 40ar 39ar dates implications for longevity of cinder cone alignments using high precision stepwise heating 40ar 39ar methods we dated volcanic products of at least 12 vents cropping out along the northern segment of the mesa butte fault in the san francisco volcanic field arizona where the relationship between fault and vent alignment is unambiguous the 40ar 39ar isochron dates indicate that the mesa butte fault hosted five episodes of volcanism over a 1 million year interval from 1400 to 300 ka the first episode occurred 1 km northwest of the trace of the mesa butte fault whereas the next two episodes were centered near the intersection of the mesa butte and cedar ranch faults the final two episodes produced red mountain 750 100 ka and shadow mountain 300 100 ka respectively ages of vents representing the third mesa butte episode of volcanism 760 50 ka to 900 80 ka indicate a systematic southwestward shift in the locus of volcanism along the mesa butte fault that contrasts sharply with overall eastward younging of volcanism within the field these data demonstrate that structures in basaltic volcanic fields are capable of hosting multiple episodes of volcanism over periods of time comparable to duration periods of composite volcanoes 106 years but at significantly lower recurrence intervals these results suggest that probability models of spatiotemporal recurrence of volcanism within basaltic volcanic fields should incorporate mapped structures particularly where these structures have either hosted previous volcanic activity or are proximal to population centers or sensitive installations such as nuclear power plants or waste repositories

decreasing magmatic footprints of individual volcanoes in a waning basaltic field the distribution and characteristics of individual basaltic volcanoes in the waning southwestern nevada volcanic field provide insight into the changing physical nature of magmatism and the controls on volcano location during pliocene pleistocene times the volumes of individual volcanoes have decreased by more than one order of magnitude as have fissure lengths and inferred lava effusion rates eruptions evolved from hawaiian style eruptions with extensive lavas to eruptions characterized by small pulses of lava and strombolian to violent strombolian mechanisms these trends indicate progressively decreasing partial melting and length scales or magmatic footprints of mantle source zones for individual volcanoes the location of each volcano is determined by the location of its magmatic footprint at depth and only by shallow structural and topographic features that are within that footprint the locations of future volcanoes in a waning system are less likely to be determined by large scale topography or structures than were older larger volume volcanoes

subsurface structure of a maar diatreme and associated tuff ring from a high resolution geophysical survey rattlesnake crater arizona abstract geophysical survey techniques including gravity magnetics and ground penetrating radar were utilized to study the diatreme and tuff ring at rattlesnake crater a maar in the san francisco volcanic field of northern arizona significant magnetic anomalies 1600 nt and a positive gravity anomaly 1 4 mgal are associated with the maar joint modeling of magnetic and gravity data indicate that the diatreme that underlies rattlesnake crater has volume of 0 8 1 km 3 and extends to at least 800 m depth the modeled diatreme comprises at least two zones of variable density and magnetization including a low density highly magnetized unit near the center of the diatreme interpreted to be a pyroclastic unit emplaced at sufficiently high temperature and containing sufficient juvenile fraction to acquire thermal remanent magnetization magnetic anomalies and ground penetrating radar gpr imaging demonstrate that the bedded pyroclastic deposits of the tuff ring also carry high magnetization likely produced by energetic emplacement of hot pyroclastic density currents gpr profiles on the tuff ring reveal long 100 m wavelength undulations in bedding planes elsewhere comparable bedforms have been interpreted as base surge deposits inflated by air entrainment from eruption column collapse interpretation of these geophysical data suggests that rattlesnake crater produced highly energetic phreatomagmatic activity that gave way to less explosive activity as the eruption progressed the positive gravity anomaly associated with the maar crater is interpreted to be caused by coherent bodies within the diatreme and possibly lava ponding on the crater floor these dense magnetized bodies have excess mass of 2 4 10 10 kg and occupy approximately 5 of the diatreme by volume magnetic anomalies on the crater floor are elongate nw se suggesting that the eruption may have been triggered by the interaction of ascending magma with water in fractures of this orientation gpr imaging of the tuff ring also suggests that substantial land slip may have occurred on the western rim perhaps causing part of the tuff ring to collapse into the crater strong radar reflections indicative of well developed weathering horizons are present as well the techniques employed at rattlesnake crater demonstrate the value of combining multiple geophysical techniques in areas where exposures are limited and invasive exploration is not an option

