recurrent quaternary magma generation at baekdusan changbaishan volcano new zircon u th ages and hf isotopic constraints from the millennium eruption abstract it is well known that baekdusan changbaishan an active 2744 m high stratovolcano located on the border between north korea and china initiated trachytic commenditic eruption in the pleistocene but the petrogenetic and temporal evolutions of this alkaline magma system remain insufficiently constrained the so called millennium eruption me in the mid 10th century which resulted in the formation of the world s highest caldera lake cheonji or tianchi atop the mountain was one of the largest and most violently explosive volcanic eruptions to have occurred on earth in the past 2000 years here we present u th disequilibrium ages and hf isotopic compositions of zircons from trachydacitic pumices ejected during the me taken together with the previous u th isotope data our laser ablation multicollector icpms analyses of the zircons yielded multiple age components consisting of an earliest holocene cluster 11 3 1 3 ka and middle late pleistocene populations ca 230 and 110 ka which indicates that baekdusan magma has been generated episodically at intervals of ca 120 100 ky if the eruption ages assigned for the samples analyzed in the previous isotopic studies are correct the consistent age pattern of zircon may suggest the persistent existence of the same magmatic plumbing system beneath baekdusan since the me the wide range of zircon ehf values 5 8 to 3 5 reflects an interaction between the em1 type primitive magma and pre existing crustal materials evidenced by the presence of a xenocrystic zircon core showing a significantly negative ehf value 21 a broad negative correlation observed between zircon ehf and the published 18o values can be ascribed to the progressive assimilation of weathered supracrustal rocks likely in the course of magma chamber expansion

prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

laser ablation coupled with icp ms applied to u pb zircon geochronology a review of recent advances abstract improvements in the technology of laser ablation and icp ms instruments make la mc icpms a rapid precise and accurate method for u pb zircon geochronology in this review we describe the main stages of the evolution of this in situ approach from the early 1990s to the present time some key points have been progressively improved the crater size has been reduced to achieve real in situ measurements the laser wavelength has been reduced as well as the duration of each pulse in order to lower inter element fractionation the blank from the gas has to be lowered as far as possible double focusing instruments and magnetic field sectors allow flat topped peaks required for precise isotope ratio measurement to be obtained the use of a multi ion counting system significantly improves the sensitivity of the method and the static mode of integration favours the precision of measurement of the transient signal originating from a noisy laser ablated particle beam combining the use of a 213 nm uv laser and a mc icpms equipped with a multi ion counting system operating in static mode the common precisions achieved for the key ratios 207 pb 206 pb and 206 pb 238 u are better than 1 and 3 2 respectively including error propagation associated with standard normalization until now the use of a zircon standard has remained necessary to ensure the accuracy of the calculated age a strategy for common pb correction is proposed according to the age of the zircon and according to the th u ratio of the grains after recording sixteen to twenty spot analyses the precision usually achieved on the age is about 1 and even significantly better for proterozoic samples in order to show the performance achieved by modern la mc icpms geochronology we tested four zircon samples covering a wide age range from 290 to 2440 ma these new age determinations can be compared in term of precision and accuracy since they have already been dated by reference methods id tims and shrimp further developments in the technology of ion counters equipping modern mc icpms and in laser systems will certainly be applied to a large field of geochronology studies in the near future as an alternative to sims for in situ age determination

late pleistocene granodiorite beneath crater lake caldera oregon dated by ion microprobe variably melted granodiorite blocks ejected during the holocene caldera forming eruption of mount mazama were plucked from the walls of the climactic magma chamber at 5 km depth ion microprobe u pb dating of zircons from two unmelted granodiorite blocks with shrimp rg sensitive high resolution ion microprobe reverse geometry gives a nominal 238 u 206 pb age of 100 78 80 ka or 174 89 115 ka when adjusted for an initial 230 th deficit shrimp rg u th measurements on a subset of the zircons yield a 230 th 238 u isochron age of 112 24 ka considered to be the best estimate of the time of solidification of the pluton these results suggest that the granodiorite is related to andesite and dacite of mount mazama and not to magmas of the climactic eruption the unexposed granodiorite has an area of at least 28 km 2 this young shallow pluton was emplaced in virtually the same location where a similarly large magma body accumulated and powered violent explosive eruptions 7700 yr ago resulting in collapse of crater lake caldera

improved spatial resolution for u series dating of opal at yucca mountain nevada usa using ion microprobe and microdigestion methods two novel methods of in situ isotope analysis ion microprobe and microdigestion were used for 230th u and 234u 238u dating of finely laminated opal hemispheres formed in unsaturated felsic tuff at yucca mountain nevada proposed site for a high level radioactive waste repository both methods allow analysis of layers as many as several orders of magnitude thinner than standard methods using total hemisphere digestion that were reported previously average growth rates calculated from data at this improved spatial resolution verified that opal grew at extremely slow rates over the last million years growth rates of 0 58 and 0 69 mm m y were obtained for the outer 305 and 740 m of two opal hemispheres analyzed by ion microprobe and 0 68 mm m y for the outer 22 m of one of these same hemispheres analyzed by sequential microdigestion these pleistocene growth rates are 2 to 10 times slower than those calculated for older secondary calcite and silica mineral coatings deposited over the last 5 to 10 m y dated by the u pb method and may reflect differences between miocene and pleistocene seepage flux the microdigestion data also imply that opal growth rates may have varied over the last 40 k y these data are the first indication that growth rates and associated seepage in the proposed repository horizon may correlate with changes in late pleistocene climate involving faster growth during wetter cooler climates glacial maximum slower growth during transition climates and no growth during the most arid climate modern data collected at this refined spatial scale may lead to a better understanding of the hydrologic variability expected within the thick unsaturated zone at yucca mountain over the time scale of interest for radioactive waste isolation

late pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at crater lake oregon abstract rhyodacite tephra and three lavas erupted 27 ka interpreted to be early leaks from the climactic magma chamber of mount mazama contain ubiquitous resorbed crystals antecrysts that were recycled from young granodiorite and related plutonic rocks of the same magmatic system the shallow composite pluton is represented by blocks ejected in the 7 7 ka climactic eruption that formed crater lake caldera plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented fe oxide needles exsolved at subsolidus conditions at least 80 of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks other crystals in the rhyodacite notably zircon also were recycled sims 238 u 230 th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between 20 ka and 300 ka with concentrations of analyses near 50 70 110 and 200 ka that correspond to periods of dacitic volcanism dated by k ar u th ages of zircon from a rhyodacite sample yield similar results no analyzed zircons from the granodiorite or rhyodacite are pre quaternary zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton survival of zircon in zircon undersaturated hydrous magma and of fe oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the 27 ka eruption of the rhyodacite the 27 ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high level magma chamber in contrast the voluminous 7 7 ka climactic rhyodacite pumice is virtually lacking in zircon indicating dissolution of any granodioritic debris in the intervening period mineralogical evidence of assimilation may be destroyed in hot vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of mount mazama

post caldera volcanism in situ measurement of u pb age and oxygen isotope ratio in pleistocene zircons from yellowstone caldera the yellowstone plateau volcanic field the site of some of the largest known silicic volcanic eruptions is the present location of ne migrating hotspot volcanic activity most volcanic rocks in the yellowstone caldera 0 6 ma which formed in response to the climactic eruption of 1000 km 3 of lava creek tuff lct have unusually low oxygen isotope ratios ion microprobe analysis of both u pb age and n 18 o in zircons from these low n 18 o lavas reveals evidence of complex inheritance and remelting a majority of analyzed zircons from low n 18 o lavas erupted inside the yellowstone caldera have cores that range in age from 2 4 to 0 7 ma significantly older than their eruption ages 0 5 0 4 ma these ages and the high n 18 o cores indicate that these lavas are largely derived from nearly total remelting of normal n 18 o huckleberry ridge tuff hrt and other pre lct volcanic rocks a post hrt low n 18 o lava shows similar inheritance of hrt age zircons the recycling of volcanic rocks by shallow remelting can change the water content and eruptive potential of magma this newly proposed mechanism of intracaldera volcanism is best studied by combining in situ analysis of oxygen and u pb isotope ratios of individual crystals fl 2001 elsevier science b v all rights reserved

gis techniques and statistical models in evaluating landslide hazard geographical information systems gis and numerical cartography may greatly facilitate the development and use of statistical models for the assessment of regional landslide hazard from a small drainage basin located in central italy relevant geological and geomorphological factors were collected and processed by applying gis technology in particular modules were used which both generate high fidelity digital terrain models and automatically partition the terrain into main slope units the resulting information was then analysed by discriminant analysis which enabled landslide hazard and risk to be evaluated in each slope unit although not lacking drawbacks the method proved to be a feasible and cost effective approach to landslide susceptibility assessment and mapping

la mc icpms and shrimp u pb dating of complex zircons from quaternary tephras from the french massif central magma residence time and geochemical implications abstract analyses of zircon grains from the queureuilh quaternary tephras pumice provide new information about their pre eruptive history u pb dating was performed in situ using two methods shrimp and la mc icpms equipped with a multi ion counting system both methods provided reliable 207pb 206pb and 206pb 238u ratios as well as u and th abundances required for u pb concordia intercept age determination after initial 230th disequilibrium correction the new la mc icpms method was validated by dating a reference zircon 61 308b and zircons from a phonolitic lava dated independently with the two techniques a time resolution of about 20 kyr for 1 ma zircon crystals was achieved for both methods the clear euhedral zircon population from queureuilh tephras is quite complex from several points of view 1 some grains are reddish or yellowish while others are colorless 2 the u and th composition changes by more than an order of magnitude and th u is generally high 1 2 3 there are three discrete ages recorded at 2 35 0 04 1 017 0 008 and 0 640 0 010 ma from the previously determined 40ar 39ar age at 0 571 0 060 ma duffell h 1999 contribution geochronologique a la stratigraphie volcanique du massif des monts dore par la methode 40ar 39ar d e a univ clermont ferrand 56 p the discontinuous zircon age populations the color of the grains and their composition we favor the following model as explanation the oldest less numerous group of reddish zircons represents xenocrystic grains resulting from assimilation of the local material during magma ascent a primitive magma chamber perhaps deep in crustal level was formed at 1 0 ma the related magma previously characterized by high th u ratio 2 2 1 1 underwent rejuvenation during ascent to a new chamber at shallow depth and or during injection of more mafic magmas during this stage at 0 64 ma the colorless zircon grains of lower th u ratio 1 3 0 5 crystallized this last stage defined the magma residence time of 70 kyr prior to eruption dated by the 40ar 39ar method however if the primitive magma is considered the magma residence time as a whole from this first stage reached 446 kyr in the light of the complex history of such magmas which commonly involves recycling of zircon grains that precipitated tens to hundreds of kyr earlier than eruptions the use of zr concentration in geochemical modeling of whole rock compositional data can be problematic

zircon u th pb dating using la icp ms simultaneous u pb and u th dating on the 0 1 ma toya tephra japan abstract simultaneous u pb and u th zircon dating using laser ablation inductively coupled plasma mass spectrometry la icp ms was performed on the 0 1 ma toya tephra hokkaido japan the method employs the concurrent measurement of 206pb 230th and 238u signals which enables the determination of 238u 206pb u pb and 238u 230th u th ages simultaneously the u pb ages revealed that the toya tephra contains zircons with ages that cluster at 0 1 ma 0 11 0 01 ma error shown as 95 confidence level and xenocrysts with ages 2 ma excluding zircons 2 ma the u th method also gave an isochron age of 0 1 ma 108 19 ka or 0 11 0 02 ma error shown as 2 therefore both u pb and u th methods result in similar ages of 0 1 ma for the toya tephra in agreement with the quartz tl age 104 30 to 118 30 ka of ganzawa and ike 2011 it was revealed that the simultaneous u pb and u th zircon dating technique using la icp ms is easy to apply and is useful for quaternary tephras in that it can give age information on the tephra itself and xenocrystic zircons in a quick and cost effective manner the double dating technique employed here has the merit that zircons yielding quaternary u pb ages can be immediately cross checked by the u th method it was also revealed that the toya tephra magma crystallized zircons and experienced eruptions instantaneously at 0 1 ma which is an important implication to decipher magmatic and eruptive processes for caldera forming large volcanic systems

