measured rates of sedimentation what exactly are we estimating and why abstract data on rates of sedimentation are essential in studies of sedimentation systems these data are obtained in three main study contexts 1 the study of sedimentation systems that are active today 2 the source to sink study of sedimentation systems that no longer are active and 3 the study of the relationship between accumulation rate and measurement timespan the aim of this paper is to question the meaning of measured rates of sedimentation and their interpretability particularly in these three contexts individual measurements of rate of sedimentation must always be interpreted with care firstly there are different definitions for instance with different statistical support or measurement dimension values that are defined differently cannot be compared directly secondly appropriate sampling schemes must have been used for the measurement this minimises sampling bias thirdly the inherent limitations of the data sources must be taken into account rates of sedimentation can usually be measured successfully in active sedimentation systems the same is not true for systems that are no longer active these can only be studied using the stratigraphic successions left behind rates of erosion can never be measured successfully in stratigraphic successions rate of sedimentation is essentially a ratio an amount of sedimentation per length of time therefore the obvious strategy to use in determining it is first to measure the amount and the time independently then to combine the values the amount can be measured in terms of thickness or volume or mass per unit area the duration of the time interval can be preset using quasi continuous measurement techniques or site reoccupation or it can be identified from interval specific sedimentary structures or it can be measured using dated horizons an alternative strategy is to use a surrogate measurement variable rates of erosion in ancient systems are usually measured in this way using cosmogenic radionuclide concentrations these two strategies are reviewed in this paper sets of measurements made in systems that are active today can certainly be used to estimate the rate of sedimentation for the system as a whole this estimation is best carried out using geostatistical estimation techniques the alternative is simply to average the measured rate values this latter approach should not be used however because the mean sedimentation rate in a system gives information only about the net sediment movement at the system boundaries it says nothing at all about how the system is operating or about its spatial and temporal variability measurements of rates of sedimentation made for source to sink studies are necessarily made in stratigraphic successions the measurements are used to estimate quantities in the sediment mass budget equation the amount of decumulation is inherently incapable of being measured in stratigraphic successions therefore there are always unknowns in the mass budget equation whenever the lithic surface at the start of the time interval considered cannot be recognised everywhere this means that the mass budget equation is applicable in practice only when all the systems involved in the study are entirely non erosional for that entire time interval a highly unrealistic situation the consistently inverse relationship documented between accumulation rates and measurement timespan is taken usually to indicate that this relationship is substantially scale invariant this in turn is often taken as indicating that the stratigraphic record is fractal in nature there are nevertheless grounds for doubt all of which relate to the ways that the data are collected and used for estimation the relationship is in fact the natural mathematical result of last in first out lifo operation and is produced in any type of system in which addition and removal processes both operate it says nothing particular about sedimentation processes the future analysis of accumulation rate data collected from stratigraphic successions will sensibly be framed in the context of estimating the parameters of a lifo model

ordered hierarchy versus scale invariance in sequence stratigraphy sequence stratigraphy has been applied in a wide range of scales of time and space from decimeter thick layers formed within hours to kilometer thick basin fills formed during hundreds of millions of years the traditional approach to practice sequence stratigraphy in this wide range of scales is to subdivide the sediment piles into an ordered hierarchy of sequence cycles of different duration and different architecture an alternative are scale invariant models with fractal characteristics published data confirm two predictions of the ordered hierarchy model sequences of very short duration 200 106 years are symmetrical transgressive regressive cycles however the sequence record in the range of 1 104 200 106 years the principal domain of sequence stratigraphy shows a rather irregular succession of sequences with variable symmetry and bounded by flooding surfaces or exposure surfaces for these time scales scale invariant models are a good first approximation particularly because the evidence for scale invariance and randomness in the stratigraphic record is strong frequency spectra of sea level change as well as rates of sedimentation and rates of accommodation change plotted against length of observation span show basic trends indistinguishable from random walk these trends combined with scale invariant sequence models may be the most efficient tools for across the board predictions on sequences and for locating islands of order in the sequence record

