major types and time space distribution of mesozoic ore deposits in south china and their geodynamic settings the ore deposits of the mesozoic age in south china can be divided into three groups each with different metal associations and spatial distributions and each related to major magmatic events the first event occurred in the late triassic 230 210 ma the second in the mid late jurassic 170 150 ma and the third in the early mid cretaceous 120 80 ma the late triassic magmatic event and associated mineralization is characterized by peraluminous granite related w sn nb ta mineral deposits the triassic ore deposits are considerably disturbed or overprinted by the later jurassic and cretaceous tectono thermal episodes the mid late jurassic magmatic and mineralization events consist of 170 160 ma porphyry skarn cu and pb zn ag vein deposits associated with i type granites and 160 150 ma metaluminous granite related polymetallic w sn deposits the late jurassic metaluminous granite related w sn deposits occur in a ne trending cluster in the interior of south china such as in the nanling area in the early mid cretaceous from about 120 to 80 ma but peaking at 100 90 ma subvolcanic related fe deposits developed and i type calc alkaline granitic intrusions formed porphyry cu mo and porphyry epithermal cu au ag mineral systems whereas s type peraluminous and or metaluminous granitic intrusions formed polymetallic sn deposits these cretaceous mineral deposits cluster in distinct areas and are controlled by pull apart basins along the south china continental margin based on mineral assemblage age and space time distribution of these mineral systems integrated with regional geological data and field observations we suggest that the three magmatic mineralization episodes are the result of distinct geodynamic regimes the triassic peraluminous granites and associated w sn nb ta mineralization formed during post collisional processes involving the south china block the north china craton and the indo china block mostly along the dabie sulu and songma sutures jurassic events were initially related to the shallow oblique subduction of the izanagi plate beneath the eurasian continent at about 175 ma but i type granitoids with porphyry cu and vein type pb zn ag deposits only began to form as a result of the breakup of the subducted plate at 170 160 ma along the nne trending qinzhou hangzhou belt also referred to as qin hang or shi hang belt which is the neoproterozoic suture that amalgamates the yangtze craton and cathaysia block a large subduction slab window is assumed to have formed in the nanling and adjacent areas in the interior of south china triggering the uprise of asthenospheric mantle into the upper crust and leading to the emplacement of metaluminous granitic magma and associated polymetallic w sn mineralization a relatively tectonically quiet period followed between 150 and 135 ma in south china from 135 ma onward the angle of convergence of the izanagi plate changed from oblique to parallel to the coastline resulting in continental extensional tectonics and reactivation of regional scale ne trending faults such as the tan lu fault this widespread extension also promoted the development of ne trending pull apart basins and metamorphic core complexes accompanied by volcanism and the formation of epithermal cu au deposits granite related polymetallic sn w deposits and hydrothermal u deposits between 120 and 80 ma with a peak activity at 100 90 ma

volcanic degassing geological society special publication 213 c oppenheimer d m pyle and j barclay editors pp 432 geological society publishing house unit 7 brassmill enterprise centre brassmill lane bath ba1 3 jn united kingdom isbn 1 86239 136 x 2003 price 100 00 volcanic degassing is one of several recent special publications on volcanoes and volcanic processes offered by the geological society this publication resulted from a two day long meeting convened in honor of the late peter francis the volume covers various aspects of magmatic volatiles and their influence on eruption dynamics and effects on the atmosphere although wide ranging this is not a comprehensive treatment of the subject rather the thematic volume is a compilation of papers which presumably were presented at the meeting consequently some of the gaps and imbalance are likely to frustrate many regular readers of economic geology particularly those who might want to improve their general knowledge on the topic the volume nevertheless

the fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma this paper describes a fluid dynamical investigation of the influx of hot dense ultrabasic magma into a reservoir containing lighter fractionated basaltic magma this situation is compared with that which develops when hot salty water is introduced under cold fresh water theoretical and empirical models for salt water systems are adapted to develop a model for magmatic systems a feature of the model is that the ultrabasic melt does not immediately mix with the basalt but spreads out over the floor of the chamber forming an independent layer a non turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock the convection continues until the two layers have almost the same temperature the compositions of the layers remain distinct due to the low diffusivity of mass compared to heat the temperatures of the layers as functions of time and their cooling rate depend on their viscosities their thermal properties the density difference between the layers and their thicknesses for a layer of ultrabasic melt 18 mgo a few tens of metres thick at the base of a basaltic 10 mgo magma chamber a few kilometres thick the temperature of the layers will become nearly identical over a period of between a few months and a few years during this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling olivines only settle after the two layers have nearly reached thermal equilibrium at this stage residual basaltic melt segregates as the olivines sediment in the lower layer depending on its density the released basalt can either mix convectively with the overlying basalt layer or can continue as a separate layer the model provides an explanation for large scale cyclic layering in basic and ultrabasic intrusions the model also suggests reasons for the restriction of erupted basaltic liquids to compositions with mgo 10 and the formation of some quench textures in layered igneous rocks

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

mineralogical and stable isotope studies of gold arsenic mineralisation in the sams creek peralkaline porphyritic granite south island new zealand at sams creek a gold bearing peralkaline granite porphyry dyke which has a 7 km strike length and is up to 60 m in thickness intrudes camptonite lamprophyre dykes and lower greenschist facies metapelites and quartzites of the late ordovician wangapeka formation the lamprophyre dykes occur as thin 3 m slivers along the contacts of the granite dyke 18omagma values 5 to 8 vsmow of the a type granite suggest derivation from a primitive source with an insignificant mature crustal contribution hydrothermal gold sulphide mineralisation is confined to the granite and adjacent lamprophyre metapelite country rocks have only weak hydrothermal alteration three stages of hydrothermal alteration have been identified in the granite stage i alteration high fo2 consisting of magnetite siderite biotite stage ii consisting of thin quartz pyrite veinlets and stage iii low fo2 consisting of sulphides quartz and siderite veins and pervasive silicification the lamprophyre is altered to an ankerite chlorite sericite assemblage stage iii sulphide veins are composed of arsenopyrite pyrite galena sphalerite gold chalcopyrite pyrrhotite rutile graphite three phases of deformation have affected the area and the mineralised veins and the granite and lamprophyre dykes have been deformed by two phases of folding the youngest of which is early cretaceous locally preserved early formed fluid inclusions are either carbonic showing two or three phases at room temperature liquid co2 ch4 liquid h2o co2 vapour or two phase liquid rich aqueous inclusions some of which contain clathrates salinities of the aqueous inclusions are in the range of 1 4 to 7 6 wt nacl equiv final homogenisation temperatures th of the carbonic inclusions indicate minimum trapping temperatures of 320 to 355 c which are not too different from vein formation temperatures of 340 380 c estimated from quartz albite stable isotope thermometry 18o values of stage ii and iii vein quartz range from 12 and 17 and have a bimodal distribution 14 5 and 16 with stage ii vein quartz accounting for the lower values siderite in stage iii veins have 18o 12 to 16 and 13c values 5 relative to vpdb unlike those from wangapeka formation metasediments 13cbulk carbon values of 24 to 19 and underlying arthur marble marine carbonates 18o 25 and 13c 0 calculated 18owater 8 to 11 at 340 c and delta 13 text c rm co 2 5 values from vein quartz and siderite are consistent with a magmatic hydrothermal source but a metamorphic hydrothermal origin cannot be excluded 34s values of sulphides range from 5 to 10 relative to cdt and also have a bimodal distribution modes at 6 and 9 correlated with stage ii and stage iii mineralisation respectively the 34s values of pyrite from the arthur marble marine carbonates range from 3 to 13 and wangapeka formation range from 4 to 9 5 indicate that they are potential sources of sulphur for sulphides in the sams creek veins another possible source of the sulphur is the lithospheric mantle which has positive values up to 14 ages of the granite lamprophyre alteration mineralisation and deformation in the region are not well constrained which makes it difficult to identify sources of mineralisation with respect to timing our mineralogical and stable isotope data does not exclude a metamorphic source but we consider that the source of the mineralisation can best be explained by a magmatic hydrothermal source assuming that the hydrothermal fluids were sourced from crystallisation of the sams creek granite or an underlying magma chamber then the sams creek gold deposit appears to be a hybrid between those described as reduced granite au bi deposits and alkaline intrusive hosted au mo cu deposits

timing of the acadian orogeny in northern new hampshire abstract new u pb geochronology constrains the timing of the acadian orogeny in the central maine terrane of northern new hampshire sixteen fractions of one to six grains each of zircon or monazite have been analyzed from six samples 1 an early syntectonic diorite that records the onset of the acadian 2 a schist a migmatite and two granites that together record the peak of the acadian and 3 a postkinematic pluton that records the end of the acadian zircon from the syntectonic wamsutta diorite gives a 207pb 206pb age of circa 408 ma the time at which the boundary between the deforming orogenic wedge and the foreland basin was in the vicinity of the presidential range this age agrees well with the emsian position of the northwest migrating acadian orogenic front and records the beginning of the acadian in this part of the central maine terrane we propose a possible acadian tectonic model that incorporates the geochronologic structural and stratigraphic data monazite from the schist migma

constraining the relative strengths of high grade metamorphic rocks using foliation refraction angles an example from the northern new england appalachians foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite facies metaturbidites in the presidential range of the white mountains in eastern new hampshire an early formed foliation developed during km scale nappe folding consistently displays larger bedding foliation angles in metapelitic units than metapsammitic units suggesting that the metapelitic units had higher effective viscosities during this deformation we collected bedding foliation angle measurements from a combination of outcrops and locally derived boulders to test the hypothesis that foliation refraction angles could be used to estimate the effective viscosity ratio between different rock types at high metamorphic grades our results show that the metapelitic layers were between 2 and 3 times more viscous than the metaspsammitic layers which we attribute to the presence of large up to 15 cm long abundant up to 30 vol effectively rigid andalusite porphyroblasts in the metapelitic layers we present a methodology for using foliation refraction angles to determine the effective viscosity ratios of different rock types address the practical limitations we encountered and suggest this method is an easy way of estimating the strength contrast between different rock types finally we hypothesize that metamorphism resulting in porphyroblast growth can strengthen large crustal volumes during orogenesis

postcumulus processes in layered intrusions during the postcumulus stage of solidification in layered intrusions fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks one mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir this process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions but could not form pure monomineralic rocks intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment dense magma emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers reactions between melt and matrix may lead to changes in mineral compositions mineral textures and whole rock isotope compositions another important mechanism for forming adcumulate rocks is compaction in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled for cumulate layers from 10 to 1000 metres in thickness compaction can reduce porosities to very low values 1 and form monomineralic rocks the characteristic time scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion during compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix both compaction and compositional convection can be interrupted by solidification in the pore spaces compositional convection will only occur if the rayleigh number is larger than 40 if the residual melt becomes lower in density and the convective velocity exceeds the solidification velocity measured by the rate of crystal accumulation in the chamber orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection

stratigraphy and ductile structure of the presidential range new hampshire tectonic implications for the acadian orogeny the presidential range of new hampshire with its unique high relief contains exceptionally well exposed rocks of the acadian orogenic hinterland these rocks are within the central maine terrane a belt of complexly metamorphosed and deformed silurian and devonian metasedimentary rocks the presidential range lies between regions to the south dominated by refolded nappes and synkinematic high grade regional metamorphism and regions to the northeast dominated by overlapping multiple deformation and contact metamorphism produced by late and post acadian plutons to better understand these complex tectonic variations and in particular to clearly and accurately determine the effects of the devonian acadian orogeny in a well exposed region the bedrock geology in the alpine zone of the presidential range was mapped in great detail the results of the stratigraphic and ductile structural analyses are presented here five metasedimentary formations have been recognized and constitute a conformable stratigraphy which from oldest to youngest are the silurian rangeley perry mountain smalls falls and madrid formations and the devonian littleton formation the littleton formation has been subdivided into fifteen different members and three submembers based on variations in bedding style of the schists and quartzites and other lithologic peculiarities the madrid smalls falls and perry mountains formations are extremely thin consistent with their deposition near the silurian tectonic hinge the rangeley formation is interpreted as an olistostromal melange and has been subdivided into three different members and six submembers on the basis of lithologic variations in the gneiss and subordinate units the presidential range preserves three phases of ductile folding f1 f2 and f3 and a single phase of thrust faulting t1 well constrained southeast east or northeast facing directions and consistent asymmetry of f1 nappe limbs suggest that nappe vergence was easterly severe refolding of some nappes resulted during f2 collapse of the nappe pile the west vergent greenough spring thrust fault truncated f1 and f2 structures and juxtaposed nonmigmatized and migmatized rocks f3 east vergent folding dominates the alpine zone and represents the final tightening of acadian structures which occurred after early granitic plutonism and prior to the emplacement of postkinematic granitic sheets the structural and sedimentological evidence supports the existence of a subduction system that was active beginning in the early silurian on the east flank of the bronson hill anticlinorium with a west dipping subducting slab geometry this system persisted throughout the acadian

disequilibrium dihedral angles in layered intrusions a microstructural record of fractionation the dihedral angle formed at junctions between two plagioclase grains and a grain of augite is only very rarely in textural equilibrium in gabbros from kilometre scale crustal layered intrusions the median of a population of these disequilibrium angles cpp varies systematically within a single layered intrusion remaining constant over large stretches of stratigraphy with significant increases and decreases associated with the addition or reduction respectively of the number of phases on the liquidus of the bulk magma the stepwise changes in cpp are present in the upper zone of the bushveld complex the megacyclic unit i of the sept iles intrusion and the layered series of the skaergaard intrusion the plagioclase bearing cumulates of rum have a bimodal distribution of cpp dependent on whether the cumulus assemblage includes clinopyroxene the presence of the stepwise changes is independent of the order of arrival of cumulus phases and of the composition of either the cumulus phases or the inferred composition of the interstitial liquid the only parameter that behaves in an exactly analogous manner to cpp is the rate of change in enthalpy with temperature h t during crystallization both h t and cpp increase with the addition of a liquidus phase and decrease with the removal of a liquidus phase the replacement of one phase by another has little effect on h t and no discernible effect on cpp an increase of h t results in an increase in the fraction of the total enthalpy budget that is the latent heat of crystallization the fractional latent heat it also increases the mass crystallized in each incremental temperature drop the crystal productivity these increases of both fractional latent heat and crystal productivity are likely to cause an increase in the time taken to form three grain junctions in the mush via thermal buffering of a thickened mushy layer we suggest these are the underlying causes of stepwise increases in cpp stepwise changes in the geometry of three grain junctions in fully solidified gabbros thus provide a clear microstructural marker for the progress of fractionation down the liquid line of descent in layered intrusions

on the interaction between convection and crystallization in cooling magma chambers abstract we investigate the interaction of thermal convection and crystallization in large aspect ratio magma chambers because nucleation requires a finite amount of undercooling crystallization is not instantaneous for typical values of the rates of nucleation and crystal growth the characteristic time scale of crystallization is about 10 3 10 4 s roof convection is characterized by the quasi periodic formation and instability of a cold boundary layer its characteristic time scale depends on viscosity and ranges from about 10 2 s for basaltic magmas to about 10 7 s for granitic magmas hence depending on magma viscosity convective instability occurs at different stages of crystallization a single non dimensional number is defined to characterize the different modes of interaction between convection and crystallization using realistic functions for the rates of nucleation and crystal growth we integrate numerically the heat equation until the onset of convective instability we determine both temperature and crystal content in the thermal boundary layer crystallization leads to a dramatic increase of viscosity which acts to stabilize part of the boundary layer against instability we compute the effective temperature contrast driving thermal convection and show that it varies as a function of magma viscosity and hence composition in magmas with viscosities higher than 10 5 poise the temperature contrast driving convection is very small hence thermal convection is weak in low viscosity magmas convective breakdown occurs before the completion of crystallization and involves partially crystallized magma the convective regime is thus characterized by descending crystal bearing plumes and bottom crystallization proceeds both by in situ nucleation and deposition from the plumes we suggest that this is the origin of intermittent layering a form of rhythmic layering described in the skaergaard and other complexes we show that this regime occurs in basic magmas only at temperatures close to the liquidus and never occurs in viscous magmas this may explain why intermittent layering is observed only in a few specific cases

deformation partitioning during transpression in response to early devonian oblique convergence northern appalachian orogen usa abstract transpressive deformation was distributed heterogeneously within the central maine belt shear zone system which formed in response to early devonian oblique convergence during the acadian orogeny in the northern appalachians straight belts are characterized by tight folds s l fabrics and sub parallel form lines and asymmetric structures that together indicate dextral se side up kinematics in contrast intervening zones between straight belts are characterized by open folds and l s fabrics within both types of zone metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade including a penetrative moderately to steeply ne plunging mineral lineation thus we interpret accumulation of plastic deformation and regional metamorphic re crystallization to have been synchronous across the central maine belt shear zone system discordance between inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in straight belt rocks records shortening by tightening of folds and greater reorientation of matrix fabrics with respect to porphyroblasts kinematic partitioning of flow was responsible for the contrasting states of finite deformation recorded in the central maine belt shear zone system perturbations in the flow were caused by serially developed thrust ramp anticlines in the stratigraphic succession immediately above the avalon like basement at which decollement of the shear zone system was initially rooted general shear deformation at the ramps involved strain softening with an enhanced component of noncoaxial flow in contrast deformation during extrusion in the intervening zones involved strain hardening with a greater component of coaxial flow part of the thickening stratigraphic succession exceeded tsolidus reflected by the occurrence of migmatites and granites the latter were partly sourced from the underlying avalon like basement that was involved in the deformation and melting

geochronology and geochemistry of the max porphyry mo deposit and its relationship to pb zn ag mineralization kootenay arc southeastern british columbia canada the max porphyry mo deposit is located at the northern end of the kootenay arc in southeastern british columbia canada molybdenum mineralization is genetically related to the variably altered trout lake granodiorite stock which has lithogeochemical affinities to the low fluorine calc alkaline granodiorite hosted porphyry mo deposit type three molybdenite samples from early and late mo bearing veins yield re os dates that overlap within analytical error with a weighted average age of 80 3 0 2 ma these dates are in close agreement with two 206pb 238u weighted average zircon ages for phases of the trout lake stock at 80 3 1 6 and 79 2 1 0 ma indicating that crystallization of the stock and hydrothermal molybdenite formation were coeval and likely cogenetic 40ar 39ar plateau ages for primary biotite in igneous dikes secondary biotite in early potassic alteration and muscovite in late phyllic alteration range from 80 to 76 ma the younger 40ar 39ar plateau ages may reflect unusually slow cooling of the magmatic hydrothermal system or a late thermal disturbance pb zn ag vein deposits are common throughout the kootenay arc and several such veins are located within and proximal to the max porphyry mo deposit the presence of rare molybdenite associated with sphalerite and argentiferous galena in some of these pb zn ag veins suggests that they represent a late stage event related to the max magmatic hydrothermal system fluid inclusions in quartz from mo and pb zn ag veins are similar and can be divided into two compositional groups type 1 aqueous liquid rich fluid inclusions and type 2 aqueous carbonic liquid rich fluid inclusions pressure corrected 1 4 1 7 kbar trapping temperatures of typical type 1 inclusions from mo bearing veins range from 430 to 460 c and for pb zn ag veins from 335 to 365 c the ranges of salinity 0 12 wt nacl equiv in fluid inclusions from both vein types are almost identical and type 2 inclusions from both vein types contain similar amounts of co2 avg xco2 0 08 with traces of ch4 the similarity of chemistry in the two vein types suggests a genetic link between porphyry mo mineralization and adjacent pb zn ag deposits however muscovite from the alteration halo around a pb zn ag vein within the max deposit yielded a 40ar 39ar plateau age of 72 2 0 5 ma this date is significantly younger than the 80 ma age of porphyry magmatism and mo mineralization suggesting either that this event is in fact unrelated to porphyry mo mineralization at 80 ma or that the k ar isotope system has been disturbed by a later thermal event

density changes during the fractional crystallization of basaltic magmas fluid dynamic implications the dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes in this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization removal of olivine and pyroxene whose values of fractionation density are larger than the density of the magma causes the density of residual liquid to decrease removal of plagioclase with fractionation density less than the magma density can cause the density of residual liquid to increase during the progressive differentiation of basaltic magma density decreases during fractionation of olivine olivine pyroxene and pyroxene assemblages when plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects in the closed system evolution of basaltic magma several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization as well as on the geometry of the chamber in an early stage of the evolution where olivine and or pyroxenes are the fractionating phases compositional stratification can occur due to side wall crystallization and replenishment by new magma with the most differentiated magma tending to accumulate at the roof of the chamber when plagioclase becomes a fractionating phase a zone of well mixed magma with a composition close to the density minimum of the system can form in the chamber the growth of a zone of constant composition destroys the stratification in the chamber a chamber of well mixed magma is maintained while further differentiation occurs unless the walls of the chamber slope inwards in which case dense boundary layer flows can lead to stable stratification of cool differentiated magma at the floor of the chamber in a basaltic magma chamber replenished by primitive magma the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber in a chamber containing well mixed plagioclase saturated magma the primitive magma can be either denser or lighter than the resident magma in the first case the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma mixing then occurs between magmas of different temperatures and compositions in the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma

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

rb sr sm nd and pb isotope systematics of pyrite implications for the age and genesis of lode gold deposits lode gold deposits commonly consist of pyrite and lesser amounts of lead and zinc sulfides accompanied by quartz and calcite their exact origin remains controversial mainly because of the scarcity of reliable geochronological data here we present direct rb sr dating of pyrite from a lode gold deposit and propose the method as a useful geochronological technique for gold mineralization for which age data are poor a positive correlation between present day 87 sr 86 sr and 87 rb 86 sr ratios of pyrite fes 2 subsamples from the linglong gold mine of china corresponds to an age of 122 123 ma which dates the age of gold mineralization the sr nd and pb isotopic compositions of pyrites and the associated rocks suggest that the hydrothermal fluids responsible for the pyrite and gold mineralization were probably derived from a mixed source i e degassing of mafic magmas and meteoric water that had leached the country rocks

supplemental material zircon and monazite geochronology in the palmer zone of transpression south central new england usa constraints on timing of deformation high grade metamorphism and lithospheric foundering during late paleozoic oblique collision in the northern appalachian orogen analytical methods for zircon u pb and monazite th pb sims geochronology zircon u pb la icp ms geochronology data tables and supporting previous work

convective exchange between pore fluid and an overlying reservoir of denser fluid a post cumulus process in layered intrusions abstract we describe experiments on buoyancy driven exchange between fluid in a porous bed of glass balls and an overlying reservoir of denser fluid fingers of the dense fluid penetrated downwards into the porous medium when the two fluids had equal viscosities the rate of convective exchange was proportional to permeability and the density difference between the fluids and was inversely proportional to viscosity when the fluids had different viscosities the rate further depended on the viscosity ratio raised to the power 0 4 if a layer of dense new melt is emplaced above a porous cumulus pile forming the floor of a magma chamber an analogous process may occur as the new melt can percolate down into the pore space and expel the less dense intercumulus liquid convective exchange may proceed at rates of metres per year to tens of metres per year in basic to ultrabasic magmatic systems this conclusion strongly depends on the porosity of the cumulus pile and is less sensitive to the magmatic viscosities chemical and textural effects will result from this process such as re equilibration and resorption of cumulus crystals when intercumulus melt exchange is incomplete lateral geochemical gradients may be preserved in cumulate rocks

the peridotite plugs of rum crystal settling and fabric development in magma conduits the rum igneous complex contains numerous circular to elongate exposures of peridotite cumulates within the layered series and the country rock the extent of thermal effects on quartzo feldspathic country rock surrounding the mam tuath and west sgaorishal plugs demonstrates that these are pipe like bodies inclined at various angles to the vertical that remained at magmatic temperatures for periods of up to several months tabular olivine grains within the plugs preserve a weak sub horizontal fabric that is stronger in the plug centres than at their margins the olivine mode increases from 50 vol at the plug margins to 85 vol at the centre the spatial arrangement of the olivine grains is consistent with their being clustered with the increase in mode towards the plug centres achieved by a combination of overgrowth and mechanical re arrangement comparison of fabrics with those of the ard mheall peridotites of the rum western layered intrusion is consistent with porosity reduction in the ard mheall cumulates also having been primarily achieved by overgrowth and mechanical re arrangement compaction via intra crystalline deformation did not occur we suggest that the rum ultramafic plugs are deep level cross sections presenting the ultimate snap shots of volcanic conduits through which olivine rich magmas flowed for substantial periods probably giving rise to fire fountains of the kind associated with hawaiian eruptions the exposed parts of the plugs are now miniature cumulate bodies that developed in situ from the settling of olivine phenocrysts once flow had ceased within the conduit

towards a model for the in situ origin of pge reefs in layered intrusions insights from chromitite seams of the rum eastern layered intrusion scotland the current debate on the origin of platinum group element pge reefs in layered intrusions centres mostly on gravity settling of sulphide liquid from overlying magma versus its introduction with interstitial melt fluids migrating upward from the underlying cumulate pile here we show that pge rich chromitite seams of the rum eastern layered intrusion provide evidence for an alternative origin of such deposits in layered intrusions these laterally extensive 2 mm thick chromitite seams occur at the bases of several cyclic mafic ultramafic units and show lithological and textural relationships suggesting in situ growth directly at a crystal liquid interface this follows from chromitite development along the edges of steeply inclined culminations and depressions at unit boundaries even where these are vertically oriented or overhanging high concentrations of pge up to 2 3 ppm pd pt are controlled by fine grained base metal sulphides which are closely associated with chromitite seams the following sequence of events explains the origin of the pge rich chromitite seams a emplacement of picritic magma that caused thermal and mechanical erosion of underlying cumulate followed by in situ growth of chromite against the base b precipitation of sulphide droplets on chromite grains acting as favourable substrate or catalyst for sulphide nucleation c the scavenging of pge by sulphide droplets from fresh magma continuously brought towards the base by convection since the rate of magma convection is 105 107 times higher than that of the solidification km year to km day versus 0 5 1 0 cm year the in situ formed sulphide droplets can equilibrate with picritic magma of thousands to million times their own volume as a result the sulphide bearing rocks are able to reach economic concentrations of pge several ppm we tentatively suggest that the basic principles of our model may be used to explain the origin of pge rich chromitites and classical pge reefs in other layered mafic ultramafic intrusions

a fundamental dispute a discussion of on some fundamentals of igneous petrology by bruce d marsh contributions to mineralogy and petrology 2013 166 665 690 marsh contrib miner petrol 166 665 690 2013 again claims that crystal free basalt magmas are unable to differentiate in crustal magma chambers and regards layered intrusions as primarily due to the repeated emplacement of crystal suspensions he ignores an earlier critique of his unconventional inferences latypov j petrol 50 1047 1069 2009 as well as a wealth of petrographic geochemical and experimental evidence supporting the dominant role of fractional crystallization in the solidification of layered intrusions most tellingly the cryptic variations preserved in the skaergaard and many other basaltic layered intrusions would require an exceedingly implausible sequence of phenocrystic magmas but are wholly consistent with in situ fractional crystallization a major flaw in marsh s hypothesis is that it dismisses progressive fractional crystallization within any magma chamber and hence prohibits the formation of crystal slurries with phenocrysts and melts that change systematically in composition in any feeder system this inherent attribute of the hypothesis excludes the formation of layered intrusions anywhere

origin of modal and rhythmic igneous layering by sedimentation in a convecting magma chamber experimental investigations of convecting particle laden fluids show two regimes for convection driven by cooling from above1 in very dilute suspensions convection will maintain a homogeneous distribution of particles throughout the convecting layer provided that particle fall velocities are small compared with turbulent fluid velocities above a critical concentration convection is unable to keep the particles suspended so the particles settle leaving behind a layer of convecting fluid virtually free of particles here we apply these results to cooling magma chambers in which crystallization leads to an increase in suspended crystal content with time discrete sedimentation events are predicted each time the concentration exceeds the critical value for common igneous minerals critical concentrations are very small typically 0 002 0 03 wt and layers of the order of centimetres to a few metres thick will result because minerals of different density and size have different critical concentrations and settling velocities complex fluctuations in sedimentation rate and mineral proportions can occur in a multi component melt this may lead to either regular repetitive cycles or more complex fluctuations the process is confined to low viscosity magmas such as basalts in which the crystals are able to separate from the active thermal boundary layer during convection

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

braided peridotite sills and metasomatism in the rum layered suite scotland the rum eastern layered intrusion eli scotland is an open system layered intrusion constructed of 16 macro rhythmic units each of the macro rhythmic units consists of a peridotite base and a troctolite gabbro top previously attributed to the fractional crystallisation of a single magma batch this classic paradigm has been challenged however with evidence presented for the emplacement of peridotite sills in units 9 10 and 14 such as cross cutting relationships upward oriented apophyses and lateral discontinuities to test whether the other major peridotites within the eli represent sills we have carried out new field petrographic and mineral chemical analyses of the peridotites in units 7 8 and 9 the peridotites display large and small scale cross cutting relationships with the overlying troctolite indicative of an intrusive relationship the peridotites also show large scale coalescence and lateral spatial discontinuities such that the eli unit divisions become arbitrary harrisite layers and cr spinel seams found throughout units 7 8 and 9 suggest the peridotites were constructed incrementally via repeated injections of picritic magma our observations allow for distinct subtypes of peridotite to be defined separated by intrusive contacts allowing for their relative chronology to be determined older poikilitic peridotite rich in clinopyroxene is truncated by younger well layered peridotite containing abundant harrisite layers in addition to the new peridotite subtypes defined in this study we find strong evidence for laterally oriented metasomatism within clinopyroxene rich wehrlites at the top of the unit 8 peridotite the wehrlites and surrounding peridotites record a complex series of metasomatic reactions that transformed thin picrite sills into clinopyroxene rich wehrlites without any evidence for the sort of vertical melt movement typically posited in layered intrusions the observations presented in this study from the eli cannot be reconciled with the classic magma chamber paradigm and are better explained by the emplacement of composite sills into pre existing feldspathic cumulate gabbro or troctolite the evidence for sill emplacement presented here suggests that the layered complex was constructed by a combination of sill emplacement and metasomatism forming many of the unusual often clinopyroxene rich lithologies that surround the sills the broad scale formation of the layered peridotites via incremental sill emplacement suggested by the occurrence of upward oriented apophyses coalescence and lateral discontinuity could be applied to much larger ultramafic intrusions which might have formed by similar mechanisms

the effects of porphyroblast growth on the effective viscosity of metapelitic rocks implications for the strength of the middle crust numerical models are used to examine the effects of porphyroblast growth on the rheology of compositionally layered rocks metapelites and metapsammites and by extension the middle crust during prograde metamorphism as porphyroblast abundance increases during prograde metamorphism metapelitic layers will strengthen relative to porphyroblast free metapelitic units and potentially relative to quartzofeldspathic metapsammitic units as metapelitic layers become stronger the integrated strength of compositionally layered successions increases potentially causing large volumes of mid crustal rock to strengthen altering the strain rate distribution in the middle crust and affecting the geodynamic evolution of an orogenic belt the growth of effectively rigid porphyroblasts creates strength heterogeneities in the layer undergoing porphyroblast growth which leads to complex strain rate distributions within the layer at the orogen scale the strengthening of large crustal volumes on the order of thousands of cubic kilometres changes the strain rate distribution which may change exhumation rates of high grade metamorphic rocks the geothermal structure and the topography of the orogen the presence of a strong zone in the middle crust causes strain rate partitioning around the zone suppressed uplift rates within and above the zone and leads to the development of a basin on the surface

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

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

modeling the three dimensional structure of macroscopic magma transport systems application to kilauea volcano hawaii we report the results of modeling the three dimensional internal structure of kilauea s magmatic passageways the approach uses a clear plexiglass model containing equally spaced levels upon which well located seismic hypocenters are plotted application of constraining geologic and geophysical criteria to this distributed volume of earthquakes permits the interpretation of seismic structures produced by fracturing in response to locally high fluid pressures four magma transport and storage structures have been identified within and beneath kilauea 1 primary conduit the conduit transporting magma into kilauea s summit storage reservoir rises from the model base 14 6 km to the 6 5 km depth level it is a zone of intense fracturing and inferred intrusion whose horizontal sections are elliptical in planform over its height the average major axis of the component horizontal sections is 3 3 km with an average minor axis of 1 7 km this yields an aspect ratio of 0 52 at the 14 6 km level the strike of the major axis is n67 e during passage from the upper mantle through the oceanic crust this axis rotates in a right handed sense until the strike is n41 w at the 6 5 km level 2 magma chamber complex floor the interval from 6 5 to 5 7 km immediately over the primary conduit is aseismic this suggests differentially high fluid to rock ratios and relatively weak pathways for further vertical transport into higher levels of the storage complex as well as lateral leakage eastward into the mauna ulu staging area for later vertical ascent beneath the upper east rift zone seismicity within the immediately subjacent rocks that form the top of the primary conduit at 6 5 km suggests that this inferred magma rich horizon forms the effective floor of the summit storage complex 3 magma chamber crown intense seismicity over the 1 1 1 9 km depth interval defines an elliptical region in plan view the top of this region has a broad apex with an average major axis of 1 5 km and an average minor axis of 1 1 km producing an aspect ratio of 0 73 this region coincides with the epicentral position of summit vertical displacement maxima during inflationary deformation and is interpreted as a region of intense diking crowning the summit storage complex 4 upper east rift zone pipe from a depth of 5 7 km a cylindrical region of seismicity rises beneath the upper east rift zone to the 1 9 km level where it merges with the subhorizontal upper east rift zone duct this pipe like zone has a mean diameter of 1 4 km and a vertical axis that pierces the surface of the volcano at the intersection of the koae fracture system and the upper east rift zone we suggest this pipe forms a principal connection between the base of the summit storage reservoir and near surface storage compartments and ducts in the upper east rift zone in this role the pipe is suggested to have contributed heavily to mauna ulu s magma supply during 1969 1974 the model of magma transport and storage thus constructed provides a context in which other types of data e g ground deformation and electrical conductivity may be interpreted as well as a means of refining petrological inferences based on lava chemistry

formation of hybrid cumulates melatroctolites in intrusion 4 of the honningsvag intrusive suite northern norway new mapping shows that the 1160 m thick layered series of the mafic to ultramafic intrusion 4 of the honningsvag intrusive suite north norwegian caledonides comprises approximately 25 feldspathic peridotite 55 melatroctolite olivine rich troctolite 5 troctolite 2 olivine melagabbro olivine rich olivine gabbro and 13 15 olivine gabbro the cumulate sequence can be divided into eight macrocyclic units the base of each unit resulting from magma replenishment compositions of olivine and plagioclase obtained from ca 1700 electron microprobe point analyses display complex and limited variations fo87 74 and an80 67 relative to stratigraphic height the origin of olivine rich cumulates such as melatroctolites 52 72 olivine and olivine melagabbros 29 37 olivine in large quantities is difficult to explain by either simple fractional crystallisation or conventional layer forming processes a new genetic model is proposed involving suspension of pre existing cumulus crystals during magma recharge in a periodically replenished magma chamber their partial resorption and simultaneous crystallisation of liquidus olivine cumulates formed in this way represent mixtures of crystals that crystallised from compositionally different magmas and are therefore termed hybrid cumulates apart from their distinctly non cotectic bulk modal proportions the hybrid nature of the olivine rich cumulates is evidenced by partially resorbed plagioclases enclosed within cumulus olivines the occurrence of resorbed plagioclase inclusions in olivine gabbros and large variations in modal plagioclase content suggest that these may also be hybrid cumulates d 2002 elsevier science b v all rights reserved

triassic mineralization with cretaceous overprint in the dahu au mo deposit xiaoqinling gold province constraints from shrimp monazite u th pb geochronology abstract the xiaoqinling terrane in central china is the second largest gold producing area in china previous application of age constraints has yielded ages for the gold mineralization ranging from paleoproterozoic through triassic indosinian to jurassic and cretaceous yanshanian shrimp u th pb dating of hydrothermal monazite in a mo mineralized quartz vein from the dahu au mo deposit in the northern xiaoqinling terrane identifies two mineralization episodes hydrothermal monazite intergrown with molybdenite yields an initial precipitation age of 216 5 ma which is identical to but more precise than the molybdenite re os isochron age of 218 41 ma a younger generation of monazite is distinguished by data falling on a mixing array it is inferred that the younger monazite was formed at the expense of the older ones during the yanshanian probably during a single event that occurred at 125 ma combining these results with available geochronological data and geological evidence we conclude that molybdenum mineralization accompanied by monazite deposition took place in the indosinian followed by a yanshanian tectono hydrothermal event that resulted in large scale gold mineralization in the xiaoqinling terrane our observation demonstrates that monazite is sensitive to hydrothermal overprint and thus is useful for dating fluid infiltration event

basaltic replenishment of the rhum magma chamber evidence from unit 14 a detailed geochemical and lithological profile through unit 14 of the rhum eastern layered series is presented and interpreted to trace the evolution of this cyclic peridotite troctolite unit the lower 15 5 m of the unit are peridotite which has an abrupt contact with the upper 35 5 m of troctolite numerous thin subsidiary peridotites occur within the troctolite variation in the modes whole rock 87 sr 86 sr ratios and ni and fo in olivine provide most information about the evolution of the unit discontinuities in these parameters divide the unit into six subunits i the lower peridotite ii the middle main peridotite iii the upper main peridotite and lowermost troctolite iv the middle troctolite v the upper troctolite and vi the disturbed troctolite these subunits are interpreted as the result of magma replenishment by picritic and by basaltic magmas field evidence suggests that the subsidiary peridotites are later sills intruded into solid or semi consolidated troctolite the olivine fo and ni values are similar to those of surrounding troctolite suggesting either re equilibration with or assimilation of the troctolite comparison with unit 10 shows that the unit 14 resident basaltic liquid was more mafic than that of unit 10 this factor along with the basaltic replenishment can explain many of the differences in geochemical and lithological variation between the two units

an ocean ridge type magma chamber at a passive volcanic continental margin the kap edvard holm layered gabbro complex east greenland the gabbros of the tertiary kap edvard holm layered serieshave a stratigraphic thickness of more than 5000 m earlier work has shown that the range in cumulus mineral compositions is restricted plagioclase an 81 an 51 olivine fo 85 fo 66 pyroxenes ca 43 mg 46 fe 11 to ca 43 mg 37 fe 20 field evidence of magma injections is common which together with the restricted range in mineral chemistry suggests that the magma chamber was frequently replenished by a less fractionated magma a detailed study of a 600 m section 900 1500 m in the lower layered series reveals a period of crystallization when the magma chamber behaved as a closed system 900 1300 m the rocks formed during this periodare well laminated olivine gabbros 900 110 m which evolved to well laminated oxide gabbros 1100 1300 m compositional trends in the cumulusminerals are towards more evolved compositions plagioclase an 64 an 58 pyroxene mg from 80 to 76 with stratigraphic height from 1300 m to 1500 m granular olivine gabbros dominate with moreprimitive mineral compositions plagioclase an 67 an 76 pyroxene mg from 78 to 82 the transition olivine gabbro to oxide gabbro at 1100m is a consequence of fractional crystallization and it is shown how changes in activities of feo and fe 2 0 3 in the magma are reflected in the total iron content of plagioclases the transition from oxide gabbro to olivine gabbro at 1300 m results from replenishment by less evolved basaltic magma on the basis of calcic pyroxene chemistry and the mineral crystallization sequence it is concluded that the kap edvard holm layered series crystallized from a tholeiitic magma similar to morb melanogabbroic units occur throughout the intrusion as discordant to subconcordant sill like bodies 0 2 2 0 m thick the melanogabbroic units consist of cr rich augite olivine plagioclase heteradcumulates and contain deformed mica crystals of pre emplacement origin these units crystallized from a wet mgo rich magma which was injected into the layered host gabbros after the formation of the cumulus pile but before the magma was completely solidified the kap edvard holm layered series has several parallels with the plutonic part of ophiolite sequences these include cumulus mineral assemblage compositions of the minerals and the restricted range in compositions with stratigraphic height field evidence of repeated replenishment of basaltic magma dyke swarms overlying the roof zone of the magma chamber and the existence of a late intrusive suite of wet mgo rich magma these parallels suggest that the processes involved in the formation of the kap edvard holm layered series were similar to those involved in the formation of the crustalpart of many ophiolites and beneath present day spreading ridges the kap edvard holm layered series is therefore believed to represent a shallow level magma chamber which acted as a reservoir for basaltic flows at the continental margin during the opening of the north atlantic ocean

porphyry mo and epithermal au ag pb zn mineralization in the zhilingtou polymetallic deposit south china the zhilingtou polymetallic deposit south china provides an excellent opportunity to investigate the magmatic hydrothermal evolution of the porphyry mo to epithermal au ag pb zn ore spectrum new zircon u pb sphalerite 40ar 39ar and pyrite rb sr ages together with previous molybdenite re os ages indicate that the granitic magmatism and mo au ag pb zn mineralization were coeval and cogenetic in the early cretaceous 113 ma fluid inclusions and h o isotope results have established the evolution from an early high temperature h2o nacl magmatic fluid system mo mineralization to a medium temperature mixed fluid system of magmatic and meteoric water au ag mineralization to a peripheral late stage low temperature h2o nacl meteoric water fluid system epithermal au ag pb zn mineralization moreover the sulfur isotopic compositions 34s 2 1 7 8 of the sulfide minerals confirm a genetic link to the granitic magma lead isotope data from pyrite and the granite rhyolite porphyries are similar indicating that they share a common pb source the geochronological geochemical and isotopic lines of evidences consistently suggest that the porphyry mo and epithermal au ag pb zn mineralization formed in one magmatic hydrothermal system the porphyry epithermal mineralization model for zhilingtou has significant importance for prospecting work in south china where mo and associated pb zn ag resources are abundant

metamorphism transient mid crustal rheology strain localization and the exhumation of high grade metamorphic rocks 1 we present a series of three dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid crustal section in the new england appalachians the natural mid crustal section records evidence for spatially and temporally variable mid crustal strength as a function of metamorphic grade during prograde polymetamorphism our models address changes in strain rate partitioning and topographic uplift as a function of strengthening weakening in the middle crust as well as the resultant changes in deformation kinematics and potential exhumation patterns of high grade metamorphic rock results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates deformation kinematics recorded in the orogen are also affected by strengthening weakening with complete reversals in shear sense occurring as a function of strengthening weakening without changes in plate boundary kinematics

distribution of orthocumulate textures in the jimberlana intrusion the p and u contents and the texture of cumulates have been used to document variations in residual porosity in the jimberlana intrusion the results show that rocks with low residual porosity adcumulate textures occur where heat loss through the floor of the chamber was low or where the viscosity of the melt from which they crystallized was low low residual porosity also correlates with the appearance of plagio clase as a cumulus phase and with zones of well developed rhythmic layering it is suggested that the initial porosity of cumulates is controlled by the balance between nucleation and crystal growth at the crystalliquid interface low initial porosity represents attainment of textural equilibrium and forms when adjacent crystals at the top of the cumulate pile continue to grow expelling the intercumulus magma in the process high initial porosities develop when new crystals nucleate at the top of the crystal pile before a high proportion of the intercumulus magma is expelled low viscosity in

an origin for harrisitic and granular olivine in the rum layered suite nw scotland a crystal size distribution study dendritic crystal morphologies occur in a number of igneous rocks and are thought to originate from the rapid growth of crystals yet many examples of dendritic morphologies are found in plutonic igneous rocks where cooling rates should be low results from crystal size distribution csd measurements on harrisitic olivines from rum scotland combined with estimated olivine growth rates suggest that the characteristic skeletal hopper and branching olivines of harrisitic cumulates that are up to centimetres long may have exceptionally short crystal growth times several hours to several hundreds of days this together with very low calculated nucleation densities for harrisitic olivine supports the interpretation of harrisite being a disequilibrium texture developed in response to supersaturation of the magma in olivine we propose that this supersaturation arose through undercooling of thin picrite sheets emplaced along the rum magma chamber floor beneath cooler resident magma it is envisaged that the picrite sheets were largely free of suspended olivine crystals coupled with the olivine enriched composition of the melt and the increasing cooling rate this allowed homogeneous nucleation of olivine to set in at deeper undercooling and greater olivine supersaturation than if there had been plentiful suspended olivines to act as heterogeneous nuclei the enhanced supersaturation caused rapid growth of olivine once nucleation began with skeletal and dendritic shapes it is suggested that the observed interlayered sequences of harrisite and cumulus peridotite found throughout the rum layered suite are a result of multiple episodes of harrisite crystallization resulting from picrite emplacement that alternated with periods of crystal growth and accumulation in the main body of magma at lesser degrees of undercooling

the dog bay liberty line and its significance for silurian tectonics of the northern appalachian orogen1this article is one of a series of papers published in this cjes special issue in honour of ward neale on the theme of appalachian and grenvillian geology 2geological survey of canada contribution 20100257 the dog bay line a silurian suture key to deciphering appalachian accretionary history was first recognized in newfoundland it marks where the ordovician tetagouch exploits ensimatic back arc basin teb which had opened within the leading peri gondwanan gander terrane finally closed here we extrapolate this suture into new england placing it between the liberty orrington miramichi inliers lom and the merrimack fredericton trough mft southeastward marine strata of the mft overlie the teb passive margin exposed in the ganderian st croix block and display southeast vergent structures transected by acadian cleavage they structurally underlie southeast vergent thrusts at the base of the lom northwestward the lom central maine matapedia trough cmmt and lower silurian igneous rocks record elements of the upper plate trench arc system respectively a subduction complex forearc basin and arc the cmmt forearc received detritus both from the northwesterly arc region and also from the

the port mouton shear zone intersection of a regional fault with a crystallizing granitoid pluton abstract the peraluminous tonalite monzogranite port mouton pluton is a petrological geochemical structural and geochronological anomaly among the many late devonian granitoid intrusions of the meguma lithotectonic zone of southern nova scotia the most remarkable structural feature of this pluton is a 4 km wide zone of strongly foliated 040 subvertical monzogranites culminating in a narrow 10 30 m straight zone of compositionally banded rocks that extends for at least 3 km along strike the banded monzogranites consist of alternating melanocratic and leucocratic compositions that are complementary to the overall composition of that part of the pluton suggesting an origin by mineral melt and mineral mineral sorting biotite and feldspar are strongly foliated in the plane of the compositional bands these compositional variations and foliations originated by a process of segregation flow during shearing of the main magma with a crystallinity of 55 75 subsequent minor brittle fracturing of feldspars twinning of microcline development of blocky sub grains in quartz and kinking of micas demonstrate overprinting by a high temperature deformation straddling the monzogranite solidus small folds and late sigmoidal dykes indicate dextral movement on the shear zone this port mouton shear zone pmsz is approximately co linear with the only outcrops of late devonian mafic intrusions in the area two of which are syn plutonic with well developed mingling textures in the marginal tonalite of the port mouton pluton also closely co linear with the mafic intrusions are a granitoid dyke that extends well beyond the outer contact of the port mouton pluton a swarm of large aligned angular xenolithic slabs a zone of thin wispy schlieren banding a large be bearing pegmatite and a breccia pipe with abundant garnetiferous metapelitic xenoliths in various ways the shear zone may control all of these features the port mouton shear zone is parallel to many other ne trending faults and shear zones in the northern appalachians probably related to the docking of the meguma zone along the cobequid chedabucto fault system

compositional convection in a reactive crystalline mush and melt differentiation fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them to elucidate some physical principles of thermosolutal convection in a reactive porous medium we describe experiments on the directional solidification of aqueous ammonium chloride solutions the addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions the phase diagram and permeability solutions were cooled from below and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid released from the boundary layer at the top of the mush the interstitial fluid within the mush also became unstable eventually the onset of convection occurring when the porous medium rayleigh number of the mush reached a critical value this threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature composition and porosity convective motions locally caused dissolution and precipitation and hence fluctuations of porosity developed dissolution occurred preferentially in the central parts of upwellings and the upflow gradually focused being ultimately channelized into narrow chimneys devoid of crystals all else being equal the areal density of chimneys was greater the lower the viscosity chimney diameter increased with increasing solution viscosity in the liquid above the growing mush the convective plumes were very similar to salt fingers depending on solution viscosity and temperature gradient they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities the base plate temperature chosen was above the eutectic and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram the spatial distribution of crystals and the final porosity of the mush were however determined by the strength of compositional convection as measured by the porous medium rayleigh number when no convection occurred mush thickness finally became equal to the initial layer thickness and the system was homogeneous when compositional convection occurred the overlying reservoir underwent chemical evolution and mush growth slowed dramatically in experiments with progressively lower solution viscosity and hence more vigorous convection final mush thickness was progressively less and final porosity lower final mush thickness was found to scale with solution viscosity to the power 0 33 during fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks we speculate that fossil chimney structures can be found in the lower zone of the bushveld ultramafic complex in the form of iron rich platinum bearing dunite pipes this could explain the adcumulate nature of the lower zone rocks and the chemical and mineralogical similarities between the pipes and the overlying merensky reef the merensky reef would be interpreted as a hiatus in mush growth the results may also have applications to the flow structure in regions of partial melting and in the earth s core

stratigraphic context geochemical and isotopic properties of magmatism in the siluro devonian inliers of northern maine implications for the acadian orogeny this paper reports detailed stratigraphic analysis whole rock geochemistry and nd isotopic ratios of basalts from the northern maine inliers these data place constraints on first order controversies about the tectonic conditions leading up to and during the early stages of the acadian orogeny in northern maine late silurian and early devonian stratigraphic sequences indicate a progressive change from shallow water and subaerial exposure ripogenus and the forks formations east branch group to a rapidly subsiding basin prior to the onset of acadian deformation subsidence was accompanied by mafic to intermediate volcanism of the west branch spider lake and fish pond volcanics and that continued with intrusion of the syn deformational greenville plutonic belt including the flagstaff lake igneous complex whose properties we report in this paper trace element geochemistry indicates that magmatism is transdiscriminant showing aspects of within plate volcanic arc and back arc affinities and nd isotopic ratios are moderately positive 2 3 to 3 8 indicating either an uncontaminated and moderately enriched mantle source or a depleted mantle source contaminated by continental crust four possible tectonic models of the acadian orogeny and the siluro devonian sedimentary volcanic sequences of northern maine are evaluated in the context of a subsiding basin associated with this magmatism developed on the amalgamated laurentian plate these include 1 slab detachment during southeast directed subduction of the laurentian continental margin 2 laramide style thrust basins above a shallow northwest dipping subduction zone 3 back arc extension followed by thin skinned shortening above a northwest dipping subduction zone and 4 moluccan style dual dipping subduction zones

the unit 12 allivalite eastern layered intrusion isle of rum a textural and geochemical study of an open system magma chamber the eastern layered intrusion of the rum central complex nw scotland comprises 16 macro units broadly defined by a basal peridotite body overlain by gabbros and troctolites the local term for the upper plagioclase bearing cumulates is allivalite the unit 12 allivalite contains five subsidiary peridotite horizons each continuous on the kilometre scale field textural and geochemical evidence points to the unit 12 allivalite forming by progressive accumulation of crystals from up to seven separate magma batches varying from picrite to basalt the thickness of the magma lens in the sill like chamber was as low as 30 m during a significant part of the development of the unit 12 allivalite and evidence supports one eruption from the chamber during its accumulation

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

petrogenesis of the ultrapotassic lincoln syenite maine late silurian early devonian melting of a source region modified by subduction driven metasomatism the late silurian early devonian 418 1 ma lincoln syenite exposed discontinuously over a distance of 75 kilometers in south central and mid coastal maine represents one of the most unusual igneous rocks in the appalachian orogen detailed field and petrographic study reveals that most of the exposed syenites have been variably affected by post magmatic recrystallization however a small proportion of the syenites in south central maine preserve original igneous minerals and textures and allow for an unobstructed study of the igneous petrogenesis the syenites are characterized by large dark colored alkali feldspar megacrysts set in a finer grained matrix dominated by alkali feldspar clinopyroxene orthopyroxene biotite oxides the rocks are intermediate in terms of sio2 content 55 59 wt yet are ultrapotassic 6 7 wt k2o and relatively primitive in terms of their mgo mg 67 cr 400 ppm and ni 150 ppm concentrations additionally the rocks exhibit large ion lithophile element enrichment relative to the high field strength elements along with elevated concentrations of light rare earth elements 200 300 chondritic abundances the syenites show limited radiogenic isotopic variability with relatively high initial 87sr 86sr values 0 707 to 0 710 relatively low initial end values 4 8 to 6 7 and initial pb isotopic compositions characteristic of igneous rocks in central maine for example 207pb 204pb 15 577 to 15 639 the geochemical characteristics of these rocks are most consistent with a model involving the partial melting of a mantle wedge previously metasomatized by subduction derived fluids crystallization of this magma was influenced by the presence of significant amounts of water that suppressed plagioclase feldspar crystallization facilitated the crystallization of mg rich biotite and ultimately resulted in the nucleation and growth of the alkali feldspar megacrysts correlation with the geochemically similar parks pond pluton in eastern maine extends the distribution of late silurian early devonian ultrapotassic magmatism in maine to an orogen parallel distance of over 150 km comparisons with rocks of similar composition in known tectonic environments suggest the lincoln syenite and by association the parks pond pluton formed as a result of lithospheric extension superimposed on a sub continental lithospheric mantle source region previously modified by subduction driven metasomatism the timing and structural position of this ultrapotassic magmatism is consistent with a geodynamic model involving slab break off or the convective removal of lithospheric mantle delamination in association with acadian convergence both processes facilitate a brief period of crustal extension and decompression melting of source rocks previously modified by subduction related processes

response to comment by mcbirney boudreau and marsh the concept of variable textural maturity at grain junctions in igneous rocks was first introduced by holness et al 2005 the papers of holness et al 2007a 2007b demonstrate how this concept can be applied to cumulates and present a hypothesis to explain the observation of excursions in textural maturity either localized or stepwise within a cumulate pile the comments by mcbirney et al refer to difficulties they have in accepting our hypothesis and we welcome the opportunity to clarify some confusions in holness et al 2007a 2007b here we identify the issues to be addressed with separate headings

the source and transport of arsenic in a bedrock aquifer new hampshire usa the geographic distribution of elevated groundwater as concentrations in a fractured silicate bedrock aquifer in central new hampshire correlates with the presence of pegmatites which border late devonian granites and intrude metasedimentary rocks as concentrations in the pegmatites averaged 9 6 mg kg which is much higher than the associated granites 0 24 mg kg and metasedimentary rocks 0 8 mg kg as was concentrated in these pegmatites by partial melting of calcareous metapelites and subsequent recrystallization as granites with low as concentrations and pegmatites with high as concentrations arsenic behaves similarly to b which was also concentrated into these late stage fluids arsenopyrite feass with an oxidation reaction rim of scorodite feaso4 2h2o was observed in aquifer materials elevated as concentrations have been observed to occur in other new england locations in pelitic metasediments intruded by anatectic plutons it is proposed that pegmatite formation from partial melting of pelitic metasediments may be an important mechanism for concentrating as in crystalline aquifer materials which can then cause local as enrichment of groundwaters groundwater as concentrations ranged from 26 nmol l to 5300 nmol l with a median value 210 nmol l that is more than 30 times higher than the median for groundwaters from all of new hampshire 6 5 nmol l high chloride concentrations 1 mmol l resulting from road salt contamination of recharge waters suggest that groundwaters are most likely young 670 nmol l or 50 g l have very low fe concentrations 7 samples with low as 7 fe oxyhydroxides form rapidly and have a neutral or negative net surface charge that does not readily adsorb as at ph 7 fe oxyhydroxide formation is slow and depends on dissolved o2 availability however the resultant fe oxyhydroxides have a positive net surface charge and appear to adsorb as readily

textural immaturity of cumulates as an indicator of magma chamber processes infiltration and crystal accumulation in the rum eastern layered intrusion textural maturity describes the extent to which rocks have approached textural equilibrium for igneous rocks this is a function of the time integrated thermal history in the sub solidus which for cumulate rocks generally translates to the cooling history via the rate of solidification of the accumulating crystal mush the most straightforward way to quantify the textural maturity of orthocumulates sensu lato is to measure the angle subtended at the corners of interstitial grains texturally mature cumulates have angle populations with median values of c 120 whereas texturally immature cumulates have lower medians inherited by the interstitial phase which infills either an impingement texture in the accumulating mush or a texturally equilibrated pore topology textural observations of allivalites from the rum layered suite demonstrate that stratigraphically localized excursions towards textural immaturity are the result of either transient relatively rapid solidification rates perhaps a result of the arrival of cold crystal laden bodies of fluid on the chamber floor or late stage infiltration of fully solidified cumulates by evolved liquids expelled upwards by compaction of underlying horizons these two processes have distinct textural and compositional signatures

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

the intrusion of fluid mechanics into geology during the last few years a number of fluid dynamicists often in collaboration with geologists have been investigating problems suggested by geology and creating the new field of geological fluid mechanics the research has uncovered many new fluid mechanical phenomena and the studies have ranged widely over theoretical contributions laboratory experiments and confirmatory field observations this paper presents an overview of part of the field with a strong emphasis on the novel fluid mechanical concepts the presentation includes discussion of some of the effects of cooling and crystallization and the resulting convective motions the flow of gravity currents at both high and low reynolds number and the erosion of solid retaining walls by hot turbulent flows

crystallization and compositional convection in a porous medium with application to layered igneous intrusions growing crystals can provide a local source of buoyancy by depleting melt of certain chemical components and vigorous convective motions in the fluid can result we report a quantitative experimental study in which saturated aqueous solutions of cuso4 and na2so4 in porous media of either glass balls or cuso4 5h2o crystals were crystallized by cooling from below the crystals growing in the pore space released light chemically depleted solution which had the potential to convect out of the porous medium and be replaced by additional saturated solution from above we explore the physical and chemical effects of compositional convection during the complete solidification of the interstitial solution by varying the permeability of the porous medium the cooling rate and the initial composition of the solution this type of convection can form adcumulate rocks by enabling continuous circulation of melt from the main magma reservoir through the pores of a cumulus pile adcumulus growth can occur by this mechanism as long as the convective velocity in the porous medium is greater than the velocity of the solidification front we scale the experimental results to magmatic conditions and infer that this process should occur commonly in layered igneous intrusions when our porous media had a small permeability compositional convection enabled the adcumulate growth of a completely solid contact between melt and cumulate trapping of dynamically unstable melt beneath this impermeable layer of crystals also occurred in these experiments and may be important geologically small shallowly emplaced intrusions and the cumulate zones at the base of komatiites are probably cooled too rapidly for significant intercumulus convection to occur

spatial constraints on magma chamber replenishment events from textural observations of cumulates the rum layered intrusion scotland the eastern layered series of the rum layered suite is formed of 16 macro units each comprising a lower peridotite and an overlying feldspar rich layer the local term is allivalite the origin of the peridotite layers is unresolved with two contrasting models the earlier of the two is based on repeated replenishment of an opensystem magma chamber with deposition of fractionated material on the chamber floor the second is based on the early formation of a troctolitic complex which is then repeatedly intruded by sills of replenishing picritic magma to form the peridotite horizons the lack of resolution of this fundamental problem is a consequence of the reliance of previously published studies on field observations i present evidence to show that the clinopyroxene in the allivalites preserves information about the distribution of the last melt to solidify permitting determination of not only the extent of supersolidus textural equilibration but also the sub solidus history of the allivalite horizons comparison of profiles of clinopyroxene plagioclase plagioclase median dihedral angle across allivalite units demonstrates that it is possible to distinguish between those that were intruded by later picrite sills and those adjacent to peridotite horizons formed by replenishment and subsequent deposition of fractionated crystals above the pre existing pile in the region studied only the main peridotite body of unit 9 was intruded into a pre existing allivalitic mush

fluid dynamic and geochemical evolution of cyclic unit 10 rhum eastern layered series lithological major element trace element and sr isotope data from cyclic unit 10 of the rhum eastern layered series are presented the lower 65 metres of the unit are peridotite subdivided on textural and geochemical grounds into a lower homogeneous portion approximately 50 metres thick and an upper heterogeneous portion approximately 15 metres thick the uppermost 16 5 metres of the unit are allivalite there are steep geochemical gradients across the peridotite allivalite boundary in ni content of olivine and whole rock sr isotope composition calculations are presented on the geochemical evolution of a rhum picritic liquid undergoing olivine precipitation both when the olivines remain suspended in the residual liquid as they precipitate and when they are continuously fractionated quenched groundmass and olivine compositions from the rhum dykes and the unit 10 peridotite olivines show good agreement with the suspension model but are inconsistent with the fractionation model the rhum chamber is thought to have been replenished with a picritic liquid from which olivine crystallized while held in suspension however replenishment by a highly olivine phyric basalt is also possible the peridotite probably accumulated rapidly as olivines were dumped out of suspension onto the chamber floor the lower part of the peridotite is a poikilitic adcumulate it is suggested that this formed by convective circulation of melt in the pores of the pile of cumulus olivines in the latter stages of adcumulus growth more fe rich and isotopically contaminated magma entered the top of the cumulus pile causing cumulus olivines to re equilibrate and giving the intercumulus plagioclase a higher sr isotope ratio than lower down the olivines in the allivalite show steep stratigraphic gradients in major element composition but not in their ni content they also show substantial variation in major element composition laterally within the allivalite it is suggested that these features are a consequence of postcumulus re equilibration of olivine with migrating intercumulus magma

petrography chemistry and distribution of platinum and palladium in ultramafic rocks of the bird river sill se manitoba canada the ultramafic portion of the archean bird river sill a mafic ultramafic stratiform intrusion in southeastern manitoba is divided into a lower and an upper series these series may be the product of two separate magmatic injection pulses that differ in their chemistry mode of crystallization and petrography

direct evolution of brine from a crystallizing silicic melt at the questa new mexico molybdenum deposit molybdenum mineralization in the southwest orebody at the questa deposit is associated with crystallization of aplite porphyry explosive brecciation and exsolution of magmatic hydrothermal fluids quartz k feldspar fluorophlogopite and molybdenite precipitated from exsolving aqueous fluids and cemented aplite porphyry and andesite wall rock clasts forming a magmatic hydrothermal breccia ore fluid inclusions in quartz in the breccia matrix trapped aqueous fluids that transported and precipitated molybdenum two fluid inclusion populations associated with molybdenite mineralization were identified the largest population commonly contains liquid vapor halite sylvite and one or more opaque daughter crystals these inclusions homogenize between 200 degrees and 500 degrees c and have a prominent mode at 360 degrees to 400 degrees c salinities vary from 31 to 57 wt percent nacl equiv low first ice melting temperatures suggest that additional fluid components possibly calcium or magnesium chlorides may be present approximately 80 percent of these inclusions homogenize by halite dissolution phase equilibria constraints require that these inclusions trapped fluids in the vapor absent field the remaining saline inclusions homogenize to liquid at temperatures above halite dissolution temperatures significantly the saline fluid did not coexist with a low density aqueous fluid a second population of inclusions contains fluids with salinities of 2 7 to 25 0 wt percent nacl equiv and homogenizes by vapor bubble disappearance vapor bubble expansion or critical behavior textural relationships and phase behavior during microthermometry indicate that these inclusions constitute a single population which formed as pressure and temperature fluctuated at near critical conditions pressure changes related to brecciation followed by crystallization and system resealing produced the observed fluids saline fluids were trapped as the system sealed following a major brecciation event these high salinity fluids were produced as the melt under pressure that increased from hydrostatic to lithostatic partitioned increasing amounts of chlorine into the exsolving aqueous fluid renewed fracturing and abrupt pressure decrease to hydrostatic conditions reduced the mass of chlorine partitioning to the exsolving fluid lower salinity fluids exsolved at near critical conditions and were trapped by inclusions that homogenize to liquid to vapor or by critical behavior the lack of cogenetic liquid and vapor rich fluid inclusions and phase equilibria constraints indicate that high salinity fluids or 57 wt nacl equiv were generated directly by the crystallizing magma and were not a product of aqueous fluid immiscibility

consistent slow exhumation in a late cenozoic glaciated landscape the presidential and carter ranges of the white mountains in new hampshire usa abstract to investigate the ability of the onset of quaternary glaciations to drive an acceleration in erosion rates we compare short timescale 103 104 yr erosion rates using the in situ terrestrial cosmogenic nuclide tcn 10be to longer time scale exhumation rates 106 107 yr using the apatite u th he method in the presidential and carter ranges of the white mountains in new hampshire usa our results are generally consistent with but refine previous studies using similar techniques resulting u th he ages and thermal modeling from mt washington in the presidential range n 4 91 7 to 117 3 ma suggest that exhumation rates from the late cretaceous to the present were slow

the contact zone of the rhum ultrabasic intrusion evidence of peridotite formation from magnesian magmas the eastern layered series of the rhum ultrabasic complex formed in situ and was not tectonically emplaced as previously suggested field relations along the contact show no evidence for the existence of a late marginal gabbro instead peridotite and allivalite layers can be traced to within 2 m of the surrounding partially melted country rocks local preservation of a chilled margin to one peridotite layer indicates that the parental magma consisted of an olivine tholeiite liquid carrying approximately 19 vol olivine crystals in suspension

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

constraints on the formation of cyclic units in ultramafic zones of large basaltic chambers petrological models for the formation of cyclic units in ultramafic zones at the base of large mafic layered intrusions are still few and simple in this study we develop simple physical constraints such as the volume balance and density relationships between the various liquids involved we consider the formation of an entire ultramafic zone made of n cyclic units due to n successive reinjections we tackle the following problems are all the injections of the same chemical composition are the n injections responsible for the formation of the whole magma chamber two end member models are examined in the first one the chamber grows with each injection and does not lose any magma variable volume model vv in the second one the chamber initially forms with a large volume of magma and remains at a constant volume with subsequent reinjections of small volumes balanced by eruptions of equal volume fixed volume model fv several scenarios for the formation of a cyclic unit are envisioned petrological data such as thicknesses of the cumulate layers and their compositions together with the fractionation density of the cumulate phases allow the calculation of the density evolution as a function of the initial density only thus the model yields constraints on the density of the initial liquid and hence on its chemical composition the volumes of the injected magmas are calculated as well as the evolution of the densities of the different involved liquids we apply the calculation to the specific case of the ultramafic series of the stillwater complex the multiple reinjection models require that the initial liquids densities vary between 2 71 and 2 72 g cm3 corresponding to mgo rich liquids for which there is no field evidence either in dikes or in chilled margins they also require that the source produces liquids of different chemical composition thus the evolution of the igneous system requires another larger reservoir where differentiation takes place prior to injection into the shallower one contrary to the multiple reinjection models a closed system model where crystallization proceeds in an isolated bottom layer does not require any specific value of density for the liquids provided of course that it is of basaltic composition on the negative side such a model is not supported by any physical processes which would account for the formation of cyclic units

silurian deformation and metamorphism of ordovician arc rocks of the casco bay group south central maine the casco bay group in south central maine consists of a sequence of late cambrian to early ordovician interlayered quartzofeldspathic granofels and pelite cape elizabeth formation overlain by early to late ordovician back arc volcanic spring point formation and volcanogenic sedimentary rocks diamond island and scarboro formations these rocks were tightly folded and subjected to low pressure amphibolite facies metamorphism in the late silurian this phase of deformation and metamorphism was followed by the development of a variety of structures consistent with a period of dextral transpression in middle devonian early carboniferous time previously dated plutons within the sequence range in age from 422 389 ma and record a period of prolonged intrusive activity in the region similarities in age volcanic rock geochemistry and lithologic characteristics argue strongly for a correlation between rocks of the casco bay group and those in the miramichi belt of eastern maine and northern new brunswic

apatite fission track evidence for the cretaceous development of kilometer scale relief and steady state tertiary topography in new england abstract apatite fission track aft ages in samples collected along the 5000 foot relief 1500 m exposed at mt washington in the presidential range of new hampshire are used to constrain the cretaceous cooling history of this area in the northern appalachians nine aft ages for samples of the littleton and rangeley formations collected along this profile range in age from 150 ma at the highest elevations of 1900 m to 100 ma at the base 500 m thermal histories based on these results and on model time temperature histories based on the distribution of fission track lengths in the higher elevations samples show three stages of cooling from 1 5 2 0 c m y 170 130 ma to 0 2 c 130 65 ma to 0 6 c m y 65 ma to the present in contrast the thermal history calculated for the lower elevations sample shows cooling delayed until 120 ma at 2 c m y 120 100 ma followed by monotonic cooling of 0 6 c m y from 100 ma to the present the convergence of these histories from high intermediate and low elevations suggests a common cooling history independent of elevation differences of 1 km structural tectonic explanations for this thermal convergence are implausible and we conclude that the most likely explanation for the common cooling history across 1 km of relief is that the relief was established by the end of the cretaceous and has persisted with steady state topography through the tertiary to the present the aft results are consistent with an earlier relief method study employing 40 ar 39 ar muscovite cooling ages geothermal gradients calculated from the results of both studies yield 40 c km suggesting that this gradient persisted throughout permian and mesozoic times

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent

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

geophysical evidence for the locations shapes and sizes and internal structures of magma chambers beneath regions of quaternary volcanism this paper is a review of seismic gravity magnetic and electromagnetic techniques to detect and delineate magma chambers of a few cubic kilometres to several thousand cubic kilometres volume a dramatic decrease in density and seismic velocity and an increase in seismic attenuation and electrical conductivity occurs at the onset of partial melting in rocks the geophysical techniques are based on detecting these differences in physical properties between solid and partially molten rock although seismic refraction techniques with sophisticated instrumentation and analytical procedures are routinely used for detailed studies of crustal structure in volcanic regions their application for magma detection has been quite limited in one study in yellowstone national park u s a fan shooting and time term techniques have been used to detect an upper crustal magma chamber attenuation and velocity changes in seismic waves from explosions and earthquakes diffracted around magma chambers are observed near some volcanoes in kamchatka strong attenuation of shear waves from regional earthquakes interpreted as a diffraction effect has been used to model magma chambers in alaska kamchatka iceland and new zealand one of the most powerful techniques in modern seismology the seismic reflection technique with vibrators was used to confirm the existence of a strong reflector in the crust near socorro new mexico in the rio grande rift this reflector discovered earlier from data from local earthquakes is interpreted as a sill like magma body in the kilauea volcano hawaii mapping seismicity patterns in the upper crust has enabled the modelling of the complex magma conduits in the crust and upper mantle on the other hand in the usu volcano japan the magma conduits are delineated by zones of seismic quiescence three dimensional modelling of laterally varying structures using teleseismic residuals is proving to be a very promising technique for detecting and delineating magma chambers with minimum horizontal and vertical dimensions of about 6 km this technique has been used successfully to detect low velocity anomalies interpreted as magma bodies in the volume range 10 3 10 6 km 3 in several volcanic centres in the u s a and in mt etna sicily velocity models developed using teleseismic residuals of the cascades volcanoes of oregon and california and kilauea volcano hawaii do not show appreciable storage of magma in the crust however regional models imply that large volumes of parental magma may be present in the upper mantle of these regions in some volcanic centres teleseismic delays are accompanied by p wave attenuation and linear inversion of spectral data have enabled computation of three dimensional q models for these areas the use of gravity data for magma chamber studies is illustrated by a study in the geysers clear lake volcanic field in california where a strong gravity low has been modelled as a low density body in the upper crust this body is approximately in the same location as the low velocity body delineated with teleseismic delays and is interpreted as a magma body in yellowstone national park magnetic field data have been used to map the depth to the curie isotherm and the results show that high temperatures may be present at shallow depths beneath the yellowstone caldera the main application of electrical techniques in magma related studies has been to understand the deep structure of continental rifts electromagnetic studies in several rift zones of the world provide constraints on the thermal structure and magma storage beneath these regions