toward an understanding of disequilibrium dihedral angles in mafic rocks the median dihedral angle at clinopyroxene plagioclase plagioclase junctions in mafic rocks cpp is generally lower than equilibrium 109 plus minus 2 observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of cpp is incorrect in several important respects firstly the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three grain junctions and hence cpp is formed during solidification the igneous process sub solidus textural modification in most dolerites and gabbros previously thought to be the dominant control on cpp is insignificant cpp is governed by mass transport constraints the inhibiting effects of small pore size on crystallization and variation in relative growth rates of pyroxene and plagioclase during rapid cooling pyroxene preferentially fills wider pores while the narrower pores remain melt filled resulting in an initial value of cpp of 78 rather than 60 which would be expected if all melt filled pores were filled with pyroxene lower cooling rates create a higher initial cpp due to changes in relative growth rates of the two minerals at the nascent three grain junction low cpp associated with cuspate clinopyroxene grains at triple junctions can also be diagnostic of infiltration of previously melt free rocks by late stage evolved liquids the metasomatic process modification of cpp by sub solidus textural equilibration the metamorphic process is only important for fine grained mafic rocks such as chilled margins and intra plutonic chill zones in coarse grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centres of oikocrysts associated with rounding of chadacrysts

isotopic and geochemical investigation of unit 10 from the eastern layered series of the rhum intrusion northwest scotland sr and nd isotopic compositions together with whole rock sr and nd abundances and olivine compositions are presented for cumulates from unit 10 of the eastern layered series of the rhum intrusion northwest scotland the results show an nd sr isotopic correlation consistent with contamination by high 87 sr 86 sr low 143 nd 144 nd crust there is a strong correlation between the degree of contamination sr and nd whole rock abundance and fe enrichment in cumulus olivine the allivalites are the most contaminated while the basal peridotites are uncontaminated this suggests that the cumulates crystallized from a primitive uncontaminated magma and the allivalites from an evolved contaminated magma subsequently movement of contaminated feldspathic intercumulus liquid has perturbed the isotopic trace element and olivine composition of the upper peridotites

comment on the stillwater complex integrating zircon geochronological and geochemical constraints on the age emplacement history and crystallization of a large open system layered intrusion by wall et al j petrology 59 153 190 2018

concentration of platinum group elements by magmatic fluids in layered intrusions evidence leading to a model which supports the formation of pt pd reefs in a layered intrusion by the action of magmatic fluids is presented as well as some of the constraints imposed on such a model the stillwater complex is used as an example of this model within the stillwater complex no laterally continuous sulfide enriched cumulates occur above the lowermost parts of the ultramafic series until one reaches the platiniferous j m reef this sulfide poor stratigraphic interval beneath the j m reef is characterized by unusually cl rich apatite although reported sulfide modes and whole rock sulfur contents in the cumulates below the j m reef are approximately an order of magnitude lower than that expected for a sulfide saturated magma sulfide saturation prior to the formation of the j m reef is suggested by relatively high pd and pt concentrations reported in the cumulates below the j m reef as well as by the presence of sulfide as inclusions in silicate and oxide cumulus minerals hence it is suggested that the footwall cumulates to the j m reef were originally sulfide saturated but later lost sulfur to a fluid phase exsolving from inter cumulus liquid sporadic sulfide bearing pegmatoids in some of these rocks are additional evidence for late loss of sulfur these observations as well as the association of the j m reef at the first significant stratigraphic break in the cl cl f ratio of apatite and the relatively high solubility of pt and pd as aqueous cl complexes are consistent with the formation of the j m reef by exsolution of cl and volatile rich fluids from footwall intercumulus liquids which leached and transported ore components both the pge and s upward to the level of the reef using published whole rock platinum group element pge concentrations for the ultramafic and lower banded series the cumulates below the j m reef must have been enriched by a factor of at most 1 24 for pt and 1 75 for pd if these rocks were to be the source for the metals in the j m reef model raleigh fractionation calculations suggest that the cumulates beneath the j m reef could have had the required metal concentrations if during the crystallization of a parent magma containing no more than 15 ppb of either metal the average bulk solid liquid distribution coefficients for pt and pd were no more than 5 these values are consistent with assumption of sulfide saturation prior to the formation of the j m reef during solidification of intercumulus liquid the separation and loss of a cl rich fluid will deplete the residual assemblage of s and pge if 0 1 to 0 5 wt percent fluid containing 5 percent s evolved during solidification of the footwall cumulates the amount of fluid expected to be exsolved by the crystallization of approximately 2 to 10 percent fluid saturated intercumulus liquid the fluid liquid distribution coefficients for pt and pd would need to be almost as large as the sulfide liquid silicate liquid distribution coefficients to remove significant amounts of pt and pd much smaller distribution coefficients are needed if sulfide is completely lost to fluid during degassing the latter possibility is consistent with the low s contents of the footwall cumulates beneath the j m reef numerical models of the degassing of intercumulus liquids suggest that thick cumulus sequences can act as chromatographic columns to separate the pge and s during the degassing of intercumulus silicate liquids the pge being enriched at a sulfide dissolution front as upward migrating sulfide undersaturated fluids resorb cumulus sulfides zones of pge and s enrichment can occur where degassing fronts encounter stratigraphic physical chemical discontinuities such as changes in the fluid saturation temperature of the intercumulus silicate liquids