stochastic and deterministic controls on stratigraphic completeness and fidelity compilations of sedimentation rates determined across a large range of time spans and different sedimentary environments attest to the discontinuous nature of sedimentation and support the reasoning that hiatuses pervade sedimentary successions at multiple spatial and temporal scales the relationship between sedimentation rate and time span of observation also emphasises how stratigraphic completeness is a time span dependent concept the scaling of stratigraphic completeness with time span alludes to the likely stochastic and fractal nature of the stratigraphic record it also implies that the relationship between stratal thickness and time is inherently non linear and this has obvious implications for the way we should expect temporal processes to be recorded stratigraphically order within the stratigraphic record may arise from deterministic processes in stratigraphy most notably through astronomically influenced sedimentation intuitively a strongly ordered stratigraphic record due to the presence of cyclicity in sedimentation increases stratigraphic completeness at time spans equal to and longer than the time span of the cyclicity and minimises the non linearity between stratal thickness and time here geological data and simple modelling are used to support this supposition but these results also highlight how stochastic variability inherent in depositional processes may sensitise certain successions to missed cycles and incompleteness even in the presence of strong cyclicity this stochastic variability has the potential to mask evidence of astronomical forcing of sedimentation it is also shown how hiatuses affect the ability of a succession to faithfully record temporal processes and this is true regardless of whether hiatuses are deterministically controlled or random or indeed whether a succession is considered complete or not consequently stratigraphic completeness and stratigraphic fidelity are not synonymous terms

recurrent hierarchical patterns and the fractal distribution of fossil localities understanding the spatial structure of fossil localities is critical for interpreting earth system processes based on their geographic distribution coordinates of marine and terrestrial sites in the conterminous united states for 17 time bins were analyzed using point pattern statistics lacunarity analysis shows that the spatial distributions of sites are fractal for almost every studied interval indicating that clumping of localities occurs at multiple scales random hierarchical multiplicative processes provide a theoretical null model for the distribution of collecting sites consistent with their occurrence being a complex product of numerous biological geological and anthropogenic processes acting at many spatial and temporal scales mechanistic models for the formation preservation and exposure of fossil localities and other geologic entities can be tested using point pattern and related spatial statistics

the ties linking rock and fossil records and why they are important for palaeobiodiversity studies a correlation exists between the quality of the rock record and the diversity of fossils recorded from that rock record but what drives that correlation and how consistent that correlation is across different environments remain to be determined palaeontologists wishing to investigate past diversity patterns need to first address issues of geological bias in their data the fossil record provides the only empirical evidence of how life has diversified over geological time but it needs to be interpreted with caution for many years the history of diversity was estimated simply by summing up the numbers of taxa species genera families palaeontologists have recorded from successive geological time intervals or extrapolated from their first and last occurrences in the geological record this time series approach led to what is now a classic view of how diversity has changed over time sepkoski et al 1981 benton 1995 sepkoski 1997 such counts take the fossil record at face value or assume that biases or errors are randomly distributed in such a way that the overall effect is negligible we now realize that this is only a first crude approximation that may be subject to a number of strong biases that arise because of the nature of the sedimentary rock record time series analysis requires that sampling be carefully controlled for best results ideally data should be collected so that sampling from each time interval is uniform or at least sampled fairly using the shareholder quorum subsampling method alroy 2010 otherwise apparent changes in diversity may arise for spurious reasons for example because a a time intervals being sampled are of variable duration longer time intervals 1 4 more recorded diversity or b a time interval has been more intensively sampled more localities specimens habitats formations sampled 1 4 more recorded diversity while for a biological survey it is easy to plan a sampling strategy that will give approximately equal effort and coverage for observations palaeontologists are faced with a much more difficult task firstly the time bins they work with are irregular and highly variable in their duration for example the durations of one of the most widely used time scales has intervals spanning two orders of magnitude gradstein et al 2004 secondly and much more critically palaeontologists are collecting from an already incomplete and highly biased set of rocks which in turn skew the range of taxa and habitats that can be sampled zuschin et al 2011 bluntly put we cannot sample what is not preserved in the rock record although phylogenetic and molecular approaches can attempt to compensate pol norrell 2006 bininda emonds et al 2007 wills 2007 o connor et al 2011 so while it may be possible to standardize for sampling effort from the rock record that remains those rocks might provide a far from uniform sample of the sediments and palaeoenvironments that were originally present palaeontologists cannot therefore assume uniform sampling of the fossil record and must try to assess how the rock record they have to work with has affected their ability to sample evenly and then develop methods that compensate appropriately for this variation in recent years therefore palaeobiodiversity studies have become more probabilistic in their approach concerned with establishing confidence limits around estimates that try to correct for uneven sampling alroy et al 2001 2008 and testing empirical patterns against model predictions foote 2001 smith mcgowan 2007 mcgowan smith 2008 if we are to improve our estimates of biodiversity over time then sampling parameters need to be better quantified and we need to better understand the complex interrelationship between rock and fossil records this is no simple task and requires a more systematic approach to recording culture as well as better documentation of the variables benton et al 2011 figure 1 summarizes the problem as we see it palaeontologists estimate past diversity from remains preserved in the rock record as with any from mcgowan a j smith a b eds comparing the geological and fossil records implications for biodiversity studies geological society london special publications 358 1 7 doi 10 1144 sp358 1 0305 8719 11 15 00 the geological society of london 2011 biological survey variation of sampling effort of the surviving rock record needs to be factored out but this rock record has already passed through a geological filter that has altered and distorted not only the rock area that survives to sample but also the proportions of facies and the preservational quality of its fossil record were this geological filter to remain approximately constant over time there would be little problem of interpreting the biological signal however it is far from uniform with amounts of rock to sample proportions of facies represented by those rocks and preservation potential all varying from one time interval to the next to interpret observed changes in sampled biodiversity estimates correctly we must therefore first understand the role of the geological filter variability of the rock and fossil records