moderate pressure metamorphism and anatexis due to anorthosite intrusion western adirondack highlands new york garnet sillimanite biotite gneiss near port leyden in the western adirondack highlands new york contains mineral assemblages and textures that formed during high temperature metamorphism and anatexis at mid crustal pressures evidence for melting includes thin plagioclase rich veins sieve textures in biotite and the presence of small euhedral garnet neoblasts hercynite silicate equilibria in combination with the solidus for biotite dehydration melting indicate metamorphic pressure was between 4 and 6 4 kbar at the temperature of melting ca 735 c the gneiss is intruded by a small discordant fe ti oxide apatite nelsonite dike reported field occurrences of nelsonite demonstrate its common association with anorthosite plutons although no anorthosite bodies are exposed in the port leyden region the presence of nelsonite is evidence of anorthositic magmatism in the western adirondacks post intrusion metamorphism has caused partial apatite recrystallization and produced a weak foliated texture in the dike u pb ages from zircon and monazite from both the gneiss and the nelsonite dike indicate that these rocks experienced a complex polymetamorphic history that we interpret to reflect two thermal episodes an older event is recorded by discordant zircons in the gneiss which indicate a minimum age of 1129 6 ma a linear best fit to the data yields an upper intercept at 1166 53 ma this range of ages coincides with anorthosite suite magmatism in the adirondacks a minimum zircon age of 1104 3 ma was obtained from the nelsonite dike lead loss or late zircon crystallization at about 1020 ma affected the u pb systematics of zircon in the dike monazite ages from both rocks also indicate high temperature metamorphism 700 c between 1040 and 960 ma the older zircon ages and textural relations in the metapelite are viewed as evidence for anatexis at ca 1150 ma and the presence of nelsonite suggests that the intrusion of anorthosite was coincident with partial melting in the gneiss p t estimates of metamorphism therefore imply that anorthosite was emplaced to about 15 km depth in the western adirondack highlands

synextensional pliocene pleistocene eruptive activity in the camargo volcanic field chihuahua mexico the camargo volcanic field is the largest mafic alkalic volcanic field in the mexican basin and range province and the relationship between volcanism and normal faulting is especially strong the camargo volcanic field lies in the northern part of the province midway between the sierra madre occidental and trans pecos texas it is formed by pliocene pleistocene 4 7 0 09 ma intraplate mafic alkalic volcanic rocks some of which contain peridotite pyroxenite and granulite xenoliths the volcanic field covers 3000 km2 and has an estimated volume of 120 km3 erupted from 300 recognized vents twenty six new 40ar 39ar age determinations for the camargo volcanic field and its environs show that volcanic activity began in the southwest part of the field and shifted toward the northeast at 15 mm yr the average magmatic eruption rate during growth of the field was 0 026 km3 k y the camargo volcanic field lies within an accommodation zone with west dipping faults and east tilted blocks to the north and east dipping faults and west tilted blocks to the south these faults are expressed in the volcanic field by a n30 w trending graben with scarps up to 100 m high through its central part volcanism and faulting were at least in part coeval and younger volcanic products commonly drape fault scarps that cut earlier lavas normal faulting is bracketed between 4 7 and 2 1 ma and may have also migrated northeastward estimated vertical slip rates on four pliocene faults range from 0 03 mm yr a likely long term rate to 1 67 mm yr interpreted as a short term rate operative during periods of active faulting northwest striking normal faults that cut alluvial fan deposits and pleistocene lavas in the northern camargo volcanic field and geomorphic evidence for recent uplift to the south of the volcanic field suggest that the region is still extending