u pb dating of discordant 0 1 ma old secondary u minerals abstract a modified tera wasserburg u pb concordia plot which takes care of initial disequilibrium of radioactive daughter nuclides has been applied to date discordant torbernite samples from hohenstein northeast bavaria to 137 11ka lower intercept and 66 27ma upper intercept the secondary u pb age is supported by the 230 th 234 u disequilibrium age of 132 4 ka

towards a definition of topographic divisions for italy abstract the study has divided italy into eight major topographic divisions provinces and 30 minor divisions sections that are similar to n fenneman s units of the united states provinces and sections are generally consistent with observed morphology the new units were defined step wise in a semi quantitative approach that combined an unsupervised three class cluster analysis of four derivatives of altitude visual interpretation of morphometric maps including shaded relief and comparative inspection of small scale geological and structural maps the four variables are altitude slope curvature frequency of slope reversal and elevation relief ratio highland upland and lowland topographic types were identified the variables were computed from a 230 m resolution digital terrain model regridded from the original 230 m dem obtained by mosaicking the entire italian mean elevation archive

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

zircon growth and recycling during the assembly of large composite arc plutons abstract concordant u pb zircon dates have been interpreted traditionally to date the crystallization ages of plutons because until recently analytical uncertainties have generally been large enough to encompass the anticipated duration of pluton growth advances in zircon tims analysis and geochronological studies by sims on zircon from young volcanic rocks along with evolving views of the evolution of magmatic systems makes it necessary to revaluate this interpretation assignment of unique pluton crystallization ages based on zircon dating is complicated by the propensity of zircon to survive multiple intrusive events that culminate in a large pluton distinctions must be made between true inherited zircon xenocrystic zircon derived from host rocks during magma ascent and or emplacement and multi stage growth of zircon during waxing and waning of magma reservoirs that solidify to form large plutons these complications are exemplified by recent high precision u pb zircon age dating from the cretaceous tuolumne and mt stuart batholiths which clearly show that the timeframe of pluton assembly was long 5 ma for mt stuart and 8 ma for tuolumne zircon crystals from samples from both batholiths exhibit appreciable concordant age dispersion for zircon several 10 5 yr up to 2 10 6 yr and assignment of statistically valid rock solidification ages is not possible from these data low zr in the rocks of each batholith indicates that magmas were initially strongly undersaturated in zircon when emplaced and inherited zircon is rare or absent recycling of zircon antecrysts during successive magmatic injections is the primary cause of the modest age dispersion of concordant zircon ages and is compatible with progressive growth of a large long lived crystal mush body because eruption of magma requires low to moderate crystal fraction and high temperatures relative to stored crystal mushes zircon crystals are more likely to be eradicated during thermal rejuvenation preceding major eruptions

residence resorption and recycling of zircons in devils kitchen rhyolite coso volcanic field california zircons from the devils kitchen rhyolite in the pleistocene coso volcanic field california have been analyzed by in situ pb u ion microprobe shrimp rg and by detailed cathodoluminescence imaging the zircons yield common pb corrected and disequilibrium corrected pb u ages that predate a previously reported k ar sanidine age by up to 200 kyr and the range of ages exhibited by the zircons is also approximately 200 kyr cathodoluminescence imaging indicates that zircons formed in contrasting environments most zircons are euhedral and a majority of the zircons are weakly zoned but many also have anhedral embayed cores with euhedral overgrowths and multiple internal surfaces that are truncated by later crystal zones concentrations of u and th vary by two orders of magnitude within the zircon population and by 10 20 times between zones within some zircon crystals indicating that zircons were transferred between contrasting chemical environments a zircon saturation temperature of 750 c overlaps within error a previously reported phenocryst equilibration temperature of 740 25 c textures in zircons indicative of repeated dissolution and subsequent regrowth are probably caused by punctuated heating by mafic magma input into rhyolite the overall span of ages and large variation in u and th concentrations combined with calculated zircon saturation temperatures and resorption times are most compatible with crystallization in magma bodies that were emplaced piecemeal in the crust at coso over 200 kyr prior to eruption and that were periodically rejuvenated or melted by subsequent basaltic injections

quaternary plutonic magma activities in the southern hachimantai geothermal area japan inferred from zircon la icp ms u th pb dating method abstract in order to constrain the timing of plutonic magma emplacements relevant to current geothermal activity in the hachimantai geothermal area northeast japan laser ablation inductively coupled plasma mass spectrometry la icp ms u th pb analyses were performed on zircons from two different origins one is from the quaternary kakkonda granite in the kakkonda geothermal field whose present temperature at 3700 m depth is 500 c and the other is from a clastic dike in the matsukawa geothermal field zircons from the kakkonda granite were obtained at a depth of 1998 m and their 238u 206pb and u th ages showed 0 1 ma which probably indicates that the kakkonda granite is the world s youngest emplaced granitic magma body ever confirmed the clastic dike in the matsukawa geothermal field contains dioritic to tonalitic xenoliths therefore zircons extracted from the dike should also be of plutonic origin 238u 206pb age of 1 08 0 08 ma after correction for initial 230th disequilibrium error shown as 95 confidence level was obtained from the zircons therefore it was found that there were at least two quaternary plutonic magma activities 1 1 ma and 0 1 ma in the hachimantai geothermal area through this study it was also found that u th pb dating using a quadrupole type icp ms coupled with excimer laser is applicable to quaternary zircons as young as 0 1 ma and fish canyon tuff zircon is usable as a standard material for u th pb dating method

uranium series accessory crystal dating of magmatic processes complex and protracted crystallization histories over geologic timescales are recorded in accessory minerals e g zircon allanite although magmatic crystallization was traditionally assumed to occur essentially instantaneously for the purposes of interpreting mineral geochronometers with low absolute time resolution for ancient samples it emerged relatively recently that magmatic crystallization can occur over extended durations this discovery arose from applying high spatial resolution accessory mineral dating techniques for uranium series isotopes to young volcanic and cognate plutonic rocks the emerging pattern from these studies is that individual crystals and crystal populations record crystallization episodes lasting from 1 000 to many hundreds of thousands of years accessory mineral dating of volcanic rocks and cognate plutonic xenoliths opens new research avenues for crystal age fingerprinting that correlates pyroclastic deposits lavas and plutonic rocks by using characteristic age dis