the concept of steady state foliation abstract a steady state foliation is thought of as a foliation with constant orientation and intensity in a rock undergoing steady state flow the problem discussed is whether such straininsensitive foliations exist in rocks and how they might be distinguished from the more familiar strain sensitive foliations steady state foliations would be useful indicators of stress or of strain rate yet misleading if used as indicators of finite strain one requirement for maintenance of steady state foliations is operation of processes that locally weaken grain shape preferred orientation or reduce grain aspect ratios or destroy lithological layering during flow these are the inverses of the processes normally discussed in connection with foliation development many such processes are known though the details of how and when they operate efficiently enough to stabilize a foliation are unknown there is however a tendency for some of them to operate more efficiently at high strains high temperatures and in flow regimes with high vorticity natural and experimental high temperature shear zones are accordingly one environment in which steadystate foliations may be encountered an experiment is described in which a thin specimen of para dichlorobenzene was given a large shearing deformation at 90 of its absolute melting point a steady state foliation was not produced but the technique is a promising one for further microstructural studies where in situ observation of material undergoing large deformations is desirable

quantitative biostratigraphy achieving finer resolution in global correlation abstract the fossil record preserves a wide range of events that might be used to build timescales and correlate strata from place to place the events include the originations and extinctions of species the occurrence of distinctive faunal assemblages magnetic field reversals changes in ocean chemistry and volcanic ash falls the fundamental task is to determine the global sequence of all these events modern computer algorithms can build high resolution timescales by sequencing and calibrating thousands of such events from hundreds of localities each successful sequencing algorithm can be understood in terms of a simple two dimensional visual device graphic correlation permutable sequences permutable matrices and slotting devices are each suited to a different data problem such as contradictory evidence of sequence a lack of information about sequence or uncertain correspondence of events algorithms based upon permutable sequences and evolutionary programming heuristics have the flexibili

lower crustal melting and the role of open system processes in the genesis of syn orogenic quartz diorite granite leucogranite associations constraints from sr nd o isotopes from the bandombaai complex namibia abstract the bandombaai complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and sphene bearing quartz diorites allanite bearing granodiorites and granites and peraluminous garnet and muscovite bearing leucogranites intrusion of the quartz diorites is constrained by a u pb zircon age of 540 3 ma quartz diorites granodiorites and granites display heterogeneous initial nd and o isotope compositions end 540 ma 6 3 to 19 8 18o 9 0 11 6 but rather low and uniform initial sr isotope compositions 87sr 86srinitial 0 70794 0 70982 two leucogranites and one aplite have higher initial 87sr 86sr ratios 0 70828 0 71559 but similar initial end 11 9 to 15 8 and oxygen isotope values 10 5 12 9 the geochemical and isotopic characteristics of the bandombaai complex are distinct from other granitoids of the kaoko belt and the central zone of the damara orogen our study suggests that the quartz diorites of the bandombaai complex are generated by melting of heterogeneous mafic lower crust based on a comparison with results from amphibolite dehydration melting experiments a lower crustal garnet and amphibole bearing metabasalt probably enriched in k2o is a likely source rock for the quartz diorites the granodiorites granites show low rb sr 1 and are most likely generated by biotite dehydration melting of heterogeneous felsic lower crust all segments of the lower crust underwent partial melting during the pan african orogeny at a time 540 ma when the middle crust of the central damara orogen also underwent high t medium p regional metamorphism and melting geochemical and isotope data from the bandombaai complex suggest that the pan african orogeny in this part of the orogen was not a major crust forming episode instead even the most primitive rock types of the region the quartz diorites represent recycled lower crustal material