mineral scale sr isotope constraints on magma evolution and chamber dynamics in the rum layered intrusion scotland sr isotopic zoning within single plagioclase crystals from rocks from unit 9 of the rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination the 87sr 86sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying unit 10 peridotite is approached models of near solidus interaction of the cumulate with a fluid or melt or large scale textural re equilibration cannot easily account for the systematic differences in 87sr 86sr between small crystals and the rims of larger crystals

the upper critical zone of the bushveld complex and the origin of merensky type ores the mg number mg mg fe atomic ratio of bronzite in each unit at the top of the critical zone of the bushveld complex decreases upward in a manner consistent with fractional crystallization the base of each unit is marked by an increase in the mg number suggesting that crystallization of fresh magma is responsible for the initiation of the unit bulk trace and major element concentrations in pegmatoids of the merensky reef which itself forms the basal part of one of a series of these cyclic units are similar to those in medium grained feldspathic bronzitite the base of each unit is also marked by pockets or a layer of sulfide with a higher ni cu and much higher pt cu ratio than the sulfides of the remainder of the unit thus the pt tenor alternates from high to low three times upward through the three units these data indicate strongly that the pt tenor was established with the stratigraphy of the rocks and are inconsistent with models calling upon upward migration of hydrothermal or magmatic fluids to enrich the sulfides in platinum group elements there are no features in the rocks to indicate that fluids could have been channeled to flow laterally for the tens to hundreds of kilometers occupied by continuous merensky reef a model based on that of campbell et al 1983 in which small amounts of sulfides form during injection of fresh magma as a turbulent plume and then settle after becoming thoroughly mixed with a large mass of magma accounts well for all of our observations and many other features of the upper part of the critical zone

rapid remobilization of magmatic crystals kept in cold storage we lack thermal histories for magma reservoirs but here the magma under mount hood oregon usa is shown to have been too cold to mobilize for most of the time it has been stored which implies that magma mobilizes at which point it can be imaged geophysically very quickly prior to eruption this paper presents a novel approach to constraining the thermal history of the subsurface magma bodies that feed eruptions by combining temporal information from a variety of sources kari cooper and adam kent combine timescales derived from uranium series disequilibria textural information and trace element zoning in crystals they show that at mount hood oregon only a small fraction of the total storage duration has been spent at temperatures above the critical crystallinity the point at which magma is easily mobilized they conclude that largely liquid bodies that could be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption the processes involved in the formation and storage of magma within the earth s upper crust are of fundamental importance to volcanology many volcanic eruptions including some of the largest result from the eruption of components stored for tens to hundreds of thousands of years before eruption1 2 3 although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes they remain relatively poorly known4 5 eruptions of crystal rich magma are often suggested to require the mobilization of magma stored at near solidus conditions6 7 8 however accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures7 8 9 what has been lacking in this debate is clear observational evidence linking the thermal and therefore physical conditions within a magma reservoir to timescales of storage that is thermal histories here we present a method of constraining such thermal histories by combining timescales derived from uranium series disequilibria crystal sizes and trace element zoning in crystals at mount hood oregon usa only a small fraction of the total magma storage duration at most 12 per cent and probably much less than 1 per cent has been spent at temperatures above the critical crystallinity 40 50 per cent at which magma is easily mobilized partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near solidus temperatures is common magma storage at low temperatures indicates that although thermobarometry calculations based on mineral compositions may record the conditions of crystallization they are unlikely to reflect the conditions of most of the time that the magma is stored our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption

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

zircon reveals protracted magma storage and recycling beneath mount st helens current data and models for mount st helens volcano washington united states suggest relatively rapid transport from magma genesis to eruption with no evidence for protracted storage or recycling of magmas however we show here that complex zircon age populations extending back hundreds of thousands of years from eruption age indicate that magmas regularly stall in the crust cool and crystallize beneath the volcano and are then rejuvenated and incorporated by hotter young magmas on their way to the surface estimated dissolution times suggest that entrained zircon generally resided in rejuvenating magmas for no more than about a century zircon elemental compositions refl ect the increasing infl uence of mafi c input into the system through time recording growth from hotter less evolved magmas tens of thousands of years prior to the appearance of mafi c magmas at the surface or changes in whole rock geochemistry and petrology and providing a new time correlated record of this evolution independent of the eruption history zircon data thus reveal the history of the hidden long lived intrusive portion of the mount st helens system where melt and crystals are stored for as long as hundreds of thousands of years and interact with fresh infl uxes of magmas that traverse the intrusive reservoir before erupting

crack density and saturation rate in the 1995 kobe earthquake region we have estimated the 3 d distribution of crack density parameter and saturation rate in the 1995 kobe earthquake region by applying a crack theory to values of seismic velocity and poisson s ratio obtained from seismic tomography our results show that and exhibit considerable heterogeneity higher and exist in the kobe fault zone in particular at the mainshock hypocenter high areas are generally consistent with high areas but high areas have a much wider distribution indicating that microcracks exist in more localized areas while fluids are permeating through wider areas in the crust a significant discrepancy between and exists beneath osaka bay where is low while is high is generally high beneath seas but low beneath land areas osaka bay and the present sea land distribution in southwest japan have existed for 2 millions years this period is long enough for the sea water to permeate down to the deep crust through the active faults that would have been ruptured during many earthquakes cycles

fluid fluxing of cumulates the j m reef and associated rocks of the stillwater complex montana olivine bearing rocks in the lower half of ob i the olivinerich rocks also define pegmatoidal pothole structures 3 rock composition texture and grain size vary considerably in olivinebearing units characterized by segregation between coarser mafic regions and medium grained felsic regions 4 primary plagioclase in olivine rich rocks has an eroded texture 5 modal variations key words layered intrusions partial melting platinum group elements are consistent with progressive incongruent melt reactions and hydrous minerals are most abundant in olivine rich rocks 6 hydrous melt inclusions now crystallized to polyphase hydrous mineral bearing assemblages are present in olivine and chromite

