granite ascent and emplacement during contractional deformation in convergent orogens based on a case study in the central maine belt of west central maine u s a it is proposed that crustal scale shear zone systems provide an eaective focussing mechanism for transfer of granite melt through the crust in convergent orogens during contractional deformation flow of melt in crustal materials at depths below the brittle plastic transition is coupled with plastic deformation of these materials the flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses within the oblique reverse central maine belt shear zone system stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain which suggests percolative flow of melt to form the migmatite leuco somes and viscous flow of melt channelized in sheet like bodies possibly along fractures cyclic fluctuations of melt pressure may cause instantaneous changes in the eaective permeability of the flow network if self propa gating melt filled tensile and or dilatant shear fractures are produced due to melt enhanced embrittlement in homogeneous migmatite and schlieric granite occur in zones of lower strain which suggests migration of partially molten material through these zones en masse by granular flow and channelized flow of melt carrying entrained residue founded on the central maine belt case study we develop a model of melt extraction and ascent using the driving forces stress conditions and crustal rheologies in convergent especially transpressive orogens ascent of melt becomes inhibited with decreasing depth as the solidus is approached for intermedi ate a h2o muscovite dehydration melting the water saturated solidus occurs between 400 and 200 mpa near the brittle plastic transition during high t low p metamorphism where the balance of forces favors sub horizontal fracture propagation emplacement of melt may be accommodated by ductile flow and or stoping of wall rock and inflation may be accommodated by lifting of the roof at shallower crustal levels and or sink ing of the pluton floor the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduits although these plutons may have locally discordant relations with country rock structures when viewed at the crustal scale granite ascent and emplacement in convergent orogens are syn tec tonic processes 1998 elsevier science ltd all rights reserved

residence of ree y th and u in granites and crustal protoliths implications for the chemistry of crustal melts a systematic study with laser ablation icp ms scanning electron microscopy and electron microprobe revealed that 70 95 wt of ree except eu y th and u in granite rocks and crustal protoliths reside within reeythu rich accessories whose nature composition and associations change with the rock aluminosity the accessory assemblage of peraluminous granites migmatites and high grade rocks is composed of monazite xenotime in low ca varieties apatite zircon thorthosilicate uraninite and betafite pyrochlore metaluminous granites have allanite sphene apatite zircon monazite and thorthosilicaie peralkaline granites have aeschinite fergusonite samarskite bastnaesite fluocerite allanite sphene zircon monazite xenotime and th orthosilicate granulite grade garnets are enriched in nd and sm by no less than one order of magnitude with respect to amphibolite grade garnets granulitegrade feldspars are also enriched in lree with respect to amphibolite grade feldspars accessories cause non henrian behaviour of ree y th and u during melt solid partitioning because elevated fractions of monazite xenotime and zircon in common migmatites are included within major minerals their behaviour during anatexis is controlled by that of their host settling curves calculated for a convecting magma show that accessories are too small to settle appreciably being separated from the melt as inclusions within larger minerals biotite has the greatest tendency to include accessories thereby indirectly controlling the geochemistry of ree y th and u we conclude that ree y th and u are unsuitable for petrogenetical modelling of granitoids through equilibrium based trace element fractionation equations

trace element modelling of pelite derived granites the presence or absence of a vapour phase during incongruent melt reactions of muscovite and biotite together with the composition of the protolith determines the trace element characteristics of the resulting melt provided that equilibrium melting occurs for those phases that host the tracc elements of interest for granitic melts rb sr and ba provide critical constraints on the conditions that prevailed during melting whereas ree are primarily controlled by accessory phase behaviour mass balance constraints for eutectic granites that are formed by the incongruent melting of muscovite in pelites indicate that melting in the presence of a vapour phase will result in a large melt fraction and deplete the restite in feldspar hence the melt will be characterized by low rb sr and high sr ba ratios in contrast vapour absent melting will result in a smaller melt fraction and an increase in the restitic feldspar consequently high rb sr and low sr ba ratios are predicted vapour absent melting will also enhance the negative eu anomaly in the melt granites that result from the incongruent melting of biotite in the source will be characterized by higher rb concentrations than those that result from the incongruent melting of muscovite the himalayan leucogranites provide an example of unfractionated crustally derived eutectic melts that are enriched in rb but depleted in sr and ba relative to their metasedimentary protoliths these compositions may be generated by the incongruent melting of muscovite as a low melt fraction f 0 1 from a pelitic source under vapour absent conditions

stable carbon isotope stratigraphy of the paleogene pedogenic series of southern france as a basis for continental marine correlation the upper cretaceous lower paleogene formations in the aix en provence basin are characterized by interfingering lacustrine carbonates and flood plain alluvium the deposits from both environments display numerous horizons of carbonate rich paleosols by using a combined magnetostratigraphy and carbon isotope stratigraphy for the pedogenic carbonate glaebules we constructed a detailed integrated stratigraphy for the entire lower paleogene this method provides a way to correlate marine and terrestrial sequences when c3 plants dominated the environment both long term and short term 13 c variations were identified in the provence series 1 the uniform carbon isotope record established through the end of the maastrichtian followed by the abruptly negative carbon isotope excursion in the lowermost paleocene provides a marker for the precise location of the cretaceous tertiary boundary in this basin 5 m below the calcaire de vitrolles formation 2 a gradual 13 c increase through the middle upper paleocene similar in amplitude to that of the marine realm 3 a short term negative 13 c excursion in the latest paleocene as seen within the calcaire de st marc formation and 4 a slow decrease during the late paleocene to early eocene with a probable hiatus in the sedimentary record the provence mammal site of palette is shown to be stratigraphically younger than the late paleocene negative 13 c excursion

40ar 39ar dating of mineral separates and whole rocks from the western ghats lava pile further constraints on duration and age of the deccan traps abstract there has been an ongoing debate for almost two decades on the age and duration of deccan trap volcanism some of the best available ages have been determined using the 40 ar 39 ar method but differences in sample location laboratory procedure type of analyzed material and ages of monitors may have been responsible for much of the apparent spread in the data we have collected and analyzed samples from close to the bottom and top of the main lava pile in the western ghats near bombay we confirm previous findings that alteration and ar recoil strongly affect these rocks particularly measurements made on whole rocks when mineral separates mostly plagioclase are analyzed the spread is much reduced we find a mean age of 65 4 0 7 ma for five flows near the base of the section jahwar and igatpuri formations and 65 2 0 4 ma for a dyke cross cutting the poladpur formation below the topmost mahabaleshwar formation this implies that at least 1800 m out of the 2500 m composite trap section were erupted in a short interval less than 1 ma long and close to 65 5 ma i e close to the cretaceous tertiary boundary we find that older ages near 67 ma found by several authors are essentially based on whole rock samples which display disturbed spectra when this can be checked i e when the spectra are published

groundwater dolocretes in a lake marginal environment an alternative model for dolocrete formation in continental settings danian of the provence basin france in the long continental history of the provence basin which extended from santonian to oligocene times a major period of palaeosol development occurred in the danian dolocretes developed within floodplain silts and partly from palustrine limestones dolocretes are overlain by pedogenic facies calcretes and palustrine limestones gradational lower and upper limits of profiles succession of nodular coalescent and massive horizons the epigenesis of quartz by dolomite the unimodal crystal size and the euhedral dolomite fabric as well as the absence of biogenic structures and vadose cements show that dolocretes formed in the phreatic zone the exclusive occurrence of dolocretes around the palaeolake or playa suggests that dolocrete formed by the mixing of groundwaters and lake brines which infiltrated the phreatic zone during periods of strong evaporation and lake level lowering the term halo dolocrete is proposed to describe this type of dolocrete deposit subsequent alteration of the dolocrete includes leaching of the central core of dolomite crystals and calcitization calcitization was either fabric destructive type i or fabric preserving type ii and took place during very early diagenesis i e concomitant with calcrete formation or palustrine limestone deposition fabric destructive calcitization is attributed to a drop in the lake level when the upper part of the dolocrete was subjected to vadose zone processes whereas the fabric preserving calcitization resulted from reactions with dilute lake and groundwaters during rise in lake level