solute sources and chemical weathering in the owens lake watershed eastern california 1 chemical and strontium isotope analyses of solutes in a series of streams representing the bulk of the water reaching the 8500 km2 owens lake watershed provide a unique opportunity to evaluate factors that control stream chemistry on a regional scale the streams drain terrain that is dominated by mesozoic granitoids of the sierra nevada batholith but that also contains irregularly distributed roof pendants of paleozoic metasedimentary sequences mass balance modeling indicates that plagioclase feldspar weathering is the dominant contributor to stream chemistry and that carbonate dissolution is also a significant source of solutes for most streams the calculated ratio of plagioclase to carbonate is highly sensitive to the assumed weathering product of plagioclase kaolinite versus smectite and we suggest that the importance of carbonate could be exaggerated in cases where only the plagioclase to kaolinite reaction is considered strontium isotope 87sr 86sr ratios range from 0 70685 to 0 71958 with a flow weighted mean of 0 70911 to the extent that this value represents the water that would be flowing into owens lake were it not diverted by the los angeles aqueduct system the flow weighted mean 87sr 86sr ratio is consistent with ratios measured in owens lake core carbonates ranging in age up to 25 kyr the overall sr budget of the watershed is determined by the relative contributions of carbonate dissolution and plagioclase feldspar weathering and is dominated by reactions related to pyrite oxidation and hydrothermal fluid input from just two subbasins the impact of these processes needs to be considered when modeling regional and global riverine sr budgets climate related shifts in the relative importance of silicate mineral weathering reactions appear to be only second order effects on a regional scale

completeness of the fossil record and the validity of sampling proxies a case study from the triassic of england and wales many studies have highlighted correlations between palaeodiversity and sampling proxies these correlations have been interpreted as evidence for bias common cause or redundancy between signals here we compare a number of sampling proxies representing sedimentary rock volume rock accessibility and worker effort with palaeodiversity through the predominantly terrestrial triassic system in england and wales we find that proxies for sedimentary rock volume and accessibility do not correlate with palaeodiversity until the removal of facies related preservational and palaeoecological factors this indicates that a weak sampling signal may be present but the effects of changing palaeoenvironments are far more important at the regional scale significant correlations between worker effort and palaeodiversity are detected although this is likely to be a result of the preferential sampling of formations already known to be rich in fossils the fact that there is little evidence for sedimentary rock bias in the fossil record of the triassic of england and wales suggests that either 1 sampling bias is not a major source of error at the regional scale or 2 sampling proxies are inadequate representations of geological and human sampling bias supplementary material correlation test results and raw time series data from the study are available at www geolsoc org uk sup18567

using remote sensing and a geographic information system to quantify rock exposure area in england and wales implications for paleodiversity studies rock outcrop area and number of sedimentary formations have been widely used as sampling proxies in paleodiversity studies and have often been found to correlate with apparent paleodiversity however rock exposure area is a better proxy for the amount of sedimentary rock available for study than the widely used measures of outcrop area i e map area or number of formations with the use of remote sensing and a geographic information system gis it is possible to quantify rock exposure area accurately on a regional scale rock exposure area does not correlate well with either outcrop area or number of sedimentary formations and the proportion of rock exposed in different areas can vary considerably with proximity to the coast bedrock age lithology land use and elevation therefore this suggests that the correlation of paleodiversity and rock volume estimates is best explained by a common cause hypothesis rather than a geologic sampling bias hypothesis gis methods offer an efficient and accurate method of quantifying regional sedimentary rock exposure and may provide a helpful approach to assessing rock record bias on patterns in the fossil record