age diversity of the deep crust in northern mexico zircons extracted from five chemically diverse granulite facies crustal xenoliths from la olivina cinder cone chihuahua mexico were analyzed by shrimp ion microprobe in order to determine the age of the deep crust in northern mexico the results demonstrate the extreme age diversity of the deep crust in this region zircon populations of ca 1 25 37 ca 200 350 1100 and 1370 ma are present nearly all samples contain 0 20 ma zircons reflecting recent granulite facies conditions in the deep crust in addition high grade metamorphism may also have occurred at 200 ma and 1 10 ga grenville the paucity of carboniferous permian ages in the xenoliths suggests that the ouachita collision event provided little thermal input to the deep crust beneath la olivina

spatial distribution and development of soils in tropical karst areas from the peninsula of yucatan mexico abstract better understanding of soil formation requires knowing the spatial distribution of the soils that allows constructing models of soil sequences in multiple directions along various types of gradients this approach was applied to comprehend the soil formation from the soil distribution in the tropical karst areas of the peninsula of yucatan mexico for soil mapping a two step methodology was followed first a geomorphic analysis was performed subsequently 382 soil profiles were reclassified and integrated into a geopedologic map additional soil survey was carried out in areas where soil information was lacking 123 soil profiles satellite images were used to identify flooded areas after conducting numerous field verifications and analyses landforms and soils were combined to make a soilscape map based on field observations and the soilscape map soil development was analyzed on soil sequences four geomorphic environments were identified karstic plains and hills with leptosols cambisols luvisols and vertisols coastal plains with arenosols regosols solonchacks and histosols fluvio paludal plains with gleysols histosols leptosols and solonchacks and tectono karstic plains and hills with leptosols cambisols luvisols and vertisols relevant soil forming factors in the peninsula of yucatan include time of emersion of the limestone platforms climate rock type and macro and micro relief other factors such as groundwater level fractures also have an influence on soil formation karst development can be considered as a complex soil and relief forming factor terra rossa soils as leptosols cambisols luvisols nitisols and vertisols in the peninsula of yucatan may be polygenic in some cases the theory of residual origin fits better the data than the theory of allochthonous origin in other cases it is the other way around

transport of magnetite and ilmenite by glaciers in the adirondack mountains of new york till collected from surface exposures within the valley of the hudson river south of the sanford hill magnetite ilmenite ore deposit in the adirondack mountains of new york is composed primarily of plagio clase pyroxene garnet magnetite and ilmenite the concentrations of magnetite and ilmenite in bulk till both decrease exponentially with increasing distance south of the ore bodies because of dilution by the entrainment of plagioclase and garnet pyroxene from the bedrock and from older till deposits in the valley evidence for comminution of magnetite and ilmenite is provided by decreasing abundances of these minerals in the coarse fractions 1000 125 m and corresponding increases in the fine fractions 125 m the apparent rate of comminution of ilmenite as a function of transport distance is significantly greater than that of magnetite which causes ilmenite to be concentrated in the fine fraction of till compared to magnetite the weight ratios of magnetite and ilmenite in the fine to coarse

lithofacies and architecture of a basinwide tuff unit in the miocene eoil basin se korea modes of pyroclastic sedimentation changes in eruption style and implications for basin configuration the miocene eoil basin se korea is a small half graben basin that was rifted by both extensional and dextral strike slip deformations during backarc opening of the east sea sea of japan the basin was filled by fluviolacustrine sediments and abundant basaltic and dacitic volcanic deposits the paljori tuff is a 2 20 m thick basinwide dacitic volcaniclastic unit that is intercalated in the topmost part of a fluviolacustrine formation the tuff consists of two pyroclastic units units ii and iii which are underlain and overlain by two resedimented volcaniclastic units units i and iv unit i at the base is composed mainly of fine grained resedimented tephra unit ii is a massive and stratified tuff produced by a dense pyroclastic density current that was ponded in the southwestern part of the basin unit iii is a massive lapilli tuff emplaced by a voluminous and turbulent pyroclastic density current that swept across the basin toward the northeast without ponding or blocking by intrabasinal highs and basin margin relief and unit iv at the top is composed of coarse grained resedimented tephra the general coarsening up of these units the lack of nonvolcaniclastic sedimentary interbeds and the offset stacking of the units toward the northeast suggest an episodic short duration eruption from the southwest that waxed in mass eruption rate and changed eruption style from phreatomagmatic to magmatic these units show basin scale variations in geometry including 1 thick accumulations of unit i in the northeastern and southwestern extremities of the basin 2 ponding of unit ii in the southwestern part of the basin 3 truncation of unit iii in the same area and 4 thickening of unit iv toward the northeastern basin margin these provide important clues to the distribution of and temporal changes in accommodation space and hence the configuration and structural evolution of the eoil basin it is inferred that the greatest subsidence occurred at the southwestern and northeastern corners of the basin the southwest northeast trending axes of the basin plunge gently toward the northeast and the major intrabasinal faults of the basin were produced before the eruption of the paljori tuff the basinwide preservation of the tuff and its fairly uniform thickness suggest that the basin was undergoing rapid extension and subsidence because of the onset of the rift climax the paljori tuff shows that important clues to and high resolution records of the large scale configuration and structural evolution of a sedimentary basin can be obtained from the study of lithofacies variations and the architecture of a single basinwide volcaniclastic unit