infiltration metasomatism of cumulates by intrusive magma replenishment the wavy horizon isle of rum scotland the eastern layered intrusion of the rum layered suite comprises paired peridotite and allivalite troctolite and gabbro layers forming 16 macro rhythmic units whereas the majority of these macro units are believed to have formed by a process of crystal liquid differentiation involving successive accumulation of crystals from multiple picritic replenishments of the chamber the unit 9 peridotite is interpreted as a layer parallel picrite intrusion closely correlated with this discontinuous peridotite body is a distinctive feature generally known as thewavy horizon which divides the overlying allivalite into a lower troctolite and an upper gabbro along a well defined undulating contact we propose that the wavy horizon is a metasomatic feature formed consequent to the removal of clinopyroxene from an original gabbroic mush foundering of the mush into the picritic sill resulted in the replacement of the original interstitial liquid by one saturated only in olivine plagioclase progressive through flow of this liquid resulted in the stripping out of clinopyroxene from the lower parts of the allivalite we interpret thewavy horizon as a reaction front representing the point at which the invading liquid became saturated in clinopyroxene the distinctive pyroxene enriched zone immediately above thewavy horizon could have formed when mixing of the interstitial liquids on either side of the reaction front formed a supercooled liquid oversaturated in pyroxene as a result of the curvature of the olivine plagioclase clinopyroxene cotectic the presence of many such approximately layer parallel features defined by differences in pyroxene content in the eastern layered intrusion of rum suggests that such an infiltration reaction process was not unique to unit 9

lateral variations in the unit 7 8 boundary zone of the rum eastern layered intrusion nw scotland implications for the origin and timing of cr spinel seam formation the rum layered suite nw scotland hosts cr spinel seams at the bases of peridotite troctolite macro rhythmic units in the eastern portion of the intrusion here we present detailed field observations together with microstructural and mineral chemical analyses for the unit 7 8 cr spinel seam and associated cumulates in the eastern layered intrusion detailed mapping and sampling reveal significant lateral variations in the structural characteristics and mineral compositions of the unit 7 8 boundary zone rocks although the cr spinel seam is laterally continuous over 3 km it is absent towards the centre and the margins of the intrusion the compositional characteristics of cr spinel and plagioclase vary systematically along strike exhibiting a chemical evolution towards more differentiated compositions with increasing distance from the main feeder conduit of the rum intrusion the long loch fault on the basis of our combined datasets we propose that the upper part of the troctolite the anorthosite layer underlying the cr spinel seam and the seam itself formed during a multi stage magma replenishment event the stages can be summarised as follows 1 peridotite schlieren and anorthosite autoliths formed following melt infiltration and cumulate assimilation in the crystal mush of the unit 7 troctolite 2 the anorthosite layer then formed from the unit 7 troctolite crystal mush by thermal erosion and dissolution due to infiltrating magma 3 subsequent dissolution of the anorthosite layer by new replenishing magma led to peritectic in situ crystallisation of the unit 7 8 cr spinel seam with 4 continued magma input eventually producing the overlying unit 8 peridotite in the central part of the rum layered suite the aforementioned assimilation of the troctolitic footwall formed the anorthosite layer however the absence of anorthosite in close proximity to the long loch fault can be explained by enhanced thermochemical erosion close to the feeder zone and its absence close to the margins of the intrusion at maximum distance from the long loch fault may be due to cooling of the magma and loss of erosion potential in line with other recent studies on pge bearing chromitites in layered intrusions we highlight the importance of multi stage intrusive magma replenishment to the formation of spatially coupled anorthosite and cr spinel seams as well as the lateral mineral chemical variations observed in the unit 7 8 boundary zone cumulates

rapid crystallization of precious metal mineralized layers in mafic magmatic systems the solidified remnants of mafic magmatic systems host the greatest concentrations of platinum group metals in the earth s crust our understanding of precious metal mineralization in these intrusive bodies is underpinned by a traditional view of magma chamber processes and crystal mush solidification however considerable uncertainty remains regarding the physical and temporal controls on concentrating these critical metals despite their importance to modern society we present high precision 87sr 86sr analyses of plagioclase and clinopyroxene from within centimetre thick precious metal enriched layers in the palaeogene open system rum layered intrusion northwest scotland isotopic heterogeneity is present between plagioclase crystals between clinopyroxene and plagioclase and within plagioclase crystals throughout the studied section on the basis of these observations we demonstrate that platinum group element mineralization formed by repeated small volume reactive melt percolation events the preservation of strontium isotope heterogeneities at 10 100 m length scales implies cooling of the melts that formed the precious metal rich layers occurred at rates greater than 1 c per year and cooling to diffusive closure within tens to hundreds of years our data highlight the importance of cyclic dissolution recrystallization events within the crystal mush and raise the prospect that precious metal bearing mafic intrusions may form by repeated self intrusion during cooling and solidification mineralization of platinum group elements in mafic intrusions occurs due to repeated self intrusion of magma according to strontium isotope heterogeneities preserved in the rum layered intrusion scotland