from sediment to granite timescales of anatexis in the upper crust abstract granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followed by formation of a grain boundary melt melt segregation into a vein network ascent of the melt through the network and finally crystallisation of the melt experimental constraints on the formation of a crustal melt from the incongruent melting of muscovite combined with geochemical studies of anatexis in the high himalaya allow the timescales required for each of these processes to be assessed discordant temperatures determined from monazite and zircon thermometry for himalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in lree implying that melts have probably been extracted in less than 10 ka experimental studies suggest that some himalayan melts are also undersaturated in zr implying segregation may have occurred within 100 years such short timescales confirm that deformation driven mechanisms are important in extracting these melts from their source the transport distances of himalayan granitic melts of 10 km may be achieved by the ascent of magma through dykes in about 1 day at such rates even the largest granite could theoretically be emplaced in 10 years crystallisation of himalayan melts involves much longer periods if emplaced as thin sheets 100 m wide a timescale of 500 years is required compared with 30 ka for single stage intrusion of the larger laccoliths for composite sheet complexes magma crystallisation rather than melt ascent comprises the rate determining step on the emplacement of the intrusion the overall timescales of melt segregation and emplacement for many orogenic granites are therefore less than 10 ka and possibly less than 1 ka in contrast the timescale required for prograde heating of the protolith is 1 ma since the melt production rate is determined by heat flow into the protolith and not by reaction kinetics for any geologically significant period we conclude that heat flow determined by both the mechanism of heating and the thermal diffusivities of crustal rocks provides the overall rate determining step of crustal anatexis

magma emplacement and remobilization timescales beneath montserrat insights from sr and ba zonation in plagioclase phenocrysts received november 22 2001 accepted december 12 2002since 1995 the soufri ere hills volcano on montserrat lesserantilles island arc has erupted crystal rich andesite magmathat is geochemically very similar to the volcanic products of atleast the last 174 3kyr 1s nomarski images of plagio clase phenocrysts from six andesites erupted between 151 4ka 1s and

continental paleotemperatures from an early cretaceous dolomitic lake gansu province china abstract the lower cretaceous strata of asia are renowned for abundant fossil resources these strata can also offer a wealth of quantitative data regarding the paleoclimate and paleoenvironment of the terrestrial cretaceous an important transition in earth history many modern groups of organisms that dominate terrestrial ecosystems today originated or expanded in the early cretaceous in addition the cretaceous period known to have been characterized by a greenhouse climate provides opportunities to understand how the climate system functioned during greenhouse climate states here we utilize the clumped isotope paleothermometer to provide paleotemperature records from a lacustrine sequence in northwestern china previously constrained to the early aptian petrographic and x ray diffraction analyses indicate that the lake sediments produced primary to early diagenetic dolomites likely microbially mediated due to high organic matter input and seasonally warm arid conditions the majority of the dolomitic carbonates formed in pore waters just below the sediment water interface or as primary shallow water precipitates clumped isotope derived temperatures from all carbonates range from 20 6 c for early diagenetic dolomites to 47 2 c for late burial calcite cements accounting for seasonal biases mean annual air temperatures are calculated to have averaged 20 2 c this is broadly consistent with local paleotemperature proxy evidence and climate model paleotemperature estimates the data presented provides further quantitative constraints on the cretaceous greenhouse world as well as insights into depositional settings that produce low temperature dolomites

late miocene movement within the himalayan main central thrust shear zone sikkim north east india in the sikkim region of north east india the main central thrust mct juxtaposes high grade gneisses of the greater himalayan crystallines over lower grade slates phyllites and schists of the lesser himalaya formation inverted metamorphism characterizes rocks that immediately underlie the thrust and the large scale south tibet detachment system stds bounds the northern side of the greater himalayan crystallines in situ th pb monazite ages indicate that the mct shear zone in the sikkim region was active at c 22 14 15 and 12 10 ma whereas zircon and monazite ages from a slightly deformed horizon of a high himalayan leucogranite within the stds suggest normal slip activity at c 17 and 14 15 ma although average monazite ages decrease towards structurally lower levels of the mct shear zone individual results do not follow a progressive younging pattern lesser himalaya sample kbp1062a records monazite crystallization from 11 5 0 2 to 12 2 0 1 ma and peak conditions of 610 25 c and 7 5 0 5 kbar whereas in the mct shear zone rock chg14103 monazite crystallized from 13 8 0 5 to 11 9 0 3 ma at lower grade conditions of 525 25 c and 6 1 kbar the p t t results indicate that the shear zone experienced a complicated slip history and have implications for the understanding of mid crustal extrusion and the role of out of sequence thrusts in convergent plate tectonic settings

a fault and stress field analysis in a coal mine gardanne bouches du rhone france abstract this paper develops an interpretation of faulting in the mining district of the arc basin where research has been conducted since 1978 in the underground workings coal winning is carried out by the houilleres de bassin du centre et du midi h b c m mining takes place in an upper cretaceous series lying ahead of an overthrust sheet etoile sheet forming the southern limit of the arc basin with a few exceptions the observations and measurements have been taken in the seam named grande mine which is the only bed being worked at present a chronological account of faulting is put forward normal faulting occurred prior to the overthrusting which brought about reverse faulting continued compression nne ssw led to strike slip faulting the last compressive phase again induced strike slip faulting along a few major faults this was a nearly e w trending compression the stress field pertaining to each period is investigated and an attempt is made to calculate the values of the principal stresses these findings on the stress history fit what is already known about the sequence of geological events in the region the present day stress field determined through in situ measurements can be at least in one part of the coal field said to result directly from the tectonic conditions it is a major cause of rock bursts in the mine workings

sedimentation in a tectonically partitioned nonmarine foreland basin the lower cretaceous kootenai formation southwestern montana the lower cretaceous kootenai formation in southwestern montana was deposited in the nonmarine cordilleran foreland basin in the united states during a period of intensified uplift in the westward adjacent but increasingly impingent sevier fold thrust belt concurrently the foreland basin was partitioned by uplift of intra foreland structural elements and incipient plutonism kootenai fluvial and fluviolacustrine depositional systems developed and evolved in response to changing basin architecture the kootenai thus provides a case study of nonmarine sedimentary responses to tectonic partitioning in a foreland basin the following criteria for recognition of tectonic partitioning of a nonmarine foreland basin are demonstrated by the kootenai 1 complex internalized drainage patterns which locally diverge from the over all regional paleoslope direction and a typical longitudinal pattern 2 intraformational and interformational unconformities both proximal to the fold thrust belt and within the foreland basin 3 evidence for cannibalization and redistribution of foreland basin sediments 4 abrupt deflections of paleodrainage axes in response to initiation of intraforeland uplifts and 5 dramatic base level alterations and consequent changes of fluvial geomorphology as a result of tectonic partitioning and segmentation of the foreland basin kootenai sedimentation can be divided into six phases 1 a tectonic pulse in the sevier fold thrust belt produced a thin but widespread sheet of gravel alluvium referred to as the basal conglomerate member which was deposited by a shallow perhaps ephemeral gravel bed braided stream system drainage was predominantly eastward across the axis of the foreland basin 2 subsequently initial activation of intra foreland structures over the present tobacco root madison gravelly and beartooth archean blocks and the boulder batholith caused reworking of the basal conglomerate in all but the westernmost foreland basin although the topographic manifestations of these uplifts were subtle the foreland basin was effectively partitioned into at least four distinct drainage basins the resulting deposit commonly called the pryor member but here referred to as the first sandstone member was deposited by sand and gravel dominant braided stream systems in the west and east respectively paleodrainage directions in the western foreland basin were southward but over all drainage was northward 3 a period of mud dominant sedimentation followed deposition of the first sandstone probably resulting from a combination of decreased tectonism influx of siliceous volcanic ash and change in source lithology 4 renewed tectonic activity in the fold thrust belt generated a second episode of coarse grained alluvial sedimentation producing the kootenai second sandstone member basin partitioning was accentuated but drainage directions remained essentially unchanged initial activation of the blacktail snowcrest uplift dammed the southern portion of the second sandstone fluvial system base level was elevated and a major anastomosed stream system developed upstream from the structural dam downstream from the axis of the uplift a transport efficient probably entrenched meandering system developed 5 the remainder of kootenai siliciclastic deposition was dominated by muddy fluvial and fluviolacustrine systems the foreland basin configuration remained essentially unchanged as drainage became ponded in response to further elevation of base level as basin partitioning continued an extensive carbonate lacustrine interval punctuated this period of sedimentation 6 gradual southward transgression of the cretaceous sea left a transitional fluviolacustrine to marine imprint on the sedimentary record of the uppermost kootenai and overlying blackleaf formation rejuvenation of orogenesis in the fold thrust belt augmented by cratonic and major intraforeland volcanic and sedimentary lithic sources renewed the supply of coarse grained detritus to the foreland basin during deposition of the blackleaf