intrusion mechanisms in a turbidite sequence the voetspoor and doros plutons in nw namibia abstract two syntectonic plutons of cambrian age intruded neoproterozoic metaturbidites in namibia at the junction of the ns trending kaoko and ew trending damara belts sinistral transpression in the kaoko belt produced km scale upright d1 folds overprinted by minor d2 folds d3 is associated with n s shortening in the damara belt the plutons show two main pulses of intrusion hornblende syenite intruded late during d1 or during d2 and biotite granite during d3 each tectonic event produced a strain shadow defined by the shape of folds and the foliation trend around the plutons the internal igneous fabric and the arrangement of wall rock xenoliths that locally make up 50 of the intrusion mass suggest that the plutons have a disk or wedge shape a marginal shear zone indicates that one of the syenite intrusions descended during emplacement with respect to the wall rock emplacement is therefore inferred to be at least partly accommodated by descent of the intrusion floor the biotite granite intruded into mega strain shadows and tension gashes alongside and in the syenite during d3 the plutons show evidence of sinistral solid state rotation with respect to the wall rocks in response to d1 d2 transpression

macrostratigraphy of north america abstract the geological record is a three dimensional mosaic of gap bound rock bodies that at any given scale of temporal resolution were emplaced more or less continuously at any geographic location the irregular alternation of processes responsible for the formation and destruction of rock bodies results in the preservation of hiatus bound rock packages that have a distinct time of first occurrence base or oldest portion a time of last occurrence top or youngest portion and a suite of defining characters e g lithologies thickness fossils etc many important aspects of the geologic record can be quantified by compiling the stratigraphic ranges of such gap bound rock packages these include the quantity and spatial and temporal distribution of preserved rock rates of rock formation sequence stratigraphic architecture and area weighted rates of expansion and contraction erosional truncation of rock emplacement settings this approach to characterizing the rock record is scalable per

covariation in macrostratigraphic and macroevolutionary patterns in the marine record of north america students of earth history have long recog nized the correlation between the quantity of preserved sedimentary rock and the diversity of life recorded as fossils but paleontologists have yet to determine whether this pattern refl ects a causal relationship or a unidirec tional sampling bias in fossil data imposed by preserved rock quantity distinguishing between these two alternatives has been com plicated by the fact that many of the basic patterns of paleontologic and lithologic co variation have yet to be quantifi ed rigorously here we present the fi rst analyses of the co variation between the macrostratigraphic and macroevolutionary histories of north america based on geographically and tem porally explicit co occurrences of rocks and fossils the analyses use independent quan titative summaries of the stratigraphic and fossil records by integrating the paleobiology database paleodb and macrostrat a mac rostratigraphy database for north america which allows a more direct comparison of the stratigraphic and biological histories of the continent than has heretofore been possible within the macrostrat database the rock re cord is divided into discrete packages of sedi ment that are bound by hiatuses resolvable at the stage level using per interval per pack age rates of sediment package initiation and truncation and genus fi rst and last appear ances herein regional origination and extinc tion we fia substantially stronger positive correlation between sediments and biology for extinction like parameters than we do for origination like parameters four of the largest coincident pulses of regional extinc tion and sediment truncation occur during the widely recognized end ordovician late permian end triassic and end cretaceous mass extinction intervals a further compari son of the global ranges of north american genera to north american macrostratigra phy indicates that the regional and global extinction of genera are more likely to oc cur in the same stage than are global and regional originations together these results suggest that our general understanding of biodiversity dynamics from the fossil record may not be strongly biased by the preserva tion of sediments and leaves open the pos sibility that certain large perturbations to the earth system are responsible for major changes of state in both the sedimentary and biological systems