a tale of two eras pliocene pleistocene unroofing of cenozoic and late archean zircons from active metamorphic core complexes solomon sea papua new guinea u pb ion microprobe analyses of zircons from gneisses and granodiorites exposed in the d9entrecasteaux islands and from conglomerate sections of the goodenough no 1 well in the adjacent trobriand basin provide constraints on the age of magmatism peak metamorphism and nature of rocks unroofed during initial stages of metamorphic core complex formation in the solomon sea the youngest populations of zircons from felsic gneisses and granodiorites indicate late pliocene 206 pb 238 u ages no inherited zircons were identified in the granodiorites and the 206 pb 238 u ages 1 65 0 18 ma 1 98 0 08 ma 2 are interpreted as crystallization ages these synkinematically emplaced granodiorites intruded into actively extending continental crust are some of the youngest known granitoids currently exposed at the earth9 surface zircon ages from felsic gneisses 2 63 0 16 ma 2 72 0 28 ma 2 are interpreted to date zircon growth subsequent to eclogite facies metamorphism felsic gneiss samples also contained zircon xenocrysts from cretaceous miocene protoliths in striking contrast zircons from igneous and metamorphic clasts from the goodenough no 1 well indicate a single population with a 207 pb 206 pb age of 2781 9 ma 2 we speculate that they are derived from basement rocks unroofed during initial stages of development of the d entrecasteaux metamorphic core complexes these results provide the first direct evidence for the existence of archean protoliths in the basement rocks of southeastern papua new guinea

age and thermal history of the geysers plutonic complex felsite unit geysers geothermal field california a 40ar 39ar and u pb study abstract sixty nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the geysers geothermal field yield 207pb 206pb vs 238u 206pb concordia ages ranging from 1 13 0 04 ma to 1 25 0 04 1 ma the weighted mean of the u pb model ages is 1 18 0 03 ma the u pb ages coincide closely with 40ar 39ar age spectrum plateau and terminal ages from coexisting k feldspars and with the eruption ages of overlying volcanic rocks the data indicate that the granite crystallized at 1 18 ma and had cooled below 350 c by 0 9 1 0 ma interpretation of the feldspar 40ar 39ar age data using multi diffusion domain theory indicates that post emplacement rapid cooling was succeeded either by slower cooling from 350 to 300 c between 1 0 and 0 4 ma or transitory reheating to 300 350 c at about 0 4 0 6 ma subsequent rapid cooling to below 260 c between 0 4 and 0 2 ma is in agreement with previous proposals that vapor dominated conditions were initiated within the hydrothermal system at this time heat flow calculations constrained with k feldspar thermal histories and the present elevated regional heat flow anomaly demonstrate that appreciable heat input from sources external to the known geysers plutonic complex is required to maintain the geothermal system this requirement is satisfied by either a large underlying convecting magma chamber now solidified emplaced at 1 2 ma or episodic intrusion of smaller bodies from 1 2 to 0 6 ma

timescales of crustal magma reservoir processes insights from u series crystal ages the dynamic processes operating within crustal magma reservoirs control many aspects of the chemical composition of erupted magmas and crystals in volcanic rocks provide a temporally constrained archive of these changing environments in this review i compile 238 u 230 th ages of accessory phases and 238 u 230 th 226 ra ages of bulk mineral separates of major phases these data document that crystals in individual samples can have ages spanning most of the history of a volcanic centre age populations for accessory phases show protracted pre eruptive crystal residence times but few crystals predate magmatic activity at a given centre these data have been interpreted in the context of residence times of the host magmas or timescales of the storage of crystals within a largely crystalline portion of the reservoir system in contrast less than half of the bulk separate 238 u 230 th 226 ra ages for major phases are more than 10 kyr older than the eruption many of these apparently conflicting observations of ages of major and accessory phases can be reconciled within the context of a model where a crystal mush was remobilized during processes leading to eruption overall the compiled data show that crystals contain rich archives of magmatic processes in crustal reservoirs especially when combined with other crystal scale geochemical data

second reply to comment of r s j sparks h e huppert and c j n wilson on evidence for long residence times of rhyolitic magma in the long valley magmatic system the isotopic record in the precaldera lavas of glass mountain