correlation of terrestrial climatic fluctuations with global signals during the upper cretaceous danian in a compressive setting provence france abstract the upper cretaceous lower paleocene terrestrial formations from the aix en provence basin offer a high resolution record of the effects of climate changes these units were deposited in a basin with low topographic relief under climate conditions that varied from subhumid to semiarid sedimentation during subhumid periods was characterized by accumulation of carbonate mud in the main lake and aggradation of floodplains during overbank floods carbonate rich paleosols which occur throughout the subhumid succession contain authigenic minerals with interstratified illite smectite and smectite although subhumid conditions were dominant during deposition the occurrence of five semiarid episodes can be documented on the basis of facies paleomagnetic signal and mineralogic associations of rocks deposited across the paleolandscape the lake margin environment was most sensitive to climate change dolomite and gypsum crystals authigenic smectite and palygorskite and secondary fine grained hematite grew within rocks deposited along the lake margin under semiarid conditions the mineralogic transformations resulted in a distinct paleomagnetic record composed of a it component 200 to 400 c and an associated chemo detrital ht component up to 600 c during the semiarid episodes sedimentation in floodplain environments was reduced allowing development of mature smectite or smectite palygorskite paleosols semiarid episode 1 occurs within the calcaire de rognac formation semiarid episodes 2 and 3 lie within the upper argiles rutilantes formation and semiarid episodes 4 and 5 are just below and within the calcaire de vitrolles formation recognition of correlative deposits with a distinct paleomagnetic signal allows correlation between the continental successions of provence and the geomagnetic polarity time scale the lithostratigraphic units as well as the distribution of dinosaur oospecies are largely diachronous representing a few millions of years semiarid episodes 1 to 3 occurred during the early maastrichtian no semiarid episodes are recorded for the cooler interval that defined the middle late maastrichtian semiarid episodes 4 and 5 correspond to the warmer periods that preceded and followed the 500 ky long interval containing the cretaceous tertiary boundary

s type granitic magmas petrogenetic issues models and evidence abstract despite a perception that it represents a perverse divergence it is perfectly possible to believe in the existence of s and i type granites and the implications for the nature of their protoliths and to disbelieve in the applicability of the restite unmixing model for chemical variation in granitic magmas white and chappell erected the s i classification with impeccable validity the isotopic evidence demands contrasting source reservoirs for s and i type granitic magmas however the major advance was not the classification but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas the restite unmixing model is commonly seen as a companion to the s i classification but it is really a separate issue this model implies that the compositions of granites image those of their source rocks in a simple way however there are other equally valid models that can explain the data and none of them represents a unique solution the most cogent explanation for the high grade metasedimentary enclaves in most s type granites is that they represent mid crustal xenoliths restitic enclaves are either rare or absent inherited zircons in s type rocks are certainly restitic however the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock zircon and zirconium behaviours are controlled by disequilibrium and kinetics and zr contents of granitic rocks can rarely be used to infer magma temperatures since the dominant ages among inherited zircons in lachlan fold belt lfb s type granites are ordovician and proterozoic it seems likely that crust of this age but geochemically different from the exposed rocks not only underlies much of the lfb but also forms a component in the granite magma sources the evidence is overwhelming that the dark microgranular enclaves that occur in both s and i type granites are igneous in origin they represent globules of quenched more mafic magma mingled and modified by exchange with the host granitic magma however magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas likewise the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and in some cases violate constraints from isotopic studies s and i type magmas commonly retain their distinct identities this suggests limited source mixing limited magma mixing and limited wall rock assimilation though intermediate types certainly exist they are probably relatively minor in volume crystal fractionation probably plays the major role in the differentiation of very many granitic magmas including most s types especially those emplaced at high crustal levels or in the volcanic environment minor mechanisms include magma mixing wall rock assimilation and restite unmixing isotopic variations within plutons and in granite suites could be caused by source heterogeneities magma mixing assimilation and even by isotopic disequilibrium however source heterogeneity coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust and crustal thickening likewise fails to provide sufficient heat generally the mantle must be the major heat source this might be provided through mantle upwelling and crustal thinning and possibly through the intra and underplating of mafic magmas upper crustal extension seems to have been common in regions undergoing granitic magmatism migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid present reactions granitic magmas are mostly formed by fluid absent processes where we do see rare evidence for arrested fluid absent partial melting the melt fraction is invariably concentrated into small shear zones veinlets and small dykes thus it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels however one major missing link in the chain is the mechanism by which melt fractions in small scale segregations occurring over a wide area can be gathered and focused to efficiently feed much wider spaced major magma conduits answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones self organization almost certainly plays a role

chemical gradient at diffusive interfaces in magma chambers the results of high pressure experiments on diffusion and soret separation in natural silicate melts show that the diffusive behaviour between natural silicic and mafic magmas can be approximately modelled as if the system were a binary mixture of sio2 and other components such as mgo feo cao steady state compositional profiles across a diffusive interface between silicic and mafic magma layers are calculated on the basis of phenomenological relationships for the fluxes of chemical species and heat in the binary mixtures using the experimental data of diffusion and soret coefficients in natural silicate melts the compositional profiles show a curvature with a minimum sio2 value within the interface due to the soret effect and temperature dependence of diffusion coefficient the compositional gradient at the lower half of the diffusive interface is similar to that resulting from the soret separation of a mafic melt regardless of the composition of the silicic magmas these results suggest that picritic magma can be formed in the interfacial region between the mafic and silicic magma layers the compositional gradient explains chemical variation of mafic to picritic inclusions in a mixed andesite of the abu volcano group japan

did the european dinosaurs disappear before the k t event magnetostratigraphic evidence abstract debate on the non catastrophic or catastrophic extinction of the dinosaurs by an asteroid impact k t event remains a controversy and is mainly based on the interpretation of the sedimentary continental sequences in the north american western interior the global aspect of this event needs to be tested in sedimentary record from all continents where continuous terrestrial deposits through the cretaceous paleogene are well preserved in the western mediterranean realm recognition of the cretaceous paleogene boundary is limited by the lack of biostratigraphic data in the upper cretaceous lower tertiary continental sedimentary sequences new magnetostratigraphic results were obtained from the analysis of two sections in southern france and compared to previous results in northern spain the last occurrence of in situ dinosaurs eggshells the only dinosaur remains found is located in chron 30n southeast france or 31n southwest france and northern spain this last occurrence could demonstrate that the extinction of the european dinosaurs occurred prior to the cretaceous paleogene boundary and would therefore support the idea of a gradual or stepwise extinction unlinked to the k t event

timescales of disequilibrium melting in the crust constraints from modelling the distribution of multiple trace elements and a case study from the lesser himalayan rocks of sikkim a numerical code has been developed to track the distribution of trace elements in crustal rocks undergoing melting the model handles diffusion with moving boundaries and accounts for the processes of diffusion dissolution and precipitation in a partially molten system among the various input parameters for modelling source composition i e modal abundance variations diffusion coefficients and partition coefficients are found to exert a significant control on the melt chemistry the other inputs such as melt reaction stoichiometry kinetics of melting and grain size of protolith have lesser influence exploration of the general behaviour indicates that for systems in which disequilibrium melting of the kind considered in this paper occurs trace element concentrations may be used to constrain the composition of the protolith or the timescales of melting depending on the specific circumstances after exploring some general features of melting in a pelitic system the model is applied to calculate trace element distributions in migmatites from the lesser himalayan rocks in sikkim india we focus on the distribution of trace elements during the initial stages of melt formation these partially molten rocks show disequilibrium distribution of trace elements and the numerical code is capable of quantitatively reproducing many of the observed patterns the results of the modelling indicate that the observed melts in this zone were formed within 50 000 years and that segregation of melts into leucosome and restite was complete between 50 000 and 250 000 years these short timescales may point to deformation enhanced melt segregation at least on a hand specimen scale it is important to distinguish between timescales of segregation over these scales and timescales of removal of melt on an outcrop scale to form plutons the latter requiring higher degrees of melting and larger distances of migration take longer

mineral leucosome trace element partitioning in a peraluminous migmatite a laser ablation icp ms study abstract low pressure peraluminous migmatites from the pena negra complex central spain stayed partially molten for 5 10 ma until the latest hercynian deformations allowed the segregation of melt as discordant leucosome veins due to long residence within its source the melt had time enough to equilibrate with residual phases other than accessories included within major refractory minerals we estimated crystal melt partition coefficients as the concentration ratios between leucosome samples appearing to be pure melts and mesosome minerals which were analyzed for trace elements with a laser probe coupled to an icp mass spectrometer our data reveal that when biotite is stable li rb cs tl sc v cr ni nb and ta become strongly compatible the role of biotite in fractionating y th u and the ree is insignificant cordierite strongly fractionates li and be and also has some effect on the hree and u garnet produces extreme fractionation of sc y and the hree the ree partition coefficients for ferromagnesian silicates increase with the atomic number this effect being progressively more important through biotite cordierite and garnet k feldspar strongly fractionates ba and pb but plays a secondary role to biotite for rb and cs k feldspar fractionation does not change the lree hree ratio but plagioclase fractionation produces a significant decrease in the lree hree ratio both feldspars greatly fractionate sr and eu monazite fractionation produces a dramatic depletion in ree th and u as well as a decrease in the lree hree and th u ratios apatite also fractionates the ree although in contrast with monazite it increases the lree hree ratio and does not affect the th u ratio zircon fractionation like apatite and garnet produces a strong depletion in the hree and a concomitant increase in the lree hree ratio in contrast with monazite zircon fractionation causes the th u ratio to increase