eruption of alkali basalts during crustal shortening in southern california the common occurrence of alkali basalts in continental rifts has led to the widespread assumption that late cenozoic alkali basalts of the western united states indicate an extensional tectonic regime however field relations of late cenozoic alkali basalt flows and cinder cones in the mojave desert area including flows that bear mantle xenoliths indicate that many were erupted in contractional or transpressional tectonic settings these observations call into question models of extensional magmatism in which lithospheric extension facilitates transport of magmas to the surface and suggest that an association of alkali basalts and coarse clastic sedimentary rocks need not indicate rifting

controls on volcanism at intraplate basaltic volcanic fields a broad range of controlling mechanisms is described for intraplate basaltic volcanic fields ibvfs in the literature these correspond with those relating to shallow tectonic processes and to deep mantle plumes accurate measurement of the physical parameters of intraplate volcanism is fundamental to gain an understanding of the controlling factors that influence the scale and location of a specific ibvf detailed volume and geochronology data are required for this however these are not available for many ibvfs in this study the primary controls on magma genesis and transportation are established for the pliocene recent newer volcanics province nvp of south eastern australia as a case study for one of such ibvf the nvp is a large and spatio temporally complex ibvf that has been described as either being related to a deep mantle plume or upper mantle and crustal processes we use innovative high resolution aeromagnetic and 3d modelling analysis constrained by well log data to calculate its dimensions volume and long term eruptive flux our estimates suggest volcanic deposits cover an area of 23 100 530 km2 and have a preserved dense rock equivalent of erupted volcanics of least 680 km3 and may have been as large as 900 km3 the long term mean eruptive flux of the nvp is estimated between 0 15 and 0 20 km3 ka which is relatively high compared with other ibvfs our comparison with other ibvfs shows eruptive fluxes vary up to two orders of magnitude within individual fields most examples where a range of eruptive flux is available for an ibvf show a correlation between eruptive flux and the rate of local tectonic processes suggesting tectonic control limited age dating of the nvp has been used to suggest there were pulses in its eruptive flux which are not resolvable using current data these changes in eruptive flux are not directly relatable to the rate of any interpreted tectonic driver such as edge driven convection however the nvp and other ibvfs used for comparison have long term eruptive fluxes that are considerably less than definitive plume related volcanic systems along with their spatio temporal patterns and other analysis it is suggested that the nvp and the vast majority of low and high flux ibvfs appear to be the result of tectonic processes without requiring additional thermal input from a deep mantle source considering a control on volcanism by tectonic processes the range of eruptive flux of ibvfs is related to variations in the rate of the effecting tectonic process mantle composition and the size of the mantle source zone where melt generation and accumulation is taking place