u pb zircon ages and pb isotope geochemistry of gold deposits in the carolina slate belt of south carolina volcanic rocks of the persimmon fork formation host the largest known gold mines of the carolina slate belt u pb shrimp zircon ages have been obtained from rocks closely associated with pyrite enargite gold deposits at brewer quartz topaz rhyolite breccia from the argillic alteration zone in the brewer pit and felsic ash flow tuff from the quartz sericite alteration zone from the disseminated and semimassive pyrite gold deposits at haile crystal lithic rhyolitic ash flow tuffs from the champion pit and from the ridgeway deposit felsic ash flow tuff from the stratigraphic host of the north pit gold deposit generally the zircons are fine grained fractured and contain crystal imperfections corrosion inclusions and pits 206 pb 238 u zircon spot ages for all deposits span a wide range mostly from 400 to 760 ma inclusions and cores indicative of inherited domains in the zircons were not found and only a few analyses range from 1 1 to 1 8 ga a distinct xenocrystic zircon population was not identified the 206 pb 238 u weighted age averages of zircon indicate the following crystallization dates for the volcanic and volcaniclastic rocks closely associated with the gold deposits 550 3 ma for brewer 553 2 ma for haile and 556 2 ma for the ridgeway deposit these zircon crystallization ages represent close estimates of the age of the original gold mineralizing events younger zircon spot ages can be attributed to the effects of paleozoic regional metamorphism pb isotope compositions of sulfide minerals galena pyrite enargite sphalerite chalcopyrite and molybdenite and silicate minerals k feldspar and sericite in the gold deposits help to constrain the sources of fluids and metals during the mineralizing events the deposits are pyrite rich containing multiple generations of pyrite including early crystallized pyrite that is closely associated with the original gold mineralizing event as well as recrystallized pyrite formed in response to paleozoic metamorphism pb isotope compositions of pyrite span a wide range including the most radiogenic values for the sulfides galena and k feldspar are not abundant but where present they are typically the least radiogenic minerals galena has a limited range of pb isotope compositions that are representative of the gold deposits as a group 206 pb 204 pb 18 020 18 326 207 pb 204 pb 15 550 15 639 208 pb 204 pb 37 605 38 286 values of 207 pb 204 pb straddle the average crustal pb growth curve consistent with contributions involving the mantle and continental crust whole rock pb isotope compositions of volcanic and volcaniclastic rocks of the persimmon fork formation nearly match the range for sulfides in the gold deposits subtle regional contrasts in pb isotope compositions exist among the deposits sulfide minerals from barite hill e g galena 206 pb 204 pb 206 pb 204 pb 18 169 haile 206 pb 204 pb 18 233 and brewer 206 pb 204 pb 18 311 in northern south carolina because pb isotope compositions of basement rocks from grenville massifs in the southern appalachians and sulfide minerals from the gold deposits do not match a direct genetic connection cannot be established diversity in values of 206 pb 204 pb and the relatively high values of 207 pb 204 pb suggest that the deposits evolved adjacent to or closely related to continental blocks perhaps linked to a back arc tectonic setting among potential younger analogues of the slate belt gold deposits are the sulfide deposits of the okinawa trough in the western pacific mantle derived isotopic contributions were more important at barite hill in southern south carolina the least radiogenic among the deposits where oceanic crust had developed than at brewer haile and ridgeway in northern south carolina where rifting thinned the continental crust

u pb zircon and geochemical evidence for bimodal mid paleozoic magmatism and syngenetic base metal mineralization in the yukon tanana terrane alaska new shrimp sensitive high resolution ion microprobe u pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic yukon tanana terrane in east central alaska help define the tectonic setting of mid paleo zoic magmatism and syngenetic hydrothermal zn pb ag mineralization along the ancient pacific margin of north america we compare data from similar greenschist facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks in the wood river area of the alaska range and the salcha river area of the yukon tanana upland and from amphibolite facies augen gneiss and mafic gneiss amphibolite in the goodpaster river area of the upland allowing for analytical uncertainties igneous crystallization age ranges of 376 353 ma 378 346 ma and 374 358 ma are indicated by 13 new shrimp u pb dates for the wood river salcha river and goodpaster river areas respectively bimodal magmatism is indicated by late devonian crystallization ages for both augen gneiss 371 3 and 362 4 ma and associated orthoamphibolite 369 3 ma in the upland and by stratigraphic interleaving of mafic and felsic rocks in the alaska range metabasites in all three study areas have elevated hfse high field strength element and ree rare earth element contents indicative of generation in a within plate extensional tectonic setting within plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the bonnifield district in the wood river area and for metarhyolite tuff interlayered with the carbonaceous nasina assemblage which hosts sedimentary exhalative sulfide occurrences in the salcha river area most of the other felsic metaigneous samples from the alaska range and the yukon tanana upland have geochemical signatures that are similar to those of both average upper continental crust and continental margin arc rocks generated in thick continental crust given the absence in our study areas of intermediate composition magmatic products generally found in most arcs and the presence of bimodal magmatism the alkalic within plate chemistry of the mafic rocks and some of the felsic rocks and the widespread occurrence of interlayered carbonaceous sedimentary rocks indicative of deposition within a restricted marine basin or submerged continental margin we consider it most likely that this prolonged late devonian to early mississippian magmatic episode resulted from attenuation of the ancient continental margin of western north america rather than development of an arc as proposed by many others

age of granitic activity associated with copper molybdenum mineralization at malanjkhand central india the malanjkhand copper molybdenum deposit in the bhandara craton central india is hosted by a granite complex which consists of regionally dominant grey granitoid and pink granitoid confined to the mineralized zone new shrimp rg data on zircons from both granite types are inferred to have crystallized during the same magmatic pulse at ca 2 48 ga the discrepancy between zircon age and earlier obtained rb sr whole rock age is attributed to modification of the rb sr system by hydrothermal overprint similarity in petrographic features and chemical affinity in combination with identical age strongly indicate that the pink granite is the hydrothermally altered variety microclinization and silicification of the grey granite the spatially associated main cu mo mineralization event at malankhand appears to be broadly contemporaneous with and genetically related to the emplacement of the host granitoids at about 2 48 ga

volcanic framework of the pliocene el dorado low sulfidation epithermal gold district el salvador the pliocene el dorado low sulfidation epithermal au ag vein system located within an 84 km2 district in north central el salvador is hosted by volcanic rocks formed along the caribbean plate margin in response to the subduction of the cocos plate during the tertiary the principal volcanic host rocks to epithermal veins and effective basement in the district consist of a 400 m thick sequence of basaltic to andesitic lava flows and volcanogenic sedimentary rocks locally porphyritic basaltic to andesitic domes and dikes intruded this sequence during the late miocene 10 7 1 9 ma the mineralized volcanic basement rocks outcropping mainly in north and central el dorado represent different parts of a complex volcanic sequence composed of multiple superimposed volcanic centers vein formation in the district occurred in the pliocene between 4 7 0 2 ma and 4 06 0 29 ma based on k ar and 40ar 39ar plateau ages on vein adularia vein formation overlapped with pliocene felsic volcanism 3 94 0 29 ma to 3 36 0 49 ma based on field relationships and 40ar 39ar geochronology pliocene felsic volcanic rock sequences crop out widely in southern el dorado at lower elevations relative to central and northern el dorado the felsic rocks have sinters intercalated with pyroclastic and sedimentary rocks at the base of the sequence that define the paleosurface of the epithermal vein system at least in this part of the district the depth of erosion in central and northern el dorado the primary au ag resource is below the paleosurface as only the low sulfidation style epithermal veins are present field relationships and geochronology suggest that the onset of extensive felsic volcanism marks the waning stage of hydrothermal activity associated with vein formation and mineralization from north to south the el dorado district represents an oblique cross section through the volcanic and epithermal vein system formed during east west extension northern and central el dorado dominated by mineralized mafic to intermediate volcanic basement rocks represent the deeper parts of the volcanic and hydrothermal system southern el dorado is the shallow level of the volcanic field and epithermal vein system preserving the surface and shallow subsurface environments including the sinters and contemporaneous to postmineral felsic volcanic rocks although we cannot preclude that the distribution of volcanic rocks represents lateral facies changes in a pliocene felsic dome field or that the variations in depth within the low sulfidation epithermal system represent simply lateral flow driven by topography in the geothermal system the preponderance of evidence suggests that the oblique section is principally due to postmineral deformation associated with formation of large pliocene and pleistocene rio titihuappa basin lying to the immediate south