the diversity of sauropod dinosaurs and their first taxonomic succession from the latest cretaceous of southwestern europe clues to demise and extinction article i nfo southwestern europe is a key setting to evaluate the diversity of non avian dinosaurs before the end of the cretaceous below the k pg boundary the ancient ibero armorican island encompassing the current regions of north east iberia and south france provides a substantial record of sauropod fossils the study of multiple sauropod femora from localities where upper campanian to uppermost maastrichtian successions are both exposed together with the integration of the information gathered from previously known localities has allowed the biodiversity of sauropods to be reassessed within a precise and clear chronostratigraphic framework from the studied sample several titanosaur forms have been distin guished including a gracile and small sized titanosaur lirainosaurus astibiae a robust medium sized titanosaur ampelosaurus atacis a gracile medium sized titanosaur atsinganosaurus velauciensis and five other indeterminate but distinct titanosaurs which span the late campanian through the entire maastrichtian the youngest of these occurs in the uppermost part of palaeomagnetic chron c30n in the latest maastrichtian 0 4 1m a before the k pg boundary representing the youngest sauropod yet doc umented in eurasia the pattern of diversity on the ibero armorican island rules out a decline in sauropod diversity at the very end of the cretaceous as with other regions during the late cretaceous the abundance and quality of the sauropod fossil record is probably influenced by multiple biases sampling ecological and environmental

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

ferruginous paleosols around the cretaceous paleocene boundary in central southern sardinia italy and their potential as pedostratigraphic markers abstract the properties of seven ferruginous paleosols formed in alluvial continental deposits of upper cretaceous paleocene age in central southern sardinia italy were examined to understand their genesis particular attention has been addressed to their paleoenvironmental and paleogeographic significance also in comparison with similar paleosols in southern france and to their potential to represent good pedostratigraphic markers for a larger area the paleosols are characterized by petroplinthic pisoplinthic plinthic or pisolithic horizons their genesis started under a warm and humid tropical climate with distinct wet and dry seasons of the latest maastrichtian lower paleocene and continued under increasingly dry conditions during the middle to upper paleocene the major differences that occur among the paleosols can be related to differences in geomorphic position parent material characteristics and drainage conditions the pisolithic paleosols of central southern sardinia show a structure and composition comparable to those of the coeval ones in southern france which were formed under similar climatic conditions on the basis of the constant stratigraphic position the large geographic extent and the supposed former lateral continuity the studied ferruginous paleosols can be considered to be a good pedostratigraphic marker in central southern sardinia and in southern france and are informally named as the monte maraconis geosol this allows extensive chronostratigraphic correlations as well as regional scale morphostratigraphic correlations and represents with its typical pedofacies a pedostratigraphic unit worthy of consideration in future regional geological mapping

application of proton microprobe data to trace element partitioning in volcanic rocks abstract in situ proton microprobe analyses are used to calculate partition coefficients d for mn fe cu zr ga rb sr y zr ba and pb and more restricted data for ni ge nb and as in pyroxenes olivines biotites hornblendes feldspars and fe ti oxides from 32 volcanic rocks all with glassy matrices these range from leucitite and picritic basalt to high silica and peralkaline rhyolites data for pigeonite aenigmatite allanite and leucite are included where these occur as additional phases coefficients have also been rayleigh fractionation corrected this correction being significant for those elements with high coefficients in specific mineral phases and where pronounced crystallisation has occurred partition coefficients are presented as plots vs nbo t ratios of coexisting glasses melts negative correlations are observed based on log transformed data for mn zn fe and y pyroxenes mn zn and fe olivines mn and zn fe ti oxides nb y sr and zr hornblendes sr plagioclase and ba cu zn and fe feldspars these element partition coefficients are not correlated with bulk mineral compositions zr and ga are not correlated with either glass or mineral compositions except zr in hornblende the correlations support the role of melt polymerisation in controlling element partitioning at least for those elements listed although possible effects of temperature cannot be uniquely isolated in the present data set the coefficients presented here are generally consistent with published values for equivalent rock compositions one difference being that some of the higher published coefficients e g sr in plagioclase are not matched in the present data set from a practical viewpoint the relative regularity of the coefficient data presented does facilitate choice of relevant coefficient values that are required for petrogenetic modelling procedures

geothermobarometry and development of inverted metamorphism in the darjeeling sikkim region of the eastern himalayan abstract the darjeeling sikkim region provides a classic example of inverted himalayan metamorphism the different parageneses of pelitic rocks containing chlorite biotite garnet staurolite kyanite sillimanite plagioclase and k feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones microprobe data of coexisting minerals show that xmg varies in the order garnet staurolite biotite chlorite white mica is a solid solution between muscovite and phengite garnet is mostly almandine rich and shows normal growth zoning in the lower part of the main central thrust mct zone and reverse zoning in the upper part of the zone chemographical relations and inferred reactions for different zones are portrayed in afm space in the low grade zones oriented chlorites and micas and rolled garnets grew syntectonically and were succeeded by cross cutting chlorites and micas and garnet rims in the upper zones sillimanite kyanite and staurolite crystallized during a static inter kinematic phase p t contitions of metamorphism estimated through different models of geothermobarometry are estimated to have been 580 c for the garnet zone to a maximum of 770 c for the sillimanite zone the preferred values of pressure range from 5 0 kbar to 7 7 kbar models to explain the inverted metamorphism include overthrusting of a hot high himalayan slab along a c 5 km wide ductile mct zone and the syn or post metamorphic folding of isograds

strontium diffusion kinetics in plagioclase feldspars abstract a pronounced systematic variation by a factor of 104 in strontium tracer diffusion kinetics has been measured in four feldspar specimens that span the range of plagioclase compositions the experiments were carried out in air using isotopically enriched 86sr and analyzed with an ion microprobe the arrhenius relations for strontium diffusion are plagioclase ab d 0 m 2 s q kj g atom sr temperature range c albite 98 1 05e 279 550 1080 oligoclase 73 7 4e 7 261 750 1100 labradorite 36 1 1e 6 295 800 1300 anorthite 4 5 7e 9 267 900 1300 no difference was found within the measurement uncertainty between these results and those for albite under hydrothermal conditions at one kbar 100 mpa strontium diffusion rates parallel to and normal to the albite twin lamellae are the same there is no effect on the strontium diffusion rate in albite between an oxygen partial pressure of 0 2 bar air and 10 19 bar graphite buffer a relation is developed that permits calculation of the strontium diffusion arrhenius parameters for any normal plagioclase composition the large variation in diffusion rate with composition has a major effect on the ease of strontium isotope exchange at high temperatures between plagioclases and coexisting phases in rocks which may prove useful in determining thermal histories

climatic influences on continental deposition during late stage filling of an extensional basin southeastern arizona climatic rather than tectonic influences on continental deposition are recorded in the late stage fill of a neogene extensional basin in south eastern arizona regional geomorphic relations low sedimentation rates and stratigraphic overlap of the principal basin bounding structures identified in a gravity anomaly profile indicate that the plio pleistocene st david formation accumulated in the san pedro valley during a period of tectonic quiescence stable isotopic composition of paleosol calcite is placed within a magnetostratigraphic framework that provides an interpretive record of variable arid to semiarid climate in the valley before 3 4 ma the valley was hydrologically closed and experienced an arid climate with seasonal precipitation drainage integration at ca 3 4 ma corresponds with a transition to wetter conditions less seasonally variable precipitation and a rising water table indicated by hydromorphic paleosols and pond deposits gradually increasing seasonality of precipitation after 2 8 ma and further decrease in winter rainfall after 2 2 ma led to establishment of a dry monsoonal climate between 1 6 and 0 6 ma the change in climate at ca 1 6 ma coincides with abrupt appearance of sheet flood dominated fan gravels above earlier vertically aggraded fluvial channel and interfluve flood plain deposits this change in facies is interpreted as the result of climatic change rather than an example of subsidence driven gravel progradation sedimentation rates were greatest when climate was relatively wet with dominant winter rainfall and about equally low for relatively dry with dominant summer rain and relatively wet but nonseasonal conditions contrary to past model simulations channel flood plain facies ratios and sedimentation rate varied directly ribbon as opposed to sheet channel bodies were favored by relatively wet climatic conditions and were less commonly formed during dry strongly seasonal times precise process response interpretation of this correlation of sedimentological variability to changing climate is limited by the restricted interpretation of paleosolisotope paleoclimate data and the poor understanding of sedimentological response to climate change during 10 5 to 10 6 yr time intervals the results of this study suggest however that caution be applied to a priori interpretations of stratigraphic variations within thick syntectonic basin fills as responses to varying subsidence rates only