vertically extensive and unstable magmatic systems a unified view of igneous processes volcanoes are an expression of their underlying magmatic systems over the past three decades the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust a transcrustal perspective must balance slow plate tectonic rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions reconciling these observations is engendering active debate about the physical state spatial distribution and longevity of melt in the crust here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt rich layers focusing particularly on conditions that cause them to destabilize ascend and accumulate in voluminous but ephemeral shallow magma chambers

a 40ar 39ar laser probe study of micas from the sesia zone italian alps implications for metamorphic and deformation histories two samples from the eclogite micaschist complex emc and the seconda zona diorito kinzigitica iidk of the sesia zone have been studied using a high spatial resolution laser probe 40ar 39ar technique with the aim of investigating the complexities of argon behaviour in metamorphic rocks and comparing their thermal histories data from a single large phengite grain from the emc show a range of ages from mid jurassic to upper cretaceous these apparent age variations are spatially related to both location within the grain and to intragrain microstructure modelling of the data shows that the profile formed by the diffusion of an excess argon component into the grain parallel to the mica cleavage profile asymmetry is explained by temporal variations in microstructural development enabling excess argon to enter the grain at different times in different places the temperatures of the initiation of deformation and the possible time scales for the deformation can be calculated as a function of cooling rate all estimates suggest deformation at greenschist facies in accord with the observed retrograde mineral assemblage absolute temperature estimates for deformation vary by less than 22 c for different cooling rates of 10 and 30 c ma 1 but vary by 80 c with different estimates of diffusion parameters the duration of deformation was for at least 2 ma at 10 c ma 1 or 0 7 ma at 30 c ma 1 biotites from the iidk sample record a permian to upper cretaceous age range that correlates with grain size the smallest grain sizes yielding the youngest ages this relationship is best explained by a partial resetting of biotites during an alpine thermal event initiated not more than 70 ma ago modelling of these data suggest that the sample never exceeded 300 c during the alpine the profoundly different thermal histories of the two units the emc recrystallized at 550 c whilst the iidk remained below 300 c suggests that they may not have been juxtaposed until much later than the eclogite facies metamorphism

successive episodes of reactive liquid flow through a layered intrusion unit 9 rum eastern layered intrusion scotland we present a detailed microstructural and geochemical study of reactive liquid flow in unit 9 of the rum eastern layered intrusion scotland unit 9 comprises an underlying lens like body of peridotite overlain by a sequence of troctolite and gabbro termed allivalite with some local and minor anorthosite the troctolite is separated from the overlying gabbro by a distinct sub horizontal undulose horizon the major wavy horizon higher in the stratigraphy is another similar horizon the minor wavy horizon that separates relatively clinopyroxene poor gabbro from an overlying gabbro to the north of the peridotite lens both troctolite and gabbro grade into poikilitic gabbro clinopyroxene habit in the allivalite varies from thin rims around olivine in troctolite to equigranular crystals in gabbro and to oikocrysts in poikilitic gabbro the poikilitic gabbros contain multiple generations of clinopyroxene with cr rich 1 1 wt cr2o3 anhedral cores with moderate ree concentrations core1 overgrown by an anhedral ree depleted second generation with moderate cr 0 7 wt cr2o3 core2 these composite cores are rimmed by cr poor 0 2 wt cr2o3 and ree poor to moderate clinopyroxene we interpret these microstructures as a consequence of two separate episodes of partial melting triggered by the intrusion of hot olivine phyric picrite to form the discontinuous lenses that comprise the unit 9 peridotite loss of clinopyroxene saturated partial melt from the lower part of the allivalite immediately following the early stages of sill intrusion resulted in the formation of clinopyroxene poor gabbro the spatial extent of clinopyroxene loss is marked by the minor wavy horizon a second partial melting event stripped out almost all clinopyroxene from the lowest allivalite to form a troctolite with the major wavy horizon marking the extent of melting during this episode the poikilitic gabbro formed from clinopyroxene saturated melt moving upwards and laterally through the remobilized cumulate pile and precipitating clinopyroxene en route this process called reactive liquid flow is potentially important in open magma chambers

peridotite sills and metasomatic gabbros in the eastern layered series of the rhum complex mapping of the eastern layered series els of the rhum ultrabasic complex on the northern flank of hallival shows that peridotite and allivalite troctolite or gabbro layers are laterally discontinuous and vary both in thickness and lithology peridotite generally has sharp upper and lower contacts against the allivalites which sometimes cut across the layering in the allivalite reaction dissolution and hybridization effects between peridotite and allivalite are developed locally some troctolite layers terminate as isolated fingered blocks in peridotite there are many small peridotite bodies which are clearly intrusive into allivalite and have previously been identified as distinct peridotite sheets and plugs they are petrographically almost identical to the major stratiform peridotites and in some cases are apophyses from them we propose that many of the peridotite layers in the els formed as thick sills of picritic magma emplaced into a partly solidified layered troctolite complex the stratiform gabbros of the els are heterogeneous layered rocks that commonly contain relicts of troctolite and anorthosite wavy metre scale contacts between gabbro and troctolite cut across pre existing grain size modal and rhythmic layering with little disruption these metasomatic gabbros mimic the textures grain size and rhythmic layering of their troctolitic protoliths we propose that many of the els gabbros formed as a result of interaction between porous troctolites and a low temperature basaltic melt residual basaltic melt segregated from solidifying peridotite may have caused this metasomatism

testing emplacement models for the rustenburg layered suite of the bushveld complex with numerical heat flow models and plagioclase geospeedometry abstract conventional models of the genesis of the rustenburg layered suite rls of the bushveld complex postulate that it is the product of a single multiply replenished magma chamber that cooled as a single massive unit i e a monolithic magma chamber this concept has been challenged by alternative hypotheses involving out of sequence emplacement of some of its major layers as sills both scenarios have partial support from high precision u pb geochronological data to test these contradictory hypotheses new zircon geothermometric data and compositional data for plagioclase from the upper critical zone are presented and the available geochronological data are tested against the predictions of the thermal evolution of the entire rls for both emplacement and cooling scenarios crystallization temperatures of zircon within units of the upper critical zone were estimated using ti in zircon thermometry and range between 799 45 c and 865 64 c the data are used in conjunction with a detailed numerical model of heat flow in the bushveld complex that is used to approximate the duration and extent of thermal perturbations felt throughout the system and delimit the time intervals during which zircon crystals could have grown at the measured crystallization temperatures the results show that prolonged cooling histories 1 myr that would be experienced near the centre of a large slowly cooled plutonic magma chamber would erase signs of plagioclase zoning at 4 kbar and moderate f h 2 o however plagioclase crystals with zoning profiles spanning at least 10 mole an are common in the upper critical zone in contrast emplacement of the major units of the rls at widely separated intervals as recorded by the existing high precision u pb dataset would impose transient thermal effects on the system against a backdrop of generally much lower ambient temperatures throughout its evolution in this scenario apparent ages of zircon crystals would record the time of cooling through temperatures near the water saturated biotite granite solidus corresponding to the final intercumulus mineral assemblage in both mafic and ultramafic macrolayers subsequent reheating of these rocks would be unable to reset primary zircon crystallization dates but might in some examples result in some amount of remelting or regrowth of zircon comparison of the evolving temperature fields of the two types of emplacement and cooling histories shows that the existing geochronological data and preservation of plagioclase zoning are consistent with out of sequence emplacement and rapid cooling of individual ultramafic sills into previously solidified lithologies of the rls but not with the conventional view of a single monolithic magma body