os and u th isotope signatures of arc magmatism near mount mazama crater lake oregon abstract interaction of mantle melts with the continental crust can have significant effects on the composition of the resulting melts as well as on the crust itself and tracing this interaction is key to our understanding of arc magmatism lava flows and pyroclastic deposits erupted from 50 to 7 7 ka at mt mazama crater lake oregon were analyzed for their re os and u th isotopic compositions mafic lavas from monogenetic vents around mt mazama that erupted during the buildup to its climactic eruption have lower 187 os 188 os ratios 0 1394 to 0 1956 and high 230 th excess 230 th 238 u 0 of 1 180 to 1 302 whereas dacites and rhyodacites tend to have higher 187 os 188 os ratios 0 2292 to 0 2788 and significant 238 u excess 230 th 238 u 0 of 0 975 to 0 989 the less radiogenic os isotope compositions of the mafic lavas can be modeled by assimilation of young 2 5 to 7 ma mafic lower crust that was modified during regional extension whereas the more radiogenic os isotope compositions of the dacites and rhyodacites can be attributed to assimilation of older 10 to 16 ma mid to upper crust that acquired its composition during an earlier period of cascade magmatism production of th excesses in the lower crust requires very young garnet formation accompanying dehydration melting in the lower crust at less than a few 100 ka by heat from recent basaltic magma injection the results from this study suggest that the combination of os and th isotopes may be used to provide insights into the timescales of evolution of the continental crust in arc settings as well as the influence of the crust on erupted magmas and suggest a link between the age and composition of the lower and upper crust to regional tectonic extension and or earlier cascade magmatism

la icp ms u pb zircon timescale constraints of the pleistocene latest magmatic activity in the sancy stratovolcano french massif central abstract it has been demonstrated for several years that the u pb zircon system can provide useful dating information for understanding the time evolution of the volcanic systems in addition in situ techniques were successfully used for determining the formation age of young pliocene and pleistocene volcanic rocks and to record the spatial and temporal distribution and the lifetime evolution of the magma chambers generally in the case of large volcanic domains here we show that it is possible to obtain a precision at the percent level on zircon from a short size and late volcanic event from the sancy stratovolcano french massif central la icp ms dating of pleistocene magmatic zircons allow to distinguish the timescale of these late magmatic events within the complex volcanic activity of this large stratovolcano a syenite xenolith enclosed into a trachyte pumice flow were dated at 277 6 3 2 ka and 260 1 3 6 ka respectively whereas an earlier trachyandesite dome was emplaced at 284 4 4 2 ka these different ages allow to propose a minimum evolution time of about 24 8 kyrs for a differentiated magma chamber which corresponds to a short lifetime associated with low magma volumes the duration of this discrete volcanic event should be compared to the hundreds of thousands of years reported for the large volcanic systems interestingly the zircon crystals from the pumices were embedded into a thin layer of glass of trachytic composition demonstrating that these zircons crystallized into the melt only a short time before eruption

water co2 cl and f in melt inclusions in phenocrysts from three holocene explosive eruptions crater lake oregon rare melt inclusions 100 m in diameter trapped near the boundaries of corroded patchy zones in plagioclase phenocrysts from plinian pumice of three holocene eruptions were analyzed by ir spectroscopy for molecular h 2o oh groups and co 2 and by electron microprobe for cl and f the three rhyodacitic eruptions each of which began with aplinian phase occurred over 200 yr the llao rock and cleetwood eruptions ended with degassed lava flows and the subsequent climactic eruption with voluminous ignimbrite groundmass glass and melt inclusion compositions anhydrous are similar inclusions with total h 2o concentrations of 1 2 3 4 wt are adjacent to fractures or are hourglass inclusions suggesting partial degassing caused by depressurization melt inclusions in phenocrysts in climactic ignimbrite may have vesiculated for the same reason intact inclusions in plinian pumices have total h 2o concentrations believed to represent magmatic h 2o contents wt llao rock 5 3 5 3 cleetwood 3 8 4 7 climactic 3 9 0 2 l n 6 ratios of or molecular h 2o indicate closure temperatures of 200 500oc that reflect syn or posteruptive cooling co 2 concentrations are 25 ppm h 2o and co 2 concentrations indicate saturation pressures of 1 0 1 8 kbar or depths 5 km six inclusions from climactic pumice average 400 40 l ppm f 1880 70 ppm cl f and cl concentrations of the other samples are similar and not well correlated with each other or with total h 2o location of melt inclusions near boundaries of patchy zones which are mantled by oscillatory zoned overgrowths suggests that their h 2o concentrations represent magmatic values significantly before eruption although mean h 2o concentrations of analyzed melt inclusions decrease for the three successive eruptions it is not certain that this indicates a corresponding change in magmatic h 2o content during the interval between eruptions

extreme u th disequilibrium in rift related basalts rhyolites and granophyric granite and the timescale of rhyolite generation intrusion and crystallization at alid volcanic center eritrea received september 20 2005 accepted june 14 2006 advance access publication july 14 2006rhyolite pumices and co erupted granophyric granite xenolithsyield evidence for rapid magma generation and crystallization priorto their eruption at 15 2 2 9 ka at the alid volcanic center inthe danikil depression eritrea whole rock u and th isotopicanalyses show