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

on balanced and unbalanced accommodation peat accumulation ratios in the cretaceous coals from gates formation western canada and their sequence stratigraphic significance coal composition was investigated by means of photometric and maceral analyses on closely spaced lithotype based strip samples over the full thickness of several paralic coal seams from the cretaceous gates formation of the western canadian sedimentary basin the aim of this investigation was to test various methods of identifying accommodation trends in coal and use them to refine sequence stratigraphic interpretation of continental sediments conventional sequence stratigraphy derives its subdivisions and significant surfaces from the records left by relative sea level oscillations these records either do not project into the continental realm or are difficult to recognise in clastic non marine sediments paralic coal seams have been selected to study this problem because they are not entirely removed from marine influence and compared with most inorganic deposits coal has stored a greater wealth of information that can be analysed at a higher level of resolution the study has led to the identification of five new surfaces with chronostratigraphic potential in the sequence stratigraphic analysis of non marine sediments two of these surfaces called paludification surface pas and terrestrialisation surface tes occur at the bases of the investigated coal seams while two other surfaces referred to as non marine flooding surface nfs and give up transgressive surface guts form the tops of the coal the fifth and probably most important new surface called the accommodation reversal surface ars is independent of any particular facies and may either coincide with some of the other surfaces or occur separately the proportion of detrital minerals has been used as the chief discriminator between different mire types and accommodation trends other useful indicators of mire type and peat dispersal have been the proportions of sporinite and inertodetrinite as well as some derived maceral and or mineral ratios e g the groundwater influence index and the tissue preservation index isometamorphic variations of telovitrinite reflectance and fluorescence as well as their coefficients of variation were also found to contribute to the identification of cyclic shifts between balanced and unbalanced accommodation peat accumulation ratios some of these cycles which are backed up by clastic stratigraphy appear to correspond to the development of shallowing upward and deepening upward parasequences superimposed high frequency low amplitude perturbations in the coal cycles relate to smaller scale accommodation cycles of sub parasequence level not always recognised in non marine strata these sub parasequence coal cycles do not always continue the shallowing upward trend typical of conventional parasequences several coals were found to contain stacks of small scale cycles with upward increasing accommodation signatures either in their lower or upper halves or over the whole seam section

rb and sr diffusion in alkali feldspars with implications for cooling histories of rocks volume diffusion of sr and rb have been measured in orthoclase feldspar as well as sr diffusion in albite using isotope exchange hydrothermal solutions and analysis by ion microprobe for water pressures of 1000 bars the kinetics of diffusion parallel to the c axis are given by the arrhenius relations strontium transport in the or is isotropic to within a factor of four the measurements use a new procedure in which only the elements in question are present in the starting fluid cooling rates of igneous rocks can now be determined from closure temperatures material balance and the rb sr dating method based on the departures of the mineral data from the whole rock isochron for cooling rates less than approximately 100 c my

magmatic residence times of zoned phenocrysts introduction and application of the binary element diffusion modelling bedm technique this paper describes a general technique binary element diffusion modelling bedm for determining single crystal residence times in magmas that relies on modelling the diffusion of two or more elements in the crystal bedm has the advantage over other diffusion based models in that it does not need a precisely defined initial compositional profile for the crystal at zero time and instead requires that the concentrations of the two elements are correlated during crystallisation any differences subsequently observed between the two elements are caused by intracrystalline diffusion during residence in hot magma these differences are removed by artificially ageing the slower diffusing of the two elements and the amount of time taken to undo the difference between the elements is simply related to the crystal residence time decoupling time at high temperatures the bedm principle is demonstrated using artificial data and is then applied to literature data for sr and ba in a zoned sanidine crystal from the bishop tuff anderson et al in j petrol 41 3 449 473 2000 for this crystal the method gives a residence time estimate of 114 ka at 800 c which is then compared with estimates from other methods in theory the method can be further expanded for use as a geothermometer as well as geochronometer however this is not easily possible with the diffusivity data currently available

introduction to special section mechanisms and consequences of melt segregation from crustal protoliths much of petrological effort has been concentrated on attempts to discern define and interpret regularly recurring patterns of igneous and metamorphic petrogenesis i do not wish to deny the existence of discernable order in petrological phenomena but every geological event is unique if we concentrate too heavily on discernment of order and pattern may we perhaps overlook or underrate the unique quality of each igneous or metamorphic episode possibly uniqueness may have as great significance as conformity to pattern when we attempt to fit the phenomena of petrology into the broad framework of geology francis j turner presidential address to the mineralogical society of america november 11 1969

loss of metals from pelites during regional metamorphism in aluminous metapelites the ratio h2o k2o decreases with increasing metamorphic grade and degree of reaction this ratio is a very practical indicator for the progress of the mineral reconstitution during progressive metamorphism with decreasing values of the ratio h2o k2o the cu concentration and the following element ratios also decrease either continuously or in stepwise fashion tl k2o ba k2o pb k2o bi k2o hg k2o sr na2o zn fe2 mg cd fe2 mg rb k2o remains approximately constant in the aluminous metapelites of the damara orogen in namibia the following losses occur between the biotite isograd and anatexis 61 cu 20 tl 34 ba 59 pb 86 bi 46 hg 30 sr 25 zn 31 cd thus the potential of regional metamorphism to form hydrothermal deposits in the low grade environment should not be neglected

quantitative modeling of granitic melt generation and segregation in the continental crust 1 we present a new quantitative model of granitic in a broad sense melt generation and segregation within the continental crust we assume that melt generation is caused by the intrusion of hot mantle derived basalt and that segregation occurs by buoyancy driven flow along grain edges coupled with compaction of the partially molten source rock we solve numerically the coupled equations governing heating melting and melt migration in the source rock and cooling and crystallization in the underlying heat source our results demonstrate that the spatial distribution and composition of the melt depends upon the relative upward transport rates of heat and melt if melt transport occurs more quickly than heat transport then melt accumulates near the top of the source region until the rock matrix disaggregates and a mobile magma forms as the melt migrates upward its composition changes to resemble a smaller degree of melting of the source rock because it thermodynamically equilibrates with rock at progressively lower temperatures we demonstrate that this process of buoyancy driven compaction coupled with local thermodynamic equilibration can yield large volumes of mobile granitic magma from basaltic and meta basaltic amphibolitic protoliths over timescales ranging from 4000 years to 10 myr the thickness of basaltic magma required as a heat source ranges from 40 m to 3 km which requires that the magma is emplaced over time as a series of sills concurrent with melt segregation these findings differ from those of previous studies which have suggested that compaction operates too slowly to yield large volumes of segregated granitic melt

alkali diffusion in plagioclase feldspar tracer diffusion kinetics at 1 atm pressure under dry conditions using 6li 41k and 87rb in four plagioclase feldspars have been determined diffusion profiles were measured using an ion microprobe sims the arrhenius parameters and 2 uncertainties for li k and rb respectively in albite are pre exponential factor log do 3 8 1 0 3 4 1 4 and 5 8 3 8 in m2 s and activation energies 146 14 296 30 and 283 85 kj mol the diffusion rate of k in albite at 700 c and 100 mpa water pressure equals that at 1 atm dry suggesting that diffusion of k is not dependent on water fugacity between 0 004 and 100 mpa the k diffusion rate depends on plagioclase anorthite content and is approximately 20 times slower in labradorite than albite li diffusion rates in albite and anorthite are identical a linear relation between log d for the alkalis measured and their ionic radius squared permits an estimate of cs diffusion kinetics diffusion rates are isotropic or nearly so in albite

chronostratigraphic synthesis of the latest cretaceous dinosaur turnover in south western europe abstract in south western europe the uppermost cretaceous continental deposits in the different sedimentary basins of iberia portugal and north and central spain the pyrenees spain and france as well as languedoc and provence southern france provide one of the few terrestrial records that allow a comprehensive study of the campanian maastrichtian dinosaur assemblages for the last years the southern pyrenees has been the target of intense geological palaeontological and geochronological research hundreds of fossil localities are now framed in high resolution lithological sections the succession of these sites most of them located in the tremp syncline is based on the physical correlation of rock bodies as permitted by the general outcropping conditions outside this syncline correlation is supported by geochronologic and biostratigraphic data mainly magnetostratigraphy and planktic foraminifera biostratigraphy the integration of the entire dataset sheds new light on the maastrichtian dinosaur turnover characterized by a shift from a sauropod dominated to a hadrosauroid dominated faunal assemblage this turnover was progressive and involved immigrants from north america eurasia and gondwana which probably reached the study area after a sea level drop this faunal change was mainly triggered by the arrival of lambeosaurine hadrosauroids a group that rapidly displaced the rest of the herbivorous clades of the region some of the extinction events suffered by the pre turnover faunas during the maastrichtian coincide with marine isotopic and sea level drop events suggesting that faunal competition was not the only cause of the observed changes in dinosaur composition despite this faunal replacement the resulting ecosystem after the turnover shows no major loss of biodiversity before the cretaceous paleogene extinction event