spatial distribution and alignments of volcanic centers clues to the formation of monogenetic volcanic fields monogenetic basaltic volcanic fields occur worldwide in tectonic environments ranging from extensional to convergent understanding similarities and differences between these fields may help to characterize key controls on their generation such volcanic fields consist of numerous volcanic centers each of which represents a pathway of magma from its source to the surface we analyzed the spatial distribution of volcanic centers in 37 monogenetic volcanic fields and assuming that the distribution of volcanic centers relative to each other is matched by a similar source pattern within the mantle applied the following methods for each 1 the poisson nearest neighbor pnn analysis representing the degree to which the distribution of the volcanic centers departs from a predicted poisson distribution and 2 a volcanic alignment analysis to ascertain the preferential pathways if any used by the magma to reach the surface this is the first comprehensive global comparison of such analyses magma pathways within the brittle upper crust are influenced to various degrees by two end member situations 1 formation of new extension fractures perpendicular to the least compressive stress 3 and 2 re activation of pre existing fractures that are near parallel to the maximum principal stress 1 the results of the pnn analysis show that independently of the tectonic environment most volcanic fields display a clustered distribution of their volcanic centers alignment analysis shows that either the ambient tectonic environment exerts a strong influence on the preferential orientations of the volcanic alignments or that it is in competition with other factors e g pre existing structures local stress changes due to older intrusions overall these results indicate that the propagation of the magma and therefore the spatial distribution of the volcanic centers within volcanic fields is the product of an interplay between deep level influences i e nature of the magma source and shallow level influences i e stress field pre existing crustal structures of the lithosphere

is there a close spatial relationship between faults and plutons abstract using spatial relationships between individual plutons and faults to support genetic correlations between faulting and magmatism is meaningless since even random magmatic or tectonic processes will result in some plutons adjacent to faults our initial analyses of populations of faults and carboniferous plutons in the armorican massif france and faults and alleghanian plutons in the southern appalachians usa indicate that plutons have broad distributions with respect to faults but with a tendency for plutons to occur away from faults maxima of integrated pluton areas occur at 1 4 appalachians and 1 2 armorican of the distance between the average fault spacing in these orogens although there is a great need for statistical evaluations of relationships between populations of igneous bodies and structures in a wide variety of settings and crustal depths our initial studies suggest that faults do not preferentially channel magma during ascent or emplacement and that these are relatively unfocused processes within orogenic belts

calcareous fossil inclusions and rock source of maya lime plaster from the temple of the inscriptions palenque mexico abstract this paper introduces for the first time the study and description of fossil remains preserved in lime plaster from a maya site the temple of the inscriptions in palenque one of the most outstanding classic period cities of the maya culture in mesoamerica plaster was removed with edta carbonate digestive technique and results based on ultrastructural microanalysis reveals marine fossil remains of shells and calcareous debris correlated with organisms of several taxa including foraminifera gastropod bivalvia and cnidarian this fossil material is associated with several tertiary fossil bearing strata in the surroundings of palenque additionally results of wdx testing on white lumps of plaster show a characteristic mgo cao ratio consistent with a dolomitic limestone source linked to the tenejapa geological unit paleocene plaster of the temple of the inscriptions is best described as a dolomitic lime plaster calcareous fossil material in plaster mixtures is present mainly as micro aggregates and lime dust such material may come from a fossil bearing rock intentionally selected that was burned and crushed this also suggests that low temperatures can cause burns to incomplete calcination of rocks and consequently that fossils can preserve their recognizable morphological features

isotopic constraints on emplacement age of anorthositic rocks of the marcy masiff adirondack mts new york the igneous emplacement age of anorthositic rocks comprising the marcy massif have been constrained by fixing minimum and maximum ages for their intrusion minimum ages have been fixed by a u pb dating of cores of air abraded zircons from the anorthosite 1113 ma b u pb dating of baddeleyite within the anorthosite 1087 ma c u pb dating of baddeleyite from an olivine metagabbro crosscutting marcy massif anorthosite 1109 ma d u pb zircon dating of jotunitic rocks crosscutting the marcy anorthosite massif 1143 pm 31 ma maximum emplacement ages are based upon a sheets of anorthositic rock crosscutting mangeritic charnockitic gneiss dated by u pb zircon at 1125 pm 10 ma 1134 pm 4 ma and 1135 pm 43 ma and b anorthosite related pyroxenite dikes crosscutting mangerite that contains xenoliths of identical pyroxenite these results constrain emplacement of the marcy anorthosite to 1113 1138 ma with the most probable age in the interval 1125 1135 ma this time of in