the eruptive history of the tequila volcanic field western mexico ages volumes and relative proportions of lava types the eruptive history of the tequila volcanic field 1600 km 2 in the western trans mexican volcanic belt is based on 40 ar 39 ar chronology and volume esti mates for eruptive units younger than 1 ma ages are reported for 49 volcanic units including volcn tequila an andesitic stratovolcano and peripheral domes flows and scoria cones volumes of volcanic units 1 ma were obtained with the aid of field mapping ortho aerial pho tographs digital elevation models dems and arcgis software between 1120 and 200 kyrs ago a bimodal distribution of rhyolite 35 km 3 and high ti basalt 39 km 3 dominated the volcanic field between 685 and 225 kyrs ago less than 3 km 3 of andesite and dacite erupted from more than 15 isolated vents these lavas are crystal poor and show little evidence of storage in an upper crustal chamber approximately 200 kyr ago 31 km 3 of andesite erupted to form the stratocone of volcn tequila the phenocryst assemblage of these la vas suggests storage within a chamber at 2 3 km depth after a hiatus of 110 kyrs 15 km 3 of andesite erupted along the w and se flanks of volcn tequila at 90 ka most likely from a second discrete magma chamber lo cated at 5 6 km depth the youngest volcanic feature 60 ka is the small andesitic volcano cerro tomasillo 2 km 3 over the last 1 myr a total of 128 22 km 3 of

direct evidence for the origin of low 18o silicic magmas quenched samples of a magma chamber s partially fused granitoid walls crater lake oregon abstract partially fused granitoid blocks were ejected in the climactic eruption of mount mazama which was accompanied by collapse of crater lake caldera quartz plagioclase and glass in the granitoids have much lower 18 o values 3 4 to 4 9 than any fresh lavas of mount mazama and the surrounding region 5 8 to 7 0 oxygen isotope fractionation between phases in granitoids is consistent with equilibrium at t 900 c following subsolidus exchange with hydrothermal fluids of meteoric origin assimilation of 10 20 of material similar to these granitoids can account for the o and sr isotopic compositions of lavas and juvenile pyroclasts derived from the climactic magma chamber many of which have 18 o values 0 5 or more lower than comparable lavas of mount mazama the o isotope data provide the only clear evidence for such assimilation because the mineralogy and chemical and radiogenic isotopic compositions of the granitoids dominantly granodiorite are similar to those of erupted juvenile magmas the granitoid blocks from crater lake serve as direct evidence for the origin of 18 o depletion in large shallow silicic magma bodies

cretaceous reduced granitoids in the goodpaster mining district east central alaska the 5 moz 1 oz troy 31 103 g gold veins of the liese zone and nearby prospects in the goodpaster mining district of east central alaska are spatially and temporally associated with late early to late cretaceous reduced granitoids that are divided into a granite suite tonalite suite and diorite suite in decreasing age synkinematic to postkinematic biotite hornblende granite granodiorite pegmatite and two mica granite with accessory garnet compose the granite suite 109 107 ma the tonalite suite 107 103 ma forms small to large bodies of postkinematic hornblende biotite granodiorite to tonalite with rare granite intruding the granite and parts of the tonalite suites are aplite and pegmatite that grade through sugary pegmatoidal or aplitic quartz veins and finally into quartz veins along strike the diorite suite 95 4 93 7 ma consists of small stocks of diorite to tonalite that intrude the older suites and are inferred to in trude the shallowly dipping auriferous quartz veins limited data indicate granitoid emplacement at 5 9 km depths consis tent with formation of the auriferous quartz veins based upon published fluid inclusion data the weakly peraluminous granite and tonalite suites are distinguished by variable amounts of monazite and zircon whereas the diorite suite is metaluminous contains 5 magnetite and lacks monazite all suites are subalkalic calc alkaline and have low mag netic susceptibilities high large ion lithophile element high field strength element lile hfse and depleted nb and ti the granite suite has higher 206 pb 204 pb values 19 4 19 6 than the diorite 19 1 overall the granite and tonalite suites likely represent melts generated late during crustal thickening that intruded along shallowly dipping faults during exhuma tion whereas the diorite suite represents postdeformation melts that underwent less interaction with the old silicic crust

volcanic stratigraphy and geochronology of the cretaceous lancones basin northwestern peru position and timing of giant vms deposits a 10 km thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the cretaceous lancones basin in northwestern peru and underlies part of the huancabamba deflection the marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin the earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar quartz porphyritic felsic volcanic rocks these volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide vms deposits which are inferred to have formed in a localized but relatively deep marine setting u pb zircon dating of felsic volcanic rocks associated with vms deposits at tambogrande indicates ages from 104 8 1 3 to 100 2 0 5 ma for the ore bearing volcanic sequence the timing of onset of rift related volcanism is not well constrained but is therefore of middle albian age or older subsequent latest albian to turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields u pb zircon ages of 99 3 0 3 to 91 1 1 0 ma these later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the lancones basin finally the basin was intruded by late cretaceous to tertiary granitoids of the coastal batholith the genesis of the cretaceous lancones basin and other equivalent volcanic rift related marginal basins in western south america including the western peruvian trough is related tectonically to the break up of gondwana early volcanism and associated vms deposits formed in the lancones basin during the albian coincided with the initial rifting stage prior to active oceanic spreading between south america and africa during this time the relatively stationary western margin of continental south america was undergoing extension and rifting due to a westward and oceanward retreating arc resembling a mariana arc type setting the mochica orogeny marks the termination of rifting subsidence and related volcanism along the western margin of south america this orogenic event also broadly coincides with the onset of spreading of the south atlantic and westward drift of the south american continent subsequent volcanism in the lancones basin was more continental arclike under an andean type scenario