exceptionally preserved fossil assemblages through geologic time and space abstract geologic deposits containing fossils with remains of non biomineralized tissues i e konservat lagerstatten provide key insights into ancient organisms and ecosystems such deposits are not evenly distributed through geologic time or space suggesting that global phenomena play a key role in exceptional fossil preservation nonetheless establishing the influence of global phenomena requires documenting temporal and spatial trends in occurrences of exceptionally preserved fossil assemblages to this end we compiled and analyzed a dataset of 694 globally distributed exceptional fossil assemblages spanning the history of complex eukaryotic life 610 to 3 ma our analyses demonstrate that assemblages with similar ages and depositional settings commonly occur in clusters each signifying an ancient geographic region up to hundreds of kilometers in scale which repeatedly developed conditions conducive to soft tissue preservation using a novel hierarchical clustering approach we show that these clusters decrease in number and shift from open marine to transitional and non marine settings across the cambrian ordovician interval conditions conducive to exceptional preservation declined worldwide during the early paleozoic in response to transformations of near surface environments that promoted degradation of tissues and curbed authigenic mineralization potential we propose a holistic explanation relating these environmental transitions to ocean oxygenation and bioturbation which affected virtually all taphonomic pathways in addition to changes in seawater chemistry that disproportionately affected processes of soft tissue conservation after these transitions exceptional preservation rarely occurred in open marine settings excepting times of widespread oceanic anoxia when low oxygen levels set the stage with these patterns non marine cluster count is correlated with non marine rock quantity and generally decreases with age this result suggests that geologic processes which progressively destroy terrestrial rocks over time limit sampling of non marine deposits on a global scale future efforts should aim to assess the impacts of such phenomena on evolutionary and ecological patterns in the fossil record

high resolution compositional mapping of matrix phases implications for mass transfer during crenulation cleavage development in the moretown formation western massachusetts abstract high resolution compositional maps provide a new tool for investigating mass transfer during cleavage formation the moretown formation of western massachusetts contains a well developed crenulation cleavage with alternating mica rich crenulation limbs and mica poor crenulation hinges compositional mapping shows two generations of plagioclase the second of which was synchronous with the crenulation cleavage a significant amount of the syn crenulation plagioclase 10 20 modally grew in hinge domains a small amount of syn crenulation plagioclase 1 and a large amount of phengitic muscovite grew in limb domains the maps also show that uncrenulated domains experienced mass transfer and reactivation of older cleavages and thus cannot be used as undeformed reference domains for comparison with crenulated regions compositional mapping facilitates a new degree of integration between petrologic and structural analysis knowledge of the structural context of compositional domains allows better selection of phases and compositions for interpreting metamorphic reactions and linking metamorphism to deformational stages knowledge of syntectonic reactions provides new insights into mass transfer and volume change during deformation in the moretown formation plagioclase and phengite producing reactions play a large role in controlling the nature and magnitude of mass transfer but microstructures control the location of reactants and products within the evolving fabric

petrogenesis of cogenetic nepheline and quartz syenites and granites northern damara orogen namibia enriched mantle versus crustal contamination abstract geochemical and nd sr o whole rock isotope data and pb isotope data from leached feldspars for a suite of synorogenic ca 540 ma syenites and granites from the northern part of the proterozoic damara orogen namibia constrain their sources and petrogenesis major rock types are mildly potassic nepheline syenites and quartz syenites that were primarily generated by fractional crystallization from a mantle derived alkaline magma even the most primitive samples show pronounced depletion in nb ti sr and p on a primitive mantle normalized diagram indicating the involvement of a recycled crustal component in the source extrapolation of the sr nd pb o isotope composition of the syenites back to a hypothetical parental melt with 8 wt 10 wt mgo suggests derivation from a moderately enriched lithospheric upper mantle documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage t

flame foliation evidence for a schistosity formed normal to the extension direction foliations are normally thought to develop approximately parallel to the xy plane of the finite strain ellipsoid i e perpendicular to the main shortening direction we present a new type of schistosity named flame foliation that forms orthogonal to the main extension direction approximately parallel to the yz plane of finite strain flame foliation consists of anastomosing biotite rich selvedges overprinting s1 in pelitic layers of metaturbitites in nw namibia the biotite crystals in the selvedges are peculiar because they are oriented oblique or orthogonal to the flame foliation itself and parallel to the previous s1 cleavage a feature no other foliation shows in some cases biotite flames flank minor quartz veinlets implying that the flame foliation developed by infiltration of fluids and biotite growth along extensional fractures perpendicular to the main d1 extension direction where overprinting ductile d2 deformation is strong the flame foliation is transformed into a normal crenulation cleavage by rotation of biotite in the flames flame foliation is found preferentially close to syntectonic syenite granite plutons and their formation in metapelitic rocks may be enhanced by fluid overpressure due to devolatilisation reactions 2006 elsevier ltd all rights reserved