labyrinthine facies architecture of the tortola fluvial system and controls on deposition late oligocene early miocene loranca basin spain abstract upper oligocene to lower miocene deposits of the tortola fluvial system loranca basin central spain display a labyrinthine stratigraphic architecture comprising isolated sandstone bodies embedded in mudstone and siltstone these sandstone bodies are subdivided into seven types which are interpreted to have formed in rivers with different discharges sediment yields and channel planform patterns on the basis of their characteristics and spatial distributions four facies zones are defined these formed a continuum along the tortola fluvial system profile each being characterized by specific depositional conditions and consequently channel pattern styles gravel rich braided streams dominated zone 1 down gradient these streams graded into wide sand dominated braided streams of zone 2 which contain large bedforms and were typified by significant discharge variations zone 3 comprises belts of mixed load dominated meandering and low sinuosity channels zone 4 represents distal floodplain areas at the toe of the fluvial system depositional conditions in a facies zone varied between an upper and lower bound or geomorphic threshold a crossing of the upper or lower threshold is indicated by relatively sudden but orderly lateral and vertical changes in channel pattern that in turn reflect superposition of facies zones a facies cycle is formed by superposition of two successive downstream facies zones superposition of facies zones is inferred to have been caused by changes in sediment supply and hydraulic regime and consequently the regional gradient of the system it is argued that the tortola fluvial system developed under a weak tectonic regime during the later stages of basin filling and was possibly affected by climatic changes changes in size of the facies zones typified the long term development of the tortola fluvial system and determined the three dimensional architecture of the succession

decadal time gaps between mafic intrusion and silicic eruption obtained from chemical zoning patterns in olivine abstract intrusion of mafic magma in a silicic crustal reservoir has been reported in numerous studies of subduction related volcanoes and is considered a main process for triggering explosive eruptions and for transferring geochemical signatures from the mantle to the crust here we determine the timescales between mafic silicic encounters and eruption by exploiting the fact that during such interactions magmas exchange crystals and their mineral compositions will strive to equilibrate to the new environmental conditions specifically we model the fe mg zoning patterns of olivine found in the dacites andesite and quenched mafic inclusions of the holocene zoned eruption of volcan san pedro 36 s chilean andes the san pedro eruption is an example where mixing between silicic and mafic magmas produced intermediate compositions e g andesite at depth in the magma reservoir for modeling the compositional profiles we took into account the anisotropy of fe mg diffusion in olivine and also the effects of diffusion in multiple dimensions aspects that have been largely neglected in previous works the retrieved timescales for olivine melt equilibration by diffusion display an array ranging from 1 year for the most silica rich dacite to 50 years for the andesite this array of timescales rather than a single value provides new insight into the details of mafic silicic mixing at depth they reveal processes that occur on a longer timescale than the days to weeks time gap for cases where the mafic intrusion triggered the eruption and they might be associated with the instances where seismic activity or significant inflation of the volcanic edifice was not immediately followed by eruption on the other hand the year to decade timescales that we report for producing magmas of intermediate composition e g andesites are much shorter than the 10 3 10 5 years inferred from u series disequilibria data for igneous differentiation by closed system fractionation to produce similar magma compositions at subduction zones

a new global paleocene eocene apparent polar wandering path loop by stacking magnetostratigraphies correlations with high latitude climatic data a new apparent polar wander path apwp from the beginning of the paleocene 65 ma to the middle of the mid eocene 42 ma is shown to be correlated with polar climatic data of the same time period rather than applying the classical method based on analysis of site based poles we stacked the apwps obtained from magnetostratigraphies magnetostratigraphies have the advantage of displaying an unbroken record of local apwps through time and for a magnetozone defined as the a combination of normal and reversed polarity intervals the instantaneous poles are synchronous seven magnetostratigraphies located on 4 different plates covered sufficient time to be used in the analysis an average apwp was then determined with respect to age at the magnetozone level for the african plate which was arbitrarily chosen as a reference frame virtual geomagnetic poles were transferred onto the african plate using ocean kinematic euler rotations the calculated apwp is characterized by a loop with two main changes of direction at magnetozones 26 25 61 5 56 5 ma and 24 22 56 5 48 6 ma distinct at a 95 level of probability and indistinct poles related to magnetozones 29 27 65 5 61 5 ma and 21 19 48 6 40 6 ma we also show that the implied rapid shift of the lithosphere with respect to the geographic pole possibly an episode of true polar wander was coeval with the time evolution of vertebrate occurrence on ellesmere island canadian arctic and with the tree ring growth rate in western antarctica

geochemistry of granitic melts produced during the incongruent melting of muscovite implications for the extraction of himalayan leucogranite magmas granites formed from the incongruent melting of muscovite preserve evidence of chemical disequilibrium both between restite and melt and within individual phases during crystallization miocene leucogranites from the himalayas exemplify melt formation from a pelitic source through muscovite breakdown trace elements rb sr and ba reside almost entirely in micas and feldspars in the protolith and therefore are appropriate for quantitative modeling equilibrium between melt and restite was established for sr under fluid phase absent conditions but some disequilibrium is suggested by rb abundances consistent with known diffusion rates for these elements abundances of zr and ce in the granites which provide essential structural constituents of the accessory phases zircon and monazite indicate that the himalayan granite melts were saturated in these elements if formed at temperatures of 670 700 c but slightly undersaturated if formed at somewhat higher temperatures ion microprobe traverses of y and heavy rare earth element abundances across single garnets from himalayan leucogranites provide evidence for disequilibrium between melt and phenocryst during garnet growth thus constraining the evolution of magmatic compositions during melt formation and crystallization in some cases progressive melting of the protolith and crystallization of garnet from the melt appear to have been concomitant indicating that minimum solidus temperatures were maintained throughout the melting event this suggests that thermal buffering rather than melt extraction provided the rate determining step in magma evolution

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

melt segregation in migmatites anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity features that have not been explained satisfactorily but which are related to physical processes of melt segregation we evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith compaction by gravity driven two phase flow results in asymmetric separation yields calculated segregation times that are slow or geologically unreasonable and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites although calculated segregation times for very wet granite might allow segregation by compaction some driving force in addition to gravity is needed to cause widespread melt segregation in the crust for segregation at low volume fraction of melt we develop a two dimensional two phase matrix and fluid viscous flow cell model length much greater than thickness with phase changes which predicts either expansion or contraction convection depending on whether volume change vr is positive or negative convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale at moderate volume fraction of melt segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress melt migration is by porous medium flow driven by differences in mean normal stress between the layers as a consequence of differences in rheology and shear enhanced matrix collapse calculated segregation rates are fast and the model yields adequate volume of leucosome the positive vr for volatile phase absent melting reactions under crustal pressures promotes melt escape unless extensional deformation facilitates melt accumulation if the rate of melt production exceeds the rate of melt escape then the increase in melt pressure may lead to melt enhanced embrittlement and fracture and melt may migrate out of the system in contrast water rich volatile phase present melt producing reactions have a negative vr which promotes melt retention after segregation unless deformation generates melt escape pathways evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy and magma may become mobile and entrain residual material restite