concentric zoning in the tunk lake pluton coastal maine the tunk lake pluton of coastal maine usa is a concentrically zoned granitic body that grades from an outer hypersolvus granite into subsolvus rapakivi granite and then into subsolvus non rapakivi granite with gradational contacts between these zones the pluton is partially surrounded by a zone of basaltic and gabbroic enclaves interpreted as quenched magmatic droplets and mushes respectively as well as gabbroic xenoliths all hosted by high silica granite the granite is zoned in terms of mineral assemblage mineral composition zircon crystallization temperature and major and trace element concentration from the present day rim interpreted as being closer to the base of the chamber to the core interpreted as being closer to the upper portions of the chamber the ferromagnesian mineral assemblage systematically changes from augite and hornblende with augite cores in the outermost hypersolvus granite to hornblende to hornblende and biotite and finally to biotite only in the subsolvus granite core of the pluton sparse fine grained basaltic enclaves that are most common in the outermost zone of the pluton suggest that basaltic magma was present in the lower portions of the magma chamber at the same time that the upper portions of the magma chamber were occupied by a granitic crystal mush however the slight variations in initial nd isotopic ratio in granites from different zones of the pluton suggest that contamination of the granitic melt by basaltic melt played little role in generating the compositional gradation of the pluton the zone of basaltic and gabbroic chilled magmatic enclaves and gabbroic xenoliths hosted by high silica granite that partially surround the pluton is interpreted as mafic layers at the base of the pluton that were disrupted by invading late stage high silica magma these mafic layers are likely to have consisted of basaltic lava layers and basalt that chilled against granitic magma to produce coarse grained gabbroic mush basaltic and gabbroic magmatic enclaves and gabbroic xenoliths are hornblende bearing suggesting that their parent melts were relatively hydrous the water rich nature of the underplating mafic magmas may have prevented extensive invasion of the granitic magma by these magmas owing to the much greater viscosity of the granitic magma than the mafic magmas in the temperature range over which magma interaction could have occurred

u pb shrimp dating of uraniferous opals abstract u pb and u series analyses of four u rich opal samples using sensitive high resolution ion microprobe shrimp demonstrate the potential of this technique for the dating of opals with ages ranging from several tens of thousand years to millions of years the major advantages of the technique compared to the conventional thermal ionisation mass spectrometry tims are the high spatial resolution 20 m the ability to analyse in situ all isotopes required to determine both u pb and u series ages and a relatively short analysis time which allows obtaining a growth rate of opal as a result of a single shrimp session there are two major limitations to this method determined by both current level of development of ion probes and understanding of ion sputtering processes first sufficient secondary ion beam intensities can only be obtained for opal samples with u concentrations in excess of 20 g g however this restriction still permits dating of a large variety of opals second u pb ratios in all analyses drifted with time and were only weakly correlated with changes in other ratios such as u uo this drift which is difficult to correct for remains the main factor currently limiting the precision and accuracy of the u pb shrimp opal ages nevertheless an assumption of similar behaviour of standard and unknown opals under similar analytical conditions allowed successful determination of ages with precisions of 10 for the samples investigated in this study shrimp based u series and u pb ages are consistent with tims dating results of the same materials and known geological timeframes

zircon xenocryst resorption and magmatic regrowth at el chichon volcano chiapas mexico abstract el chichon volcano is the only active volcano located within the chiapanecan volcanic arc in southern mexico which lies between the trans mexican volcanic belt and the central american volcanic arc previous studies have shown that 12 eruptions have occurred at el chichon within the last 8000 years forming a complex of lava domes with a central crater and surrounding pyroclastic deposits here we report the discovery of zircon in holocene el chichon rocks which were analyzed by high spatial resolution imaging color cathodoluminescence ccl and isotopic secondary ionization mass spectrometry sims methods to resolve core and rim crystallization ages pumice samples from five proximal pyroclastic flow and fall out deposits were collected based on published stratigraphy two of the samples were further re classified by new 14c dates in addition we sampled two lavas from the 1982 eruption and from remnants of the older somma lava complex zircon crystals were dated using 230th 238u disequilibrium u th and u pb geochronology u th zircon ages fall between near eruption ages and ca 84 ka with overlapping ages in all samples by contrast zircon core u pb ages range between ca 290 ma and 1 9 ga these ages are consistent with xenocrystic origins and their heterogeneity indicates derivation from clastic country rocks strong age contrasts between inherited xenocrystic and young magmatic domains in individual zircon crystals are evidence for arrested assimilation of crustal rocks where initially zircon undersaturated magmas cooled rapidly to form a crystal mush or subsolidus amalgamate as a crustally contaminated boundary layer this layer contributed zircon crystals to eruptible magma during episodic recharge events followed by partial melt extraction mixing and homogenization zircon overgrowths are significantly older than major minerals whose u series ages and sharp zonation boundaries suggest crystallization only within a few ka before eruption this implies that zircon can detect magmatic longevity which is obscured in the major mineral record

fluvial facies architecture and sequence stratigraphy of the tertiary duchesne river formation uinta basin utah u s a continental sequence stratigraphy in upstream fluvial environments can be complex due to the interplay of source tectonics local and global climate change and topography the tertiary duchesne river formation utah u s a represents the last stage of intermontane basin fill in the uinta basin it is surrounded by sediment source mountain ranges of the uinta mountains to the north and the sevier fold thrust belt ftb to the west excellent basin scale exposure of the duchesne river formation allows the vertical and lateral characterization of facies architectures in the upstream environments the four members of the duchesne river formation are distinctive lithological units the lower three members comprise a typical upward fining unconformity bounded fluvial sequence from the basal coarse grained unit into the overlying fine grained units the fourth uppermost coarse grained member records the onset of another upward fining cycle the sequence stratigraphy of these unconformity bounded upward fining cycle s was primarily tectonics driven due to uplift of the uinta mountains the duchesne river formation records a distinct change in fluvial lacustrine styles between the western and eastern part of the basin demonstrating the variable allogenic controls of tectonic subsidence and discharge related to the local climate and source terrain input within the basin specifically the western degradational fluvial system with a high net sandstone to gross thickness ratio of the basal member was controlled by high discharge due to a wet climate and two source terrain inputs i e uinta mountains and sevier fold thrust belt whereas the eastern aggradational fluvial system with a low net sandstone to gross thickness ratio was controlled by low discharge due to a dry climate and single source terrain input i e uinta mountains the development of local lacustrine environments on the alluvial plain of the third member present in the western uinta basin was controlled by differential tectonic subsidence in the basin the duchesne river formation preserves a valuable example of an upstream fluvial sequence and demonstrates how internal facies architectures at the basin scale were dictated by allogenic controls