siliciclastic sequence stratigraphic patterns in foreland ramp type basins stratal patterns within shelf depositional sequences are dependent on tectonically controlled subsidence rates and their regional patterns in active tectonic basin settings regional subsidence patterns can be very different from passive continental margin settings resulting in substantial modifications of the basic sequence stratigraphic model in ramp type foreland basins on the tectonically active side subsidence rates decrease seaward in contrast with passive continental margins where the opposite subsidence pattern exists analysis of the interplay between eustasy and subsidence suggests the existence of two tectono stratigraphic zones occurring proximally and distally with respect to the basin margin zone a is defined as the region within which the rate of subsidence always exceeds the rate of eustatic fall consequently relative sea level rises continuously during a eustatic cycle albeit at varying rates zone b is defined as the region within which the rate of eustatic fall periodically exceeds the rate of subsidence resulting in an interval of relative sea level fall during a eustatic cycle on the tectonically active sides of foreland basins zone a lies on the landward side of the basin margin proximal to the orogenic belt and zone b lies seaward off zone a away from the orogenic belt where subsidence rates are lower on passive continental margins because of the opposite subsidence patterns zone a lies seaward of zone b where subsidence rates are greater the location of the shoreline relative to these zones determines the stacking patterns and stratal discontinuities within a depositional sequence if the shoreline remains in zone a then only type 2 sequence boundaries will occur if sufficient sediment flux is available allowing the shoreline to prograde into zone b then type 1 sequence boundaries can occur in foreland basins these type 1 sequence boundaries would become type 2 sequence boundaries updip in zone a the pattern of seaward decreasing subsidence on the tectonically active sides of foreland basins results in characteristic longitudinal facies and stratal patterns when the two zones occur the updip region of the basin proximal to the orogenic belt is characterized by nearly continuous nonmarine deposition albeit at varying rates the downdip region is characterized by forced regressions and deposition primarily of lowstand and transgressive systems tracts a transitional region containing deposits of all three systems tracts commonly occurs between the updip and downdip areas

the cuyama strike slip basin california u s a an exemplar of contrasting syntectonic and post tectonic strata abstract miocene strata in the cuyama basin of southern california exemplify syntectonic and post tectonic stratigraphic development in a strike slip basin integrated seismic well log and outcrop data provide a detailed portrayal of the seismic stratigraphy and sedimentary facies in the previously unstudied middle and upper miocene sequences results show sharp contrast among syntectonic and post tectonic strata from composition and texture of sediments to lateral distribution and thickness of facies seven post tectonic sequences were mapped across the basin sequences contain eolian to fluvial terrestrial facies wave and tide dominated nearshore facies biosiliceous dolomitic and phosphatic outer shelf and slope mudstones and shale rich submarine fans in basinal environments authigenic dolomites and phosphates are pervasive in outer shelf and slope facies and indicate the possibility of restricted sediment supply post tectonic sequences are thin but laterally continuous and they exhibit little depositional asymmetry minor erosion is distributed equally among regressive and transgressive relative sea level changes asynchronous changes in tectonism sediment supply and eustatic sea level change created dramatic differences among syntectonic and post tectonic sequences interbasinal tectonic quiescence led to the abrupt end of stratigraphic mismatches and the accumulation of sediment during brief episodic periods multiple points of progradation persisted through several million years of post tectonic basin development and highlight the importance of extrabasinal sediment supply to the development of all sequences the eventual loss of most accommodation space promoted the importance of eustatic sea level change and reduced the prominence of asymmetric facies and abrupt lateral facies changes

gneiss domes and orogeny many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression the decreasing buoyancy associated with increasing melt fraction drives further decompression at near isothermal conditions as the partially molten crust rises diapirically this combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep seated migmatite terranes and inferred for active orogens in exhumed orogens the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite granite the large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis

nd isotope disequilibrium during crustal anatexis a record from the goat ranch migmatite complex southern sierra nevada batholith california geological and geochemical studies of a pelitic migmatite complex within the isabella pendant of the southern sierra nevada batholith california suggest that the leucosomes represent the products of partial melting of the metapelite host driven by the emplacement of the adjacent goat ranch pluton ca 100 ma the leucosomes preserve a record of largemagnitude nd isotope disequilibrium with respect to their pelitic source the leucosomes have a wide range of nd 100 ma from 26 0 to 211 0 as compared to 28 7 to 211 3 for the source they can be subdivided into two groups based on their major elements and sr and nd isotope geochemistry group i leucosomes have higher p2o5 contents and nd 100 ma values than those of group ii the nd 100 ma values of group i leucosomes are significantly higher than those of metapelites and migmatites by two to four epsilon units suggesting that group i leucosomes are in nd isotope disequilibrium with their sources correlations among p2o5 contents nd 100 ma values and sm nd ratios in the leucosomes suggest that apatite or monazite has played a dominant role in fractionating sm from nd and generating nd isotope disequilibrium dissolution of apatite or monazite might play a critical role in regulating the behavior of the sm nd isotope systems and thus the nd isotope compositions of melts generated during crustal anatexis especially in metasedimentary protoliths

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

diffusion in solid silicates a tool to track timescales of processes comes of age understanding how processes occurring on a wide range of temporal and spatial scales combine to produce a stable dynamic earth is a major goal of the earth scientist determining durations of processes is a key step toward attaining that goal records of incomplete diffusive equilibration preserved in minerals are uniquely suited for the purpose of unraveling timescales of a variety of processes compositional zoning in minerals is like the tracks of a cd that can be decoded with suitable technology this review discusses the causes for the limited use of this tool until recently and how these hindrances are being overcome examples are presented to illustrate that diffusion modeling can clock processes that last from only a few days to those that last over tens of millions of years recorded in rocks that range in age from current volcanic eruptions to condensates from the early solar nebula

exhumation history of a section of the sikkim himalayas india records in the metamorphic mineral equilibria and compositional zoning of garnet the exhumation history of pelitic migmatite samples from the high himalayan crystalline complex hhc near the south tibetian detachment system in the sikkim darjeeling section has been determined on the basis of thermo barometric analyses retrograde breakdown reactions and compositional zoning of garnet the peak metamorphic condition is estimated to be 10 4 kbar 800 c from thermo barometric and phase equilibrium constraints the observed retrograde breakdown of garnet to spinel and cordierite requires near isothermal and hence extremely rapid 15 mm yr exhumation up to the depth of 15 km numerical modeling suggests that the initial rapid exhumation must have been followed by a much slower process 2 mm yr up to at least 5 km depth to lead to the development of the observed compositional zoning of garnet the dramatic change of the exhumation velocity vz might reflect a process of tectonic thinning followed by erosion and or horizontal flow at shallow depth assuming that the vz did not change significantly below 5 km depth these results suggest that the hhc samples studied in this work exhumed from a depth of 34 km within 8 ma

coupling of anatectic reactions and dissolution of accessory phases and the sr and nd isotope systematics of anatectic melts from a metasedimentary source advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the rb sr and sm nd isotope systems in anatectic melts from a muscovite bearing metasedimentary source modeling results show that 1 the sm nd ratios and nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt and 2 the relative proportion of micas muscovite and biotite and feldspars plagioclase and k feldspar that enter the melt is a key parameter determining the rb sr and 87 sr 86 sr ratios of the melt furthermore these two factors are not in practice independent in general nonmodal partial melting of a pelitic source results in melts following one of two paths in e nd 87 sr 86 sr ratio space a higher temperature fluid absent path path 1 represents those partial melting reactions in which muscovite biotite dehydration and apatite but not monazite dissolution play a significant role the melt will have elevated rb sr 87 sr 86 sr sm nd and e nd values in contrast a lower temperature fluid fluxed path path 2 represents those partial melting reactions in which muscovite biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue the melt will have lower rb sr 87 sr 86 sr sm nd and e nd values than its source the master variables controlling both accessory phase dissolution and hence the sm nd system and melting reaction and hence the rb sr systematics are temperature and water content the complexity in sr nd isotope systematics in metasediment derived melts as suggested in this study will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component

styles of post subduction collisional orogeny influence of convergence velocity crustal rheology and radiogenic heat production abstract a number of models of continental collision already exist but the role of foregoing oceanic plate subduction or variable coupling between plates still remains to be explored in addition heat generation by radioactive decay may be quite variable for example our geochemical data from low to high grade metamorphic rocks of the lesser and higher himalayan sequences reveal measured heat production values on the order 4 5 w m 3 which is considerably higher than normal inferred upper crustal values 3 therefore we performed 2d numerical experiments of oceanic continental subduction followed by continental collision for varying continental lower crustal strength convergence velocity and radiogenic heating rate oceanic subduction produces thinning and upwarping of the overriding lithospheric wedge such as is observed in andean type collisional settings cascadia and central andes after collision two main orogenic styles develop depending on the values of key parameters either a slow convergence rates weak shear heating and consequently strong rheological coupling between plates and or a weak lower crust generate a two sided thick and narrow symmetrical collisional zone or b high convergence rates strong shear heating and therefore weak rheological coupling between plates and a strong lower crust produce a one sided thin and broad asymmetrical collisional zone in this case a vertical lithospheric body divides two upper crustal wedges that develop distinct deformation patterns in both cases enhanced radiogenic heating favors the formation of midcrustal partially melted channels that propagate toward the surface and the pro foreland partial melting triggers lateral extrusion of metamorphic rocks from different depths 15 23 kbar 700 c on top of coherent lp lt units the geometry of metamorphic isograds inverted shapes of p t paths at different grades and exhumation rates are similar to those observed in parts of the himalayan chain indicating that the models capture the essence of real systems the model results demonstrate that internal heat production rate is a crucial parameter in determining the timing of slab breakoff polarity reversal and exhumation of the inverted metamorphic sequence with respect to the timing of partial melting late stages of some numerical experiments showed interesting geodynamical evolution of the collisional zone with the appearance of polarity reversal roll over of both plates and roll back of the subducting slab irrespective of these details hundreds of km of continental lithosphere are recycled into the mantle in all models supporting the idea that continental crustal consumption is indeed a common and considerable process during collision this affects inferences about extents of convergence and or shortening during orogenesis moreover crustal recycling into the deep mantle during collisional orogeny needs to be considered in models of crustal growth and evolution

time scales of magmatic processes from modeling the zoning patterns of crystals the advent of polarized light microscopy in the middle of the 19th century allowed mineralogists and petrologists interested in igneous rocks to recognize the widespread occurrence of fine scale heterogeneities in the optical properties of minerals e g see young 2003 for details the interpretation of mineral zoning patterns as archives of magmatic processes has been with us for some time e g larsen et al 1938 tomkeieff 1939 the development of the electron microprobe in the 1960 s allowed mineral zoning profiles to be quantitatively analyzed and modeled e g bottinga et al 1966 moore and evans 1967 enhanced textural observations permitted recognition of many kinds of detailed structures during the growth and dissolution of magmatic minerals fig 1 e g anderson 1983 pearce and kolisnik 1990 and these processes were also explored using experimental and numerical models e g albarede and bottinga 1972 lofgren 1972 kirkpatrick et al 1976 loomis 1982 major element zoning patterns are routinely measured and with the arrival of the ion microprobe the identification of heterogeneous distribution of trace elements opened the window for more realistic and sophisticated scenarios and models fig 1 e g kohn et al 1989 blundy and shimizu 1991 singer et al 1995 we now measure abundances of naturally occurring isotopes at the scale of tens of micrometers e g davidson et al 2007a ramos and tepley 2008 and this allows in situ dating of crystals e g cooper and reid 2008 figure 1 a nomarski dic image of a plagioclase crystal from a gabbroic xenolith of volcan san pedro costa et al 2002 the crystal shows a well defined core and a rim separated by an oscillatory zoned interval ion microprobe analysis pits and the trace of the electron microprobe traverse between the pits are evident b major

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

rare earth diffusion kinetics in garnet experimental studies and applications abstract we determined the diffusion coefficient of sm in almandine garnet as function of temperature at 1 bar and fo 2 corresponding to that of wustite iron buffer and to a limited extent that of a few other selected rare earth elements in almandine and pyrope garnets both garnets were demonstrated to have metastably survived the diffusion annealing at conditions beyond their stability fields the experimental diffusion profiles were analyzed by secondary ion mass spectrometry and in addition by rutherford back scattering spectroscopy for two samples transmission electron microscopic study of an almandine crystal that was diffusion annealed did not reveal any near surface fast diffusion path using reasonable approximations and theoretical analysis of vacancy diffusion the experimental data were used to develop an expression of rare earth element ree diffusion coefficient in garnet as a function of temperature pressure fo 2 ionic radius and matrix composition calculation of the closure temperature for the sm nd decay system in almandine garnet in a metamorphic terrain shows very good agreement with that constrained independently modeling of the ree evolution in melt and residual garnet suggests that for dry melting condition the ree pattern in the melt should commonly conform closely to that expected for equilibrium melting however for much lower solidus temperatures that would prevail in the presence of a h 2 o co 2 fluid the concentration of light ree in the melt could be significantly lower than that under equilibrium melting condition a reported core and rim differences in the ree content of a garnet crystal in a mantle xenolith in kimberlite have been reproduced by assuming that the ree zoning was a consequence of entrapment in a magma derived from an external source for 32 000 yr before the eruption

high p t polymetamorphism dehydration melting and generation of migmatites and granites in the higher himalayan crystalline complex sikkim india the higher himalayan crystalline complex hhc in sikkim india consists of pelitic migmatites interlayered with calc silicate rocks and minor metabasites microstructural relationships between the mineral phases and deformational fabric elements and zoning characteristics of garnet indicate a prolonged and complex polymetamorphic history for the hhc the pelitic rocks in the upper part of the hhc contain the assemblage plagioclase quartz garnet k feldspar biotite sillimanite and are devoid of muscovite most of the mineral phases grew syn to post tectonically mineral growth coincided with development of a pervasive fabric s 2 during prograde metamorphism m 2 in the early stages of the collisional event some garnet grains display texturally distinct cores and rims which are separated by calcic plagioclase this texture suggests an earlier metamorphic episode m l m may represent pre himalayan metamorphism and decompression of the hhc later the collisional event led to renewed burial of the hhc and m 2 reactions m 2 is reflected by dehydration melting of muscovite and biotite to form granitic melts which either crystallized in situ to form leucosomes or migrated from their source regions to form larger granitic bodies geothermobarometric estimates for peak m 2 conditions indicate p 10 12 kbar t 800 850 c a subsequent metamorphic event m 3 occurred because of 5 kbar of decompression m 3 is recorded by the breakdown of porphyroblastic garnet in all hhc lithologies higher temperature and pressure estimates come from progressively higher structural levels of the complex the thermal gradient of 5 5 c km is anomalous and may be a consequence of thermal buffering during melting however the pressure gradient of 0 25 kbar km resembles a normal lithostatic gradient which suggests that the hhc in sikkim represents an inverted barrovian sequence this inverted zonation of the hhc is probably the result of large scale structural inversion and or tectonic juxtaposition because of ductile shearing

the role of melt in the uplift and exhumation of orogenic belts abstract in some orogenic belts rapid pulses of uplift occur when melt is present detailed work on the relation of melt to structural and metamorphic features in several localities shows that hot metamorphic rocks and especially anatectic migmatite can be physically transported from the lower to middle crust on decollements the decollements form in the lower crust because even though the lower crust is structurally weak throughout due to thermally activated crystal plastic deformation mechanisms the crust is substantially weakened where melt forms or is intruded melt in the lower crust is present long enough to accomodate strain whereas in the upper crust it crystallizes too fast to be a factor in strain localization following crustal loading due to thrusting exhumation results in the formation of additional melt in the migmatites due to decompression melting support of this sequence of events includes 1 evidence for near isothermal decompression of high temperature metamorphic rocks with early high pressure minerals having formed pre or synchronous with penetrative deformation and with lower pressure minerals having formed after the deformation 2 the occurrence of magma sheets that occur along decollements between high temperature metamorphic rocks and lower temperature metamorphic rocks in either thrust or normal configurations 3 nearly concordant k ar cooling dates for biotite and hornblende suggesting rapid cooling rates and 4 post tectonic intrusions of leucogranite or leucotonalite that may represent the products of decompression melting the crystalline slabs of the high himalaya the compressional surges of the coast orogen of british columbia and alaska the cordillera of southern british columbia and the southern belt of the hercynian chain are examples where melt assisted rapid pulses of uplift appear to have occurred during the terminal phases of uplift and following uplift these orogenic belts were exhumed during extension while melt was still present

development of tectonic cyclothems in rift pull apart and foreland basins sedimentary response to episodic tectonism the thickest part of asymmetric rift pull apart and foreland basin fills commonly consists of large scale hundreds to thousands of metres thick tectonically generated cyclothems of fine grained marine lacustrine or longitudinal fluvial deposits and coarse grained transverse braid plain or alluvial fan deposits the appearance of coarse clastics in these basin fills is typically noted as the time of tectonically rejuvenated source area uplift based on the conceptual tie between relief and coarse grain size and on the application of the davis theory of landform development we propose the opposite interpretation that the commencement of fine grained sedimentation above coarse grained deposits in a tectonic cyclothem is the best indicator of renewed tectonic activity this reinterpretation is more consistent with 1 modern examples 2 the consideration of source area and sedimentary basin geomorphology 3 the disparity between the reaction rates of the various environments to subsidence 4 the disparity between the rates of tectonic uplift and erosion and 5 the controls on elastic wedge progradation in our model extensive coarse grained clastic wedge progradation is an indicator of tectonically quiescent phases

paleoclimate controls on stratigraphic repetition of chemical and siliciclastic rocks climate is a primary control on sediment flux from continental sources into sedimentary systems in warm climates siliciclastic input is greatest under highly seasonal rainfall nonseasonal conditions favor formation of end member chemical rocks perennially wet climates are conducive to coal formation whereas dry climates produce carbonates and or evaporites stratigraphic repetition of siliciclastic and chemical rocks therefore appears to be related to paleoclimate cycles as well as to transgressive regressive events and tectonics

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods