mechanisms of crustal anatexis a geochemical study of partially melted metapelitic enclaves and host dacite se spain to shed light on the mechanisms of crustal anatexis a detailed geochemical study has been conducted on minerals and glasses of quenched anatectic metapelitic enclaves and their host peraluminous dacites at el hoyazo se spain anatectic enclaves composed of plagioclase biotite sillimanite garnet glass k feldspar cordierite graphite formed during the rapid heating and overstepped melting of a greenschist facies metapelite and finally equilibrated at 850 508c and 5 7 kbar glass appears as melt inclusions within all mineral phases and in the matrix of the enclaves and has a major element composition similar to that of peraluminous leucogranites melt inclusions and matrix glasses have normative quartz orthoclase albite compositions that plot in the vicinity of h2o undersaturated haplogranite eutectics melt inclusions show some compositional variability with high li cs and b low y first row transition elements frte and rare earth elements ree and zircon and monazite saturation temperatures of 665 7508c they are interpreted as melts produced by muscovitebreakdown melting reactions at the onset of the process of rapid melting and mostly under h2o undersaturated conditions compared with melt inclusions matrix glasses show less compositional variability lower large ion lithophile element contents higher y frte and ree and higher zircon and monazite saturation temperatures 695 8158c they are interpreted as former melts recording the onset of biotite dehydration melting matrix glasses in the dacite are compositionally different from glasses in the enclaves hence the genetic connection between metasedimentary enclaves and dacite is not as straightforward as previous petrographic and bulk major element data suggest this opens the possibility for some alternative interpretation this study shows the following 1 melt inclusions provide a window of information into the prograde evolution of anatexis in the enclaves 2 melting occurred for the most part under h2o undersaturated conditions even if because of the rapid heating the protolith preserved most of the structurally bound h2o contained at greenschist facies up to the beginning of anatexis such that the excess h2o maximized the amount of h2o undersaturated melt generated during anatexis 3 although a large proportion of accessory minerals are currently shielded within major mineral phases they have progressively dissolved to a considerable extent into the melt phase along the prograde anatectic path as indicated by the relative clustering of accessory mineral saturation temperatures and closeness of these temperatures to those of potential melting reactions 4 the dacite magma was probably produced by coalescence of melts

mica liquid trace elements partitioning and the granite pegmatite connection the st sylvestre complex western french massif central abstract we constrain the genetic relation between granite and pegmatite parental melts from the variscan saint sylvestre leucogranite complex and associated pegmatite bodies massif central france through compositions of micas using mica trace element concentrations and available partition coefficients for li rb ba cs and f we calculated the trace element contents of granitic and pegmatitic melts at equilibrium with micas biotite in leucogranites and pegmatites ranges mostly from fe biotite to siderophyllite more evolved facies contain protolithionite and zinnwaldite and lepidolite occurs in the most fractionated pegmatite white micas have homogeneous compositions from muscovite to li phengite in granites biotite and muscovite trace element distributions are clustered in pegmatites mica trace element contents globally follow differentiation from the least to the most evolved body the reconstructed pegmatitic and granitic melts present strong compositional similarities such as enrichments in li rb cs and depletion in ba that suggest a similar origin however micas are shown to have selectively equilibrated with the last melt or fluid in contact and so trace element concentrations of early magmatic liquids are rarely preserved inversion of the mica data constrains the composition of parental melts and the trace element evolutions during crystallization leucogranite and pegmatite melts show mutual incompatible element evolutions inconsistent with a parent daughter genetic relation the data and the modelling suggest that they represent non cogenetic melts generated by discrete episodes of partial melting differentiation is thus inherited from source processes rather than being the consequence of fractional crystallization of a common parental melt magma we suggest that both the leucogranites and the pegmatites originate from partial melting of a heterogeneous source rather than pegmatites being the product of granite crystallization

geochronology petrogenesis and tectonic significance of the jitang granitic pluton in eastern tibet sw china abstract the jitang granitic pluton which is situated in the southern margin of the north qiangtang block in eastern tibet may provide critical information about the source and derivation of the north qiangtang block during gondwana breakup and dispersion in this paper we report relevant data such as zircon u pb age whole rock major and trace element abundances and sr nd isotopes for the jitang pluton the major rock types of the pluton are granodiorite and biotite granite whole rock major element data reveal that the jitang pluton is a peraluminous s type granitic pluton the u pb age of zircons from the pluton is 219 1 1 7 ma which is 10 ma younger than the age of high pressure metamorphism in the longmu co shuanghu collisional suture between the north and south qiangtang blocks the jitang granitoids show pronounced negative ba eu sr anomalies high initial 87sr 86sr ratios from 0 7266 to 0 7389 and low end t values for from 11 1 to 13 2 which are remarkably similar to the gneisses and meta sedimentary rocks from the indian craton as well as the north qiangtang block the results from this study indicate that the jitang granitoids formed by melts derived from a crustal source with sr nd isotopic compositions similar to those of the indian cratonic crust we concur with the previous interpretation based on detrital zircon records that the north qiangtang block was derived from the indian gondwana

miocene andalusite leucogranite in central east himalaya everest masang kang area low pressure melting during heating abstract the studied miocene andalusite bearing leucogranites intrude the upper part of the high himalayan crystallines hhc and the north himalayan domes and outcrop in an area stretching from mt everest to the kula khangri massif bhutan towards the east the leucogranites constitute both dykes as well as sills and parts of larger andalusite free leucogranite plutons e g makalu they represent mainly of two mica muscovite biotite tourmaline cordierite andalusite sillimanite dumortierite leucogranite and tourmaline muscovite tourmaline biotite andalusite sillimanite garnet kyanite spinel corundum leucogranites microstructures reveal several generations of andalusite from residual peritectic early magmatic to cotectic late magmatic even in the same sample the occurrence of residual and or peritectic andalusite together with inclusions of sillimanite biotite in cordierite indicates that melts formed by dehydration melting of biotite at t 660 700 c during prograde heating at low pressure conditions p according to current models leucogranites are produced by dehydration melting of muscovite and or biotite during exhumation of the hhc in this case micas are consumed in the sillimanite stability field as a consequence these models cannot explain the occurrence of residual and or peritectic magmatic andalusite conditions for anatexis in the andalusite field may have been achieved by heat transfer within the exhuming extruding hhc from structurally lower and hotter rocks towards upper and colder fertile lithologies

dissolution of corundum and andalusite in h2o saturated haplogranitic melts at 800 c and 200 mpa constraints on diffusivities and the generation of peraluminous melts of h2o in the aluminum alkali eigenvector could not be obtained the mechanisms and kinetics of equilibration between peraluminous from our results we propose that some h2o may provide charge minerals and granitic melt were investigated experimentally by balance for excess aluminum multicomponent diffusion models the dissolution of corundum and andalusite into h2o saturated yield eigenvalues for the corresponding aluminum alkali eigenvector metaluminous haplogranitic melt at 800 c and 200 mpa mineral of 0 3 10 to 1 9 10 cm s calculated effective and haplogranitic glass rods were juxtaposed inside platinum binary diffusion coefficients for al2o3 are higher 1 0 10 to capsules and then subjected to experimental conditions for times 3 6 10 cm s these diffusion coefficients are used to ranging from 12 to 2900 h upon melting the mineral melt calculate equilibration times between peraluminous minerals and interface retreats with the square root of time the composition of granitic melts in a model system in which two grains of corundum the melt at the interface changes with time but its asi aluminum or andalusite 1 cm apart are connected via a narrow melt column saturation index molar al2o3 cao na2o k2o at the conditions of the experiments diffusive equilibrium through remains constant at 1 20 after 480 h this value is close to initially metaluminous h2o saturated melt would be attained within the asi of an h2o saturated haplogranitic melt in equilibrium with 10 10 years at h2o activity well below saturation of the corundum or andalusite which may indicate that the composition of melt lower diffusivity of all components may increase the time to the melt at the interface follows the liquidus of these minerals in equilibration to 10 10 years approaching possible time frames the system sodium and potassium diffuse rapidly uphill across the for the generation and extraction of crustal melts entire length of the charge towards the interface resulting in a rapid and uniform increase in asi throughout the entire melt column this uphill diffusion of alkalis is due to coupling with excess aluminum entering the melt at the interface and involves long range

microchemical and sr isotopic investigation of zoned k feldspar megacrysts insights into the petrogenesis of a granitic system and disequilibrium crystal growth k feldspar megacrysts kfm are used to investigate the magmatic evolution of the 7ma monte capanne mc monzogranite elba italy dissolution and regrowth of kfm during magma mixing or mingling events produce indented resorption surfaces associated with high ba contents diffusion calculations demonstrate that kfm chemical zoning is primary core to rim variations in ba rb sr li and p support magma mixing i e high ba and p and low rb sr at rims but more complex variations require other mechanisms in particular we show that disequilibrium growth related to variations in diffusion rates in the melt may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber initial 87 sr 86 sr ratios isr obtained by microdrilling decrease from core to rim inner core analyses define a mixing trend extending towards a high isr rb sr melt component whereas the outer cores and rims display a more restricted range of isr but a larger range of rb sr lower isr at the rim of one megacryst suggests mixing with high k calc alkaline mantle derived volcanics of similar age on capraia trace element and isotopic profiles suggest 1 early megacryst growth in magmas contaminated by crust and refreshed by high isr silicic melts as seen in the inner cores and 2 later recharge with mafic magmas as seen in the outer cores followed by 3 crystal fractionation with possible interaction with hydrothermal fluids as seen in the rim the model is compatible with the field occurrence of mafic enclaves and xenoliths

the distribution of halogens f cl br in granitoid rocks abstract the halogen contents of granitoid rocks and their distribution between apatite biotite muscovite and amphibole from two different localities german part of the bohemian massif and central aegean greece are investigated in detail whole rock f and cl contents and f cl mass ratios vary systematically with the magmatic halogen bearing mineral assemblage in which biotite and apatite are present in all rock samples muscovite bearing granites are f rich mostly 1100 2000 g g but up to 3100 g g and are relatively cl poor 50 130 g g in contrast amphibole bearing rocks reach high cl contents 50 900 g g and are relatively f poor 500 1400 g g only biotite and apatite bearing granites show intermediate f 750 1100 g g and cl 120 350 g g contents bromine contents of all rock samples are below the detection limit of about 0 6 g g in general f and cl are preferentially incorporated in apatite over biotite and biotite usually contains higher amounts of f and cl than muscovite similarly biotite prefers f over amphibole but the distribution of cl between biotite and amphibole varies between different rock samples bromine contents in muscovite are below the detection limit of 0 4 g g but can reach up to 0 8 g g in biotite and up to 2 5 g g in apatite and amphibole near solidus effects such as fluid exsolution diffusive re equilibration and hydrothermal overprint may change the initial halogen distribution in the rock importantly significant amounts of the whole rock s f up to 60 and cl up to 90 may be accommodated in post magmatic alteration phases fluorite and na k ca bearing sheet silicates and fluid inclusions therefore great care has to be taken when interpreting halogen contents of whole rocks and the influence of post magmatic halogen carriers needs to be taken into account

liquidus equilibria in the system k2o na2o al2o3 sio2 f2o 1 h2o to 100 mpa ii differentiation paths of fluorosilicic magmas in hydrous systems we investigated phase equilibria in the six component system na2o k2o al2o3 sio2 f2o 1 h2o at 100mpa to characterize differentiation paths of natural fluorine bearing granitic and rhyolitic magmas topaz and cryolite are stable saturating solid phases in calcium poor systems at 100mpa the maximum solidus depression and fluorine solubility in evolving silicic melts are controlled by the eutectics haplogranite cryolite h2o at 6408c and 4 wt f and haplogranite topaz h2o at 6408c and 2 wt f topaz and cryolite form a binary peralkaline eutectic at 6608c 100mpa and fluid saturation the low temperature nature of this invariant point causes displacement of multiphase eutectics with quartz and alkali feldspar towards the topaz cryolite join and enables the silicate liquidus and cotectic surfaces to extend to very high fluorine concentrations more than 30 wt f for weakly peraluminous and subaluminous compositions the differentiation of fluorine bearing magmas follows two distinct paths of fluorine behavior depending on whether additional minerals buffer the alkali alumina ratio in the melt in systems with micas or aluminosilicates that buffer the activity of alumina magmatic crystallization will reach either topaz or cryolite saturation and the system solidifies at low fluorine concentration in leucogranitic suites precipitating quartz and feldspar only the liquid line of descent will reach topaz or cryolite but fluorine will continue to increase until the quaternary eutectic with two fluorine bearing solid phases is reached at 5408c 100mpa and aqueous fluid saturation the maximum water solubility in the haplogranitic melts increases with the fluorine content and reaches 12 5 0 5 wt h2oat the quartz cryolite topaz eutectic composition a continuous transition between hydrous fluorosilicate melts and solute rich aqueous fluids is not documented by this study our experimental results are applicable to leucocratic fluorosilicic magmas in multicomponent systems however the presence of calcium may severely limit enrichment of fluorine by crystallization of fluorite

extreme fractionation in a granite pegmatite system documented by quartz chemistry the case study of tres arroyos central iberian zone spain abstract the tres arroyos granite pegmatite system is located in the sw margin of the nisa alburquerque variscan batholith two granitic facies monzogranite and marginal leucogranite and three types of aplite pegmatite dykes barren intermediate and highly evolved li rich have been distinguished in the area with a zoned distribution from the granite southwards trace elements in quartz from the five facies have been analysed by la icp ms in order to obtain information about the petrogenetic links among the different lithologies of this system as well as to better understand the regional and individual fractionation processes that led to the distinct rocks aluminium ti li and ge show continuous trends from the monzogranite through the marginal granitic facies the barren and intermediate aplite pegmatites up to the most evolved li rich dykes titanium and ge contents respectively decrease and increase gradually with fractionation in contrast al and li show a more complex trend with an initial descending trend to the marginal granitic facies and then showing the highest al and li contents in the quartz from the most fractionated li rich aplite pegmatites this suggests the influence of different competing factors controlling the incorporation of these trace elements in quartz such as the chemical composition of the melt the p and t conditions and the rate of crystallization based on the good correlation between al and li the substitution si4 al3 li seems to be the dominant mechanism of li incorporation into quartz the negligible amount of other trace elements suggests that the remaining al was mainly compensated with h ions via the si4 al3 h substitution a continuous fractionation trend from the monzogranite up to the most fractionated aplite pegmatites is inferred from geochemical modelling by applying the rayleigh equation for fractional crystallization fractionation rates over 50 are needed to obtain the marginal granite and the barren aplite pegmatites compositions and over 99 for the most evolved dykes no pattern in the chemical variation of the trace elements in quartz from different layers in the layered aplite pegmatites has been found suggesting the lack of internal fractionation processes most probably due to the rapid crystallization of the pegmatitic melt intruded into a colder country rock

late collisional granites in the variscan erzgebirge germany the late collisional erzgebirge granites 325 318 ma were introduction emplaced at shallow crustal levels in the variscan metamorphic the erzgebirge is a ne sw trending antiformal structure basement shortly after large scale extension caused by orogenic exposing variscan crystalline basement rocks at the northcollapse these granites comprise mildly peraluminous transitional ern margin of the bohemian massif the region is famous i s types and strongly peraluminous s type rocks which can be among economic geologists for its extensive and highly subdivided into three major groups low f biotite granites low f variable metallic ore deposits which have been mined two mica granites and high f high p2o5 li mica granites the since the middle ages e g stemprok seltmann 1994 highest degree of differentiation is reached in the li mica granites tischendorf forster 1994 and for the classic studies which exhibit strongly elevated concentrations of p f li rb cs in the fields of economic geology mining and metallurgy ta sn w and u but very low ti mg co ni sr ba y zr chemistry and mineralogy e g werner 1791 breithaupt hf th and rare earth elements crystal melt fractionation is the 1849 agricola 1974 dominant process controlling the bulk composition of all groups of as in many of the other variscan provinces in central granites however metasomatic processes involving late stage residual europe granitic rocks make up a large proportion of the melts and high t orthomagmatic fluids became increasingly more exposed rocks in the erzgebirge and a great variety of important in highly evolved units and have modified the abundances compositional and textural types occur fig 1 knowof mobile elements p f li rb cs ba sr in the li mica ledge of the processes involved in the generation and granites particularly isotopic and geochemical characteristics suggest evolution of these silicic magmas is essential for underthat the three granite groups cannot be derived from a common standing heat and mass transport during the variscan precursor magma their discrete compositions are source related orogeny and also for the formation of metallic ore and are attributed to melting of quartzo feldspathic and pelitic deposits several lines of evidence indicate that early crustal lithologies in different proportions granites are common in high temperature sn w deposits are associated with the the central european variscides but the erzgebirge is unusual for emplacement of highly evolved volatile rich granitic the predominance of evolved li mica granites associated with melts tischendorf 1986 forster tischendorf 1992 economically important sn w and u deposits the abundance of stemprok 1993 the role of the granites in formation of li mica granites is attributed to a combination of favourable factors later permian u pb zn and younger ore mineralization 1 low degrees of anatectic melting of crustal protoliths 2 wide f ba is controversial but good evidence exists that the distribution of fertile lithologies rich in large ion lithophile elements vein type u deposits owe their uranium to leaching from and ore elements 3 extended magmatic differentiation by crystal the uraninite rich granites e g tischendorf forster melt fractionation and subsequent autometasomatism 1994

lithium isotopic systematics of a type granites and their mafic enclaves further constraints on the li isotopic composition of the continental crust article i nfo lithium concentrations and isotopic compositions of 39 a type granites and mafic enclaves from 11 plutons in northeast china and the north china craton are used to constrain their genesis and to characterize further the average composition of the continental crust lithium concentrations 2 8 to 80 ppm in 29 a type granites are slightly higher than those of average i type granites and lower than those of average s type granites reflecting source differences lithium isotopic compositions 7 li 1 8 to 6 9 of a type granites fall within the range of worldwide i type and s type granites by contrast 10 mafic enclaves have significantly higher li concentrations 32 to 179 ppm and less variation in li isotopic compositions 7 li

crystallization of the little three layered pegmatite aplite dike ramona district california subhorizontally layered pegmatite aplite bodies are characterized by fine grained sodic to granitic aplite that is usually juxtaposed abruptly above by much coarser grained commonly graphic potassic pegmatite although well studied there currently is little concensus as to how such dikes form the little three dike near ramona california is representative of such zoned bodies in this and other regions and contains discontinuous miarolitic pockets near the base of the graphic pegmatite zone tourmaline garnet biotite and muscovite show no changes in major or minor element compositions indicative of progressive magmatic fractionation until the immediate vicinity of the main miarolitic zone where they record abrupt and extreme enrichments in li f and mn there is no correlation of chemical changes in the dike with the appearance of small miarolitic vugs well below the main miarolitic zone nor is there any indication that the aplite graphic pegmatite or miarolitic pockets represent separate magma injections the chemistries of tourmaline garnet and micas however preclude conventional models of rayleigh fractionation or traditional zone refining textural features and modeled cooling histories indicate that the dike cooled quickly and might have solidified partially or totally to glass before crystallization commenced geothermometry based on the compositions of coexisting plagioclase and homogeneous nonperthitic k feldspar indicates inward crystallization of the dike from 400 435 c at the margins to 350 390 c within 20 30 cm of the pocket horizon then a sharp decrease to 240 275 c in the pockets where k feldspar is perthitic we interpret the feldspar geothermometry except perhaps in the miarolitic cavities to reflect the temperatures at crystallization fronts that advanced into the pegmatite first from the foot wall and eventually joined by a similar front downward from the hanging wall crystallization down from the hanging wall may have commenced after 70 80 of the foot wall aplite had crystallized the very abrupt increases of li mn and f in tourmaline and garnet near the miarolitic zone appear to be explained best by the process of constitutional zone refining in which a fluxed crystallization front sweeps an incompatible element enriched boundary layer through a solid or semi solid after these two highly fluxed boundary layers merged near the main miarolitic zone compositional evolution could have proceeded by crystal melt fractionation

an experimental study of element partitioning among biotite muscovite and coexisting peraluminous silicic melt at 200 mpa h2o

mineralogical evidence for fractionation processes in the himalayan leucogranites of the ramba dome southern tibet abstract himalayan leucogranites provide some of the best examples in the world of highly felsic and strongly peraluminous granites and they have been paid special attention with regard to investigating the evolution of continental crust here we describe our mineralogical investigations of a late miocene leucogranite complex within the ramba dome of the tethyan himalaya belt that have helped us decipher the petrogenesis of these high silica peraluminous granites the ramba leucogranites range from two mica granites biotite muscovite to muscovite leucogranites muscovite garnet tourmaline with the two mica granites constituting the main part of the pluton and the muscovite leucogranites commonly occurring as late stage dikes that are located within the pluton or invaded the country rocks variations in the mineral assemblages of the two types of leucogranite are associated with systematic changes in bulk rock geochemistry and the distinctive mineralogical features demonstrate a genetic relationship between the two types of leucogranite that involved fractional crystallization with the muscovite leucogranites being more evolved than the two mica granites chemical data for the minerals in the two mica granites record a long period of differentiation during which a normal granitic melt developed into a highly evolved magmatic system with high flux components e g h2o f b in contrast the mineralogical data for the muscovite leucogranites record the crystallization of a very late residual melt i e a volatile rich highly fluid melt according to recent studies of differentiation processes in silicic magma reservoirs we can interpret the ramba leucogranites as having undergone a high degree of differentiation by in situ fractional crystallization in a crystal mush environment the two mica granites represent a congealed crystal mush that was composed of cumulate crystals and a trapped interstitial liquid and the muscovite leucogranites represent the almost pure liquid that was extracted from the crystal mush modeling using the trace elements sr and ba shows that the extraction probably occurred when the crystallinity of the mush was 59 64 at least for the most evolved muscovite leucogranite sample the two mica granite corresponds to a residual crystal mush that had a terminal porosity of 20 26 filled with a trapped interstitial liquid collectively fractional crystallization can account for the geological mineralogical petrological and geochemical variations between the two types of leucogranite this work highlights the important role of crystal fractionation in the generation of himalayan leucogranites

gahnite chrysoberyl and beryl co occurrence as accessory minerals in a highly evolved peraluminous pluton the belvis de monroy leucogranite caceres spain abstract gahnite znal 2 o 4 chrysoberyl beal 2 o 4 and beryl be 3 al 2 si 6 o 18 have been found as accessory minerals in the external highly fractionated leucogranitic unit within the hercynian reversely zoned belvis de monroy pluton westernmost part of the montes de toledo batholith caceres spain the highly felsic sio 2 72 wt and peraluminous acnk 1 2 character of this leucogranite together with the high content of some incompatible elements f li b and p seems to be a primary consequence of fractional crystallization in a magmatic closed system the high be contents and zn fe total ratio 0 01 are relevant factors which have favoured the precipitation of these minerals moreover the si al p b and f activities might be high favouring the magmatic crystallization of such exotic mineral phases together with be rich cordierite f rich micas sillimanite and al rich phosphates in fact the interplay between the silica and alumina activities likely controls the stabilization and the preferential crystallization of gahnite chrysoberyl or beryl chrysoberyl assemblages in mm sized microdomains the p t crystallization conditions are constrained by the muscovite and sillimanite stability fields and the minimum granite al 2 o 3 saturated solidus and have been estimated at temperatures between 670 and 700 c and pressures between 1 and 2 kbar

petrogenesis of the dasuji porphyry mo deposit at the northern margin of north china craton constrains from geochronology geochemistry and isotopes characteristics abstract the dasuji porphyry mo deposit is located at the northern margin of north china craton mo mineralization occurs mainly within the intrusions of alkali feldspar granite granite porphyry and quartz porphyry which yield weighted mean 206pb 238u ages 2 of 227 6 1 5 225 7 1 5 and 224 3 1 8 ma respectively these ages are indistinguishable to the molybdenite re os isochron age of 223 9 1 5 ma 2 the dasuji granites show the petrological mineralogical geochemical and geotectonic characteristics of a type granites they exhibit low negative end t values 16 7 to 12 6 with corresponding tdm2 nd ages of 1705 2000 ma variable but mainly negative ehf t values mostly 21 0 to 7 9 with corresponding tdm2 hf ages of 1715 2728 ma and pb isotopic compositions indicating that the primary magmas were dominantly derived from partial melting of ancient lower crust in chondrite and mantle normalized trace element diagrams all the granitic rocks show negative anomalies in ba sr and eu and a positive anomaly in pb the ratios of lree hree and la yb n and eu values decrease from the alkali feldspar granite to the granite porphyry and quartz porphyry these data are attributed to fractional crystallization processes fractionation of feldspar biotite and possibly k feldspar produced the variations in major elements and rb sr and ba and the distribution of ree are controlled mainly by fractionation of zircon allanite minor apatite and possibly a small quantity of monazite together with the regional geology the data infer that the dasuji porphyry mo deposit formed in a post collision extensional setting during the late triassic

granitoid magmas preserved as melt inclusions in high grade metamorphic rock this review presents a compositional database of primary anatectic granitoid magmas entirely based on melt inclusions mi in high grade metamorphic rocks although mi are well known to igneous petrologists and have been extensively studied in intrusive and extrusive rocks mi in crustal rocks that have undergone anatexis migmatites and granulites are a novel subject of research they are generally trapped along the heating path by peritectic phases produced by incongruent melting reactions primary mi in high grade metamorphic rocks are small commonly 5 10 m in diameter and their most common mineral host is peritectic garnet in most cases inclusions have crystallized into a cryptocrystalline aggregate and contain a granitoid phase assemblage nanogranitoid inclusions with quartz k feldspar plagioclase and one or two mica depending on the particular circumstances after their experimental remelting under high confining pressure nanogranitoid mi can be analyzed combining several techniques emp la icp ms nanosims raman the trapped melt is granitic and metaluminous to peraluminous and sometimes granodioritic tonalitic and trondhjemitic in composition in agreement with the different formula 1 conditions of melting and protolith composition and overlap the composition of experimental glasses produced at similar conditions being trapped along the up temperature trajectory as opposed to classic mi in igneous rocks formed during down temperature magma crystallization fundamental information provided by nanogranitoid mi is the pristine composition of the natural primary anatectic melt for the specific rock under investigation so far 600 nanogranitoid mi coming from several occurrences from different geologic and geodynamic settings and ages have been characterized although the compiled mi database should be expanded to other potential sources of crustal magmas mi data collected so far can be already used as natural starting point compositions to track the processes involved in formation and evolution of granitoid magmas 1 embed mml math 1 gif

fluorine as a tool in the petrogenesis of quartz bearing magmatic associations applications of an improved f oh biotite apatite thermometer grid abstract fluorine contents of whole rock and biotites allow to discriminate quartz bearing magmatic associations through the nature of protoliths and petrogenetic processes f and rb are used to discriminate crustal and mantle derived melts with the exception of calcalkaline association in which mantle contribution may be significant the other quartz bearing associations are essentially derived from crustal melting by biotite and or amphibole vapor absent reactions based on experimental data and on an improved biotite apatite thermometer grid closure temperatures from near liquidus to near solidus conditions as well as fh2o fhf evolution during cooling are determined for the different magmatic associations it appears that the liquidus temperature and fh2o fhf decreases in the order high silica alkaline high k calcalkaline and calcalkaline associations while the solidus temperature follows an inverse order the fh2o fhf values for the peraluminous associations vary over a wide range and overlap the fh2o fhf values of the high k calcalkaline and calcalkaline associations

trace element geochemistry by laser ablation icp ms of micas associated with ta mineralization in the tanco pegmatite manitoba canada in the tanco pegmatite one of the world s major ta deposits tantalum mineralization shows a complexity that reflects the complex petrogenesis of its host pegmatite micas are common in most of the pegmatite units and are intimately associated with the successive stages of ta mineralization from the wall zone to the central zones where micaceous replacement is pervasive different generations of micas both primary and secondary associated with ta oxides were selected for electron microprobe and laser ablation icp ms investigation their chemical trends are used to constrain the magmatic versus hydrothermal processes that played a role in their crystallization and their associated ta mineralization micas range from dioctahedral muscovite to trioctahedral lepidolite through al li substitution unexpectedly the most evolved compositions low k rb ratios and high li contents occur in the wall zone they are interpreted to reflect nonequilibrium crystallization from an undercooled melt with or without boundary layer effects in the central zones the fine grained mica quartz assemblage hosts some coarser grained li muscovite which has the highest ta contents up to 400 ppm these li f a rich micas are interpreted to originate from a magmatic metasomatic event which was also at the origin of the mqm style ta mineralization at tanco however the li ta poor muscovite end member compositions of fine grained alteration micas suggest crystallization from an aqueous fluid during a metasomatic hydrothermal event involving late pegmatitic fluids the low ta concentrations around 50 ppm of this fine grained muscovite suggest that this fluid transported at least small amounts of ta

two mica and tourmaline leucogranites from the everest makalu region nepal tibet himalayan leucogranite genesis by isobaric heating in the higher himalaya of the region from cho oyu to the arun valley northeast of makalu the miocene leucogranites are not hosted only in the upper high himalayan crystallines hhc a network of dykes also cuts the lower hhc and the lesser himalayan crystallines lhc the plutons and dykes are mainly composed of two mica muscovite biotite tourmaline cordierite andalusite sillimanite leucogranite with tourmaline 2 6 and biotite 1 5 modal and tourmaline muscovite tourmaline biotite sillimanite garnet kyanite andalusite spinel corundum leucogranite with tourmaline 2 2 and biotite 1 5 modal both leucogranite types were produced by partial melting in the andalusite sillimanite facies series under lp ht conditions constrained by the occurrence of peritectic andalusite and cordierite the geochemical features of the leucogranites suggest that tourmaline leucogranite was produced by muscovite dehydration melting in muscovite rich metapelites at p 350 mpa and t 640 c whereas two mica leucogranite was produced by biotite dehydration melting in biotite rich metapelites at p 300 mpa and t 660 710 c melting in fertile muscovite rich metapelites of the top of both the hhc and lhc produced magmas which were emplaced at the same structural level in which they had been generated melting in the biotite rich gneiss of both the hhc and lhc produced hotter magmas which were transported upwards by dyking and eventually coalesced in the plutons of the upper hhc a similar process also produced a network of two mica granite at the top of the lhc in the ama drime nyonno ri range northeast of makalu the prograde character of leucogranite melt producing reactions in the everest makalu area suggests that here the generation of miocene leucogranites took place in a regime of nearly isobaric heating following nearly adiabatic decompression

late pleistocene deposits at block fen cambridgeshire england organic sediments in a gravel quarry at block fen cambridgeshire form a sheet dividing lower from upper gravels analyses of pollen macroscopic plant remains and molluscs from these organic sediments are presented they indicate the presence of temperate freshwater and slightly brackish fine floodplain sediments which on the basis of the palaeobotany are correlated with the temperate ipswichian stage the freshwater sediments ascribed to ipswichian substage iib occur at ca 3 m od marine influenced tidal sediments ascribed to ipswichian substage iii occur at ca 6 m od no evidence was found for the presence of more than one temperate stage in the sequence the lower gravels are then correlated with the cold wolstonian stage and the upper gravels with the cold devensian stage in contrast to the woodland environments indicated by the palaeobotany of the ipswichian organic sediments post ipswichian pollen diagrams and macroscopic plant remains in the upper suite of sands and gravels indicate open tree less vegetation typical of the cold devensian stage they also contain a typical cold stage mollusc fauna the sediments containing these floras and faunas are associated with thermal contraction cracks indicating the presence of permafrost the final sand and gravel aggradation in the devensian forms the block fen terrace near 0 m od the evidence indicates that it is younger than the lacustrine sediments resulting from the blocking of the fenland at the wash by late devensian ice at ca 18 5 ka bp the sequence at block fen is related to nearby ipswichian and devensian sediments at chatteris march wimblington and mepal and to deposits at wretton on the east margin of fenland the correlation permits an outline reconstruction of the history of the valley carrying the river great ouse between the isle of ely and the chatteris and march islands from the time of a gravel aggradation before the ipswichian to the flandrian the reconstruction shows the time and level of the ipswichian marine incursion into the middle level of fenland and the extent of aggradation and erosion in the devensian

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

rb depletion in biotites and whole rocks across an amphibolite to granulite facies transition zone tamil nadu south india abstract relatively low concentrations of rb and high k rb ratios are characteristic of many granulite facies terranes this depletion in rb has been attributed to both the removal of a partial melt and exchange with a metamorphic fluid phase these models have been tested using rb concentrations in biotites and whole rocks from intermediate and felsic gneisses collected along a traverse from just north of krishnagiri to just north of salem in tamil nadu state south india along this traverse the northern amphibolite facies zone gives way to a clinopyroxene zone in which clinopyroxene appears in intermediate and felsic gneisses further south is the lowland charnockite zone characterised by the presence of orthopyroxene and the scarcity of clinopyroxene in intermediate to felsic gneisses the abundance of orthopyroxene increases southwards and it is the dominant ferromagnesium silicate in the highland charnockite zone there is a good correlation between rb in biotite and whole rock rb in samples collected throughout the traverse intermediate and felsic gneisses in the northern portion of this traverse have relatively high modal abundances of biotite low ti concentrations in the biotites high whole rock rb concentrations low k rb ratios and high rb concentrations within the biotites ti concentrations in the biotites increase southward into the clinopyroxene zone and then remain relatively constant high k rb ratios first appear at the southern boundary of the clinopyroxene zone in the lowland and highland charnockite zones the majority of the rocks have relatively low rb concentrations and high k rb ratios low rb concentrations in biotites at or near the detection limit of 65 ppm first appear in the lowland charnockite zone and persist into the highland charnockite zone a smaller group of rocks in the highland charnockite zone contain biotites with moderate rb concentrations most of these rocks also contain anomalously high biotite concentrations and low k rb ratios it is not possible to produce rb depleted rocks containing low rb biotites by simple equilibrium partial melting models beginning with a rock with the average composition of the amphibolite facies gneisses 70 ppm and using distribution coefficients from the literature it is possible to produce such rb depleted rocks if the melt is removed in batches however it requires large amounts of partial melting 40 low rb biotites can be produced by both equilibrium and batch melting if the original rocks have low whole rock concentrations of rb 10 20 ppm to begin with attempts to model rb depletion by exchange with a metamorphic fluid phase are hampered by a general lack of experimentally determined mineral fluid distribution coefficients recent experimentally determined exchange coefficients between phlogopite and a k rb cl rich supercritical brine at high pressure and temperature indicate that it would require a fluid rock mass ratio of approximately 0 125 to produce rb depleted rocks with low rb biotites by exchange with a kcl rich fluid alternatively biotites low in rb may have formed shortly after the peak of metamorphism in the rocks after they were depleted in rb but while they were still under relatively high grade conditions

mineral chemistry of ti rich biotite from pegmatite and metapelitic granulites of the kerala khondalite belt southeast india petrology and further insight into titanium substitutions precise chemical composition including fe 3 and h of biotite from a pegmatite dike and its host granulite from the kerala khondalite belt of se india has been determined using a multi technique approach involving emp sims mossbauer and c h n elemental analysis biotite in these rocks formed at t 800 850 c and p 5 1 kbar the full analyses were normalized on the basis of o 12 x y z oh x cl y f z biotite in the pegmatite is ti f and cl rich 0 33 0 46 and 0 16 apfu respectively h 2 o poor oh 0 86 pfu has x mg 0 49 and fe 3 fe tot 3 the low octahedral vacancies 0 06 pfu and the high oxygen content in the hydroxyl site oh f cl 1 49 pfu confirm the role of the ti oxy substitution as a major exchange vector in these high t biotites in the host granulite fine grained biotite is fe 3 free has low cl 0 03 apfu and more variable composition with ti f and x mg in the ranges 0 26 0 36 0 52 0 67 and 0 67 0 77 respectively the number of octahedral vacancies is relatively large 0 10 0 18 pfu and the sum of volatiles oh f cl varies from 1 71 to 2 06 pfu systematic variations of x mg are a function of the microstructural position and are in agreement with retrograde exchange reactions biotite included in or in contact with garnet has the maximum values whereas crystals in the matrix have the minima titanium has systematic negative correlations with f x mg and oh f cl whereas al and octahedral vacancies are virtually constant these trends indicate that the ti vacancy along with substitutions involving al cannot explain the observed short scale variations conversely the ti oxy exchange appears to be active resulting from combination of two vectors the more conventional hydroxylation ti 4 2o 2 fe mg 2 2oh and the fluorination ti 4 2o 2 fe mg 2 2f the systematic retrograde redistribution involves not only fe and mg as commonly observed but also ti f and h in a way such to eliminate the primary ti oxy component of biotite

structural control magmatic hydrothermal evolution and formation of hornfels hosted intrusion related gold deposits insight from the thaghassa deposit in eastern anti atlas morocco in the moroccan eastern anti atlas the thaghassa intrusion related gold deposit is hosted in hornfelsed metasedimentary rocks that lie adjacent to the ikniwn granodiorite fields studies reveal three tectono magmatic stages controlling the formation of the deposit i the first stage refers to the top to the south asymmetry and the syn kinematic ikniwn pluton emplacement controlled by a compressional or transpressional strain regime ii the second stage is characterized from older to younger and further away from the intrusion by metatexite with leucocratic stromatic bands aplo pegmatite sills intermediate veinlets composed of quartz k feldspar and muscovite and then gold bearing striped foliation veins all these features are assumed to have been emplaced during a large scale ene wsw dextral shearing process that results from an ese wnw shortening direction during transtensive tectonics we suggest that the progressive and continuous shearing was initiated since the aplo pegmatite stage and achieved during the hydrothermal phase the existence of intermediate veins characterized by quartz rich core and apatite muscovite feldspar rich rims demonstrates the progressive evolution from the magmatic to the hydrothermal stage and evidence for the persistence of the magmatic character at least until the onset of the hydrothermal process iii the late stage developed large volcanic dyke swarm and brittle faulting zircon u pb la icp ms dating yields a concordia age of 563 5 6 3 ma for the ikniwn granodiorite intrusion the fluid inclusions data besides the mineral thermometry indicate that two main types of fluids can been highlighted i a hot aquo carbonic h2o nacl co2 fluid with n2 and ch4 evolving from vapour rich n2 and ch4 inclusions for the magmatic stage 550 c to ch4 co2 biphased inclusions for the ongoing hydrothermal stage 450 300 c and ii an always secondary low t 200 c saline aqueous type probably nacl free from volatiles with very variable salinity eventually we highlight that in the hornfels hosted irg deposits fluid sources may originate from both magmatic processes and devolatization of the metamorphic host rocks

tectonic record magmatic history and hydrothermal alteration in the hercynian guerande leucogranite armorican massif france abstract the guerande peraluminous leucogranite was emplaced at the end of the carboniferous in the southern part of the armorican massif at the scale of the intrusion this granite displays structural heterogeneities with a weak deformation in the southwestern part whereas the northwestern part is marked by the occurrence of s c and mylonitic extensional fabrics quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both e w and n s stretching therefore during its emplacement in an extensional tectonic regime the syntectonic guerande granite has probably experienced some partitioning of the deformation the southwestern part is characterized by a muscovite biotite assemblage the presence of restites and migmatitic enclaves and a low abundance of quartz veins compared to pegmatite dykes in contrast the northwestern part is characterized by a muscovite tourmaline assemblage evidence of albitization and gresenization and a larger amount of quartz veins the southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone the granite samples display continuous compositional evolutions in the range of 69 8 75 3 wt sio 2 high initial 87 sr 86 sr ratios and low e nd t values suggest that the peraluminous guerande granite a cnk 1 1 was formed by partial melting of metasedimentary formations magmatic evolution was controlled primarily by fractional crystallization of k feldspar biotite and plagioclase an20 the samples from the apical zone show evidence of secondary muscovitization they are also characterized by a high content in incompatible elements such as cs and sn as well as low nb ta and k rb ratios the apical zone of the guerande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement u th pb dating on zircon and monazite revealed that the guerande granite was emplaced 309 7 1 3 ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca 303 ma these new structural petrological and geochronological data presented for the guerande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime magmatic differentiation and hydrothermal alteration and provide a general background for the understanding of the processes controlling some mineralization in the western european hercynian belt

large scale melt depletion in granulite terranes an example from the archean ashuanipi subprovince of quebec this study uses field petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane and to determine what the grain and outcrop scale melt flow paths were during the melt segregation process the ashuanipi subprovince located in the north eastern superior province of quebec is a large 90 000 km2 metasedimentary terrane in which 85 of the metasediments are of metagreywacke composition that was metamorphosed at mid crustal conditions 820 900 c and 6 7 kbar in a late archean dextral transpressive orogen decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction biotite quartz plagioclase melt orthopyroxene oxides using melt orthopyroxene ratios for this reaction derived from experimental studies the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol partial melting the metagreywackes are enriched in mgo cao and feot and depleted in sio2 k2o rb cs and u have lower rb sr higher rb cs and th u ratios and positive eu anomalies compared to their likely protolith these compositions are modelled by the extraction of between 20 and 40 wt granitic melt from typical archean low grade metagreywackes a simple mass balance indicates that about 640 000 km3 of granitic melt was extracted from the depleted granulites the distribution of relict melt at thin section and outcrop scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary porous flow from the site of melting across the layers and into bedding planes between adjacent layers in other rocks pervasive grain boundary flow of melt occurred along the layers for a few to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits visible as the net like array of leucosomes in some outcrops the leucosomes contain very little residual material 5 biotite orthopyroxene indicating that the melt fraction was well separated from the residuum left in situ as melt depleted granulite only 1 3 vol percentage melt remained in the melt depleted granulites hence the extraction of melt generated by biotite dehydration melting in these granulites was virtually complete under conditions of natural melting and strain rates in a contractional orogen

the geochemical differentiation of s type pegmatites constraints from major trace element and li b isotopic composition of muscovite and tourmaline pegmatites often exhibit regional as well as internal zonation in this study we use major and trace element and li b isotopic composition of muscovite and tourmaline from internally zoned and un zoned s type pegmatites and their host granites to characterize the geochemical and isotopic fractionation associated with their formation the internally zoned pegmatites comprise three distinct textural zones namely wall intermediate and core muscovite and tourmaline occur in all the zones of these pegmatites the trace element concentrations ratios of muscovite from the three zones form well defined differentiation trends marked by enrichment of incompatible elements such as rb cs sr b zn nb ta p and the depletion of ni co v sc ti ba from the wall zone through the intermediate zone to the core zone this is suggestive of a strong role of fractional crystallization in producing the compositional diversity in the internally zoned pegmatites alkali element ratios such k rb and k cs in muscovite exhibit near exponential decline from the wall to the core zone which is suggestive of rayleigh type fractional crystallization fractional crystallization modelling reveals that the formation of the wall zone requires 69 crystallization the intermediate zone 85 95 and the core zone ca 99 crystallization leading to extreme enrichment of rb and cs and other incompatible elements muscovites and tourmaline from the un zoned pegmatites display similar compositional trends as the internally zoned ones but with a significant compositional gap between the host granite and the pegmatite lithium isotopic composition of muscovites and b isotopic composition of tourmalines become progressively lighter from wall to core zone of the internally zoned pegmatite and from the granite to the pegmatite for the un zoned pegmatites this is suggestive of an important role of vapour exsolution in the formation of the pegmatites taken together the geochemical and isotopic trends in the pegmatites can be explained by rayleigh fractional crystallization operating in tandem with vapour exsolution

the extent of equilibration between melt and residuum during regional anatexis and its implications for differentiation of the continental crust a study of partially melted metapelitic enclaves thisstudy presentsthefirst precise evaluation of the extent ofchem ical equilibrium between felsic melt and crystalline residuumduring the crustal anatexis of metasediments the high precision ofthese resultsstemsfrom thefactthat in thiscase the meltsare repre sented by pristine glasses that occur in the matrices of partiallymelted metapelitic enclaves and as abundant glassy melt inclusionstrapped by different minerals in the same enclaves in previous stu dies of anatexis the composition of the melt has been approximatedby that of leucosomes in migmatites which do not necessarily repre sent the composition of the melt very well the present study isbased on laser ablation analyses of 35 trace elements in the glassand principal minerals of enclaves included within neogene peralu minous dacites exposed at el hoyazo se spain the enclaves which contain the assemblage plagioclase biotite sillimanite garnet glass ilmenite graphite k feldspar cordieritequartz represent fragments of metapelitic continental crust thatwaspartiallymeltedatpressuresof5 7kbarduringaregionalmeta morphic event matrix melt and melt inclusions within the enclaveswere quenched to glass upon eruption of the dacite the glasses inthe melt inclusions are interpreted as the remains of melt producedby an initial melting reaction involving a muscovite rich assemblageat 700 7508c whereasthe matrixglassesare consistent with con tinuedmeltingand oralatermeltdominatedbytheincipientmeltingof biotite at 800 8508c the distribution of trace elements be tween residuum and melt duringanatexis of the enclaves isdifferentfrom that predicted by disequilibrium models that consider effectivedistribution coefficients close to unity instead the bulk melt andthe major minerals plagioclase biotite alkali feldspar cordierite were close to equilibrium except for the case of garnet duringthe generation of the melt inclusions and matrix glasses thequenchedmelts meltinclusionsandparticularlythematrixglasses were not in equilibrium with accessory zircon and monazite andare depleted in zirconium and the light rare earth elements recrystallization of mineralsand diffusion in the melt were import ant controls during the redistribution of trace elements betweenthe solids and the bulk melt local disequilibrium processes alsotook place at mineral melt interfaces during the crystallizationof peritectic minerals and the recrystallization of residual phases as shown by the depletion of compatible elements in the meltinclusions

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

experimental silicate phosphate equilibria in peraluminous granitic magmas with a case study of the alburquerque batholith at tres arroyos badajoz spain with the actual mineralogy and whole rock p in the granite experimental results presented here establish the p2o5 content of aplite pegmatite facies at tres arroyos peraluminous silicic melt for three potential buffering reactions at 200 mpa h2o and t from 525 to 850 c 1 biotite sarcopside 2 spessartine sarcopside 3 petalite amblygonite montebrasite quartz the first two equilibria are moderately insensitive to t and buffer the p content of melt between 1 30 1 57 wt

melt generation and fluid flow in the thermal aureole of the bushveld complex granite sheets emplaced into the migmatite zone of the eastern contact aureole of the bushveld complex resulted from fluid enhanced incongruent biotite melting of the underlying silverton formation shales during prograde metamorphism ba concentrations are extreme in both the sheets 1000 ppm and the hornfels 800 ppm into which they have been emplaced we conclude that a ba rich hydrothermal fluid induced melting in the aureole and that fluid transport of ba2 and to a lesser extent sr2 and eu2 persisted in the melt zones under subsolidus conditions sr isotope systematics from high ba localities define an errorchron of 2161 106 ma with an initial 87sr 86sr ratio of 0 705 0 001 metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids there is no evidence of regional homogenization oxygen isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact suggesting the infiltrating fluid equilibrated isotopically with the metasediments their elevated 18o values 11 3 12 1 are consistent with a fluid source from devolatilization of the sedimentary lithologies textural analysis at both outcrop and thin section scale of arkose and psammites in the aureole lakenvalei and magaliesberg formations shows that the extent of melting was highly heterogeneous even on the grain scale and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt which preserve textures inherited from the regionally metamorphosed protolith simple finite difference thermal modelling of the aureole suggests that the width of the melt zone 500 m is inconsistent with conductive heat transfer and hence that the thermal structure has been modified by fluid advection

petrologic and geochemical links between the post collisional proterozoic harney peak leucogranite south dakota usa and its source rocks abstract the proterozoic terrane of the black hills south dakota includes the composite harney peak leucogranite and associated pegmatites that were emplaced into metamorphosed pelites and graywackes available dates indicate that granite generation post dated regional metamorphism and deformation that have been attributed to collision of the wyoming and superior cratons at 1760 ma previous radiogenic and stable isotope work indicates that the exposed metasedimentary rocks are equivalent to sources of the leucogranites in this study whole rock and mineral compositions of the metasedimentary rocks were used to calculate the likely average residue mineralogies and melt fractions that would be generated by muscovite dehydration melting of the rocks these were then used to model observed trace element compositions of the granites using published mineral melt distribution coefficients model trace element melt compositions using pelitic and graywacke protoliths yield similar results the models reproduce well the observed depletion of transition metals and ba in the granites relative to metasedimentary protoliths the depletion is due mainly to high proportion of biotite with variable amounts of k feldspar in the model residue sr is also depleted in the granites compared to source rocks but to a lesser relative extent than ba this is because of the low biotite melt distribution coefficient for sr and because high proportion of plagioclase in the residue is compensated by high sr concentrations in protoliths rubidium cs and ta behaved as slightly compatible to incompatible elements and therefore were not strongly fractionated during melting of the considered elements only b appears to have been highly incompatible relative to residue during melting the protoliths had sufficient b to allow tourmaline crystallization in those parts of the harney peak granite in which ti concentration was sufficiently low not to enhance crystallization of biotite the reproducibility of observed trace element concentrations in the harney peak granite by the models supports the often made proposition that metapelites and metagraywackes are common sources for leucogranites this argues against mass input from the mantle into metagraywacke and metapelitic crustal sources or melting of amphibolites to generate the post collisional harney peak and other similar peraluminous granite suites

estimating abundances of volatile and other mobile components in evolved silicic melts through mineral melt equilibria to variable degrees of accuracy from source partitioning between and earths residual minerals and anatectic melts through melt fractionation partitioning between igneous minerals and residual melts to the end stages of magma solidification where higher concentrations of normally trace elements may promote saturation in their crystalline phases the magmatic abundance of rare alkalis and alkaline earths

experimental simulations of anatexis and assimilation involving metapelite and granitic melt abstract assimilation of foreign material into a granitic magma body entails mixing between two end member components the initially solid assimilant and the original magma whose composition is mostly represented by a granitic liquid in this study we assess the interactions between a haplogranite liquid made to the composition of the minimum at 200 mpa h 2 o ab 38 23 or 28 72 qtz 33 04 crn 00 01 with a schist composed of muscovite and quartz with minor biotite and garnet and traces of ilmenite plagioclase apatite and graphite from 700 to 800 c at 200 mpa aqueous carbonic fluid the primary focus of this work is to assess the changes in compositions of granitic liquids that form in each reservoir as a result of the assimilation of schist by granitic magma before doing so we evaluate the experimental database for the compositions of granitic liquids in equilibrium with peraluminous mineral assemblages most experiments that entail partial melting of peraluminous mineral assemblages begin with finely milled powders this method yields melt pools glass too small for accurate analysis by electron beam methods as a result most published analyses of glasses from such experiments are more aluminous and less sodic than their actual compositions the analytical errors are large up to 50 relative to the mean value for some elements in addition the experimental melts are commonly more hydrous than reported and vapor saturated even in experiments that are cited as vapor or fluid absent experiments that combine sand sized mineral grains and powdered granitic glass yield the best approach to chemical equilibrium and facilitate accurate analysis of the glass glasses from such experiments define the compositions of liquids that may be derived either from the anatexis of aluminous metasediments or by the assimilation of aluminous rocks into a granitic magma whose composition is approximately represented by the minimum composition in the haplogranite system ab or qtz h 2 o new experiments designed to simulate the assimilation of metapelites by granitic melt juxtaposed a solid core of anhydrous granitic glass with a solid core of mica schist both as described above at comparable conditions melting within the mica schist alone is far less extensive than when coupled with the granitic liquid this is because field diffusion the long range and highly organized migration of h na and k throughout the entire volume of interconnected melt quickly brings the composition of silicate liquid to chemical equilibrium across the metapelite granite interface with respect to these components and in regard to the asi aluminum saturation index and the h 2 o content of the melt the most significant change entails the diffusion of na from granitic liquid into the metapelite which shifts the bulk composition of the metapelite toward the minimum in the granite system and in turn promotes a greater proportion of melting within the metapelite a weaker counter flow of k back into the haplogranite raises its k molar k na k and lessens the increase in asi molar al na k due to na loss the solubility of h 2 o in granitic liquid is positively correlated with the asi value hence changes in the asi of melt that follow from field diffusion of h na and k ions also strongly affect the a h 2 o within the coupled metapelite granite system

b and li in proterozoic metapelites from the black hills u s a implications for the origin of leucogranitic magmas the abundance and distribution of b and li in metasedimentary rocks and in the harney peak granite hpg in the black hills south dakota u s a elucidate the behavior of these two elements during 1 regional metamorphism that began at 1760 ma 2 subsequent contact metamorphism associated with emplacement of the hpg and associated pegmatites at 1715 ma and 3 anatexis leading to production of the granite there is no clear evidence for changes in b and li concentrations with progressive regional metamorphism of the metapelites from chlorite biotite grade up to staurolite grade rocks outside the pegmatite aureole that surround the hpg have average b contents of 80 ppm there is no correlation of b with other elements in the pelitic schists which indicates that none of the major phases is the dominant host for b boron content is mainly controlled by small amounts of randomly distributed tourmaline that was identified in alpha track maps of thin sections in the aureole of the granite b concentrations are depleted in many samples the depletion is attributed to consumption of tourmaline during interaction of the rocks with alkalic fluids the average li contents in metapelites far away from the hpg are also 80 ppm near the hpg li concentrations reach roughly 190 ppm indicating significant metasomatism by fluids that emanated from the granite and pegmatites the concentrations of b and li in the low grade metapelites are sufficient to explain their abundances in the hpg if the granite formed by partial melting of the metapelites and if there was total breakdown of tourmaline stabilization of tourmaline in leucogranites is not necessarily related to sources enriched in b but may be related to its competition for fe and mg with biotite

geochemical and isotopic constraints on the petrogenesis of early ordovician granodiorite and variscan two mica granites from the gouveia area central portugal abstract a biotite granodiorite and seven sn bearing two mica granites crop out in the gouveia area central portugal a shrimp u th pb zircon age from the granodiorite and monazite ages from four of the two mica granites show that they are of early ordovician 480 ma and permo carboniferous i e variscan 305 and 290 ma age respectively the variscan two mica granites are late and post d3 major and trace element variation in the granitic rocks and their biotite and muscovite indicate mainly individual fractionation trends the granitic rocks are mostly depleted in hree relative to lree the biotite granodiorite is probably derived from igneous lower crust as evidenced by low initial 87 sr 86 sr 0 7036 high e nd t 2 5 and moderate 18 o 8 8 the two mica granites are probably derived by partial melting of heterogeneous mid crustal metasediments mainly metapelite and some metagraywacke as evidenced by their high initial 87 sr 86 sr 0 7076 0 7174 18 o 10 7 13 4 and major element compositions however variation diagrams for major and trace elements from two of the muscovite biotite granites and their micas define fractionation trends rb sr whole rock analyses from the two granites are perfectly fitted to a single isochron and the rocks have sub parallel ree patterns the younger granite is derived from the older by fractional crystallization of quartz plagioclase biotite and ilmenite tested by modelling major and trace elements most of the sn bearing granites are derived from distinct magma batches they result from partial melting of a heterogeneous mid crustal metasediment they do not represent a crustal anomaly in tin fractional crystallization is responsible for the increase in the sn contents of the granites and their micas muscovite has a higher sn content than coexisting biotite and is the principal host mineral for sn in these rocks

crystal chemistry of al rich biotites coexisting with muscovites in peraluminous granites a comparison was made between single crystal structure refinements electron microprobe analyses and octahedral site populations of seven biotite crystals and data obtained previously for coexisting muscovite in peraluminous granites using the same methods both micas from several plutons of northern victoria land antarctica and sardinia italy show significant octahedral substitutions biotite has a composition characterized by relatively high 6 al content 0 32 6 al 0 59 apfu whereas muscovite is characterized by phengite like substitutions 0 12 6 mg fe ti mn 0 35 mean bond lengths and electron count data for six biotites 1 m space group c 2 m agreement factor 2 7 r obs 3 6 and a biotite 2 m 1 space group c 2 c r obs 2 8 show that al substitutes for divalent cations in the octahedral m2 site and that the fe and mg distribution is disordered the mean tetrahedral bond lengths determined for biotite 2 m 1 reflect al si disorder in coexisting muscovite 2 m 1 crystals small positive electron density residuals close to m1 site position as well as the increase in m2 mean atomic number is in agreement with the presence of a significant phengitic component reduction in biotite unit cell dimensions with the increase of al follows a pattern similar to that of associated muscovites and the octahedral site volumes of both micas are influenced by the al saturation index asi of the rock these results and the calculated partition coefficients between biotite and muscovite for elements in m sites are consistent with continuous reaction and re equilibration of biotite and muscovite during crystallization of peraluminous granitic melts

genesis of yellow manganese rich elbaite from the canary mining area lundazi zambia abstract the most important source of yellow gem elbaite is the canary mining area in the lundazi district of eastern zambia the tourmaline has been mined since 1983 from both pegmatite and eluvial alluvial deposits in colors typically ranging from yellow green to yellow to orange and brown much of the orange to brown material is heated to attain a golden or canary yellow color the elbaite is mn rich up to 9 18 wt mno documented in the literature and contains small amounts of ti and little or no fe the distinctive composition of this tourmaline is probably the result of the early crystallization of abundant schorl from an unusual b rich li poor pegmatite melt which depleted fe while conserving mn until the late stage crystallization of gem pockets the simple mineralogy of the pegmatite consists of feldspars quartz and tourmaline the lack of micas phosphates or li minerals and the presence of very little garnet allowed mn to fractionate to high levels during pegmatite crystallization the presence of abundant gem tourmaline in a li poor pegmatite is highly unusual

non pegmatitic deposits of beryllium mineralogy geology phase equilibria and origin non pegmatitic occurrences of be minerals constitute a diverse set of geologic environments of considerable mineralogical and petrological interest they currently provide the majority of the world s be ore and emeralds and they contain the greatest resource of these commodities of the approximately 100 be minerals known see chapter 1 by grew appendix a most occur in hydrothermal deposits or non pegmatitic igneous rocks where their distribution varies systematically with the setting and origin table 1 fig 1 view this table table 1 beryllium minerals in non pegmatitic deposits formulas use and occurrence 1 figure 1 chemography of the principal solid phases in the beo al2o3 sio2 h2o f2o 1 bash system with the projected positions of helvite group and alkali be silicates also shown are generalized fields for some of the major types natural of occurrences cf table 1 fig 4 see text for discussion beryllium minerals are best known from geologic systems associated with felsic magmatism they also occur in a variety of settings that lack evident igneous associations environments range from the surface to the deep crust and host rocks range from feldspathic to carbonate to ultramafic in composition genetically related igneous rocks are felsic and share low calcium and high f contents but are diverse in composition setting and origin compositions range from strongly peraluminous to peralkaline and can be silica undersaturated beryllium minerals also occur in metamorphic and basinal environments and are redistributed by surface processes table 2 summarizes the types and significance of major groups of occurrences by their lithologic associations figure 2 shows the global distribution of some important examples and regional belts for most types at least one example has been described in some detail and can be used to help evaluate general patterns however

partitioning of fluorine and chlorine between biotite and granitic melt experimental calibration at 200 mpa h 2 o experiments from 640 to 680 c 200 mpa h2o at fo2 nno employing a natural f rich vitrophyric rhyolite from spor mountain utah assessed the effect of variable mg 100mg mg mn fe on the partitioning of fluorine and chlorine between biotite bt and melt over this temperature interval bt fluorite quartz is the sole liquidus phase partition coefficients for fluorine between biotite and glass dfbt melt show a strong dependence on the mg of bt dfbt melt varies from 1 5 to 7 2 over the range of mg from 21 to 76 a strong linear correlation between dfbt melt and mg has a slope of 9 4 and extrapolates through the origin i e dfbt melt 0 at mg 0 an annite siderophyllite solid solution in these experiments dclbt melt values 1 to 6 in the same experiments vary inversely with mg the al content of biotite does not vary with the aluminum saturation index asi molar al2o3 alkali and alkaline earth oxides of melt but two exchange mechanisms involving al appear to operate in these micas 1 alvi aliv siiv mgiv and mgiv 2aliv 2siiv iv the effects of other components such as li or other intensive parameters including fo2 have yet to be evaluated systematically at comparable mg of bt however the spor mountain rhyolite yields higher dfbt melt values than an li rich strongly peraluminous melt previously investigated the results indicate that the mg of bt exerts the principal control on halogen partitioning with asi and t as second order variables the experimental partition coefficients compare well with other experimental results but not with most volcanic rocks magmatic bt from most rhyolites records higher dfbt melt due to reequilibration with degassed h2o depleted magma and perhaps with f2o 1 exchange that may accompany oxidation fe3 o fe2 oh 1 this behavior is evident in magmatic biotite from a zoned peraluminous rhyolite complex near morococala bolivia bt is sharply zoned with f rich rims but bt core melt inclusion pairs fall on our experimental curve for dfbt melt these experimental data can be used in part to assess the preservation of magmatic volatile contents in plutonic or volcanic silicic rocks for plutonic rocks the actual f content of melt not a relative activity ratio involving hf species can be reasonably estimated if the mica has not undergone subsolidus reequilibration this information is potentially useful for some shallow level ca poor magmas that are thought to be rich in f e g a and s type granites but do not conserve f well as rocks

mixing of magmatic hydrothermal and metamorphic fluids and the origin of peribatholitic sn vein type deposits in rwanda abstract the fluid sources of granite related sn quartz vein deposits are commonly obscured by fluid mixing or fluid rock interactions as a result fluid inclusions minerals and isotopes in these veins indicate an intermediate composition between magmatic and metamorphic but the degree of mixing between these endmembers is currently unquantified this study presents a novel quantitative approach to assess the degree of mixing between magmatic hydrothermal and external metamorphic fluids in the formation of peribatholitic sn quartz veins in particular fluid mixing in the sn mineralized rwamagana musha ntunga pegmatite quartz vein field in east rwanda has been evaluated by the following four methods quartz stable isotopes muscovite geochemistry fluid inclusion microthermometry and la icp ms and geochemical modelling the quartz stable isotope data 18o 13 1 to 15 8 v smow d 27 6 to 59 7 v smow cannot uniquely differentiate between a metamorphic fluid origin or an initial magmatic hydrothermal fluid origin with subsequent metamorphic fluid mixing or host rock interaction however granitophile element concentrations in magmatic muscovite from pegmatites and hydrothermal muscovite from associated sn quartz veins are equally high indicating a close genetic link rb 530 8740 ppm li 110 1990 ppm sn 87 810 ppm cs 62 420 ppm primary h2o co2 n2 nacl medium saline magmatic fluid inclusions in quartz of pegmatites 12 7 wt nacleq and h2o co2 n2 nacl low saline fluid inclusions in barren metamorphic quartz veins 4 9 wt nacleq were analyzed by la icp ms these results show an enrichment in li rb sn and cs for the magmatic fluid while the metamorphic fluid is characterized by low granitophile element concentrations and high sr and ba contents the expected rb cs and rb sn signature of the sn quartz vein muscovite was modelled using the measured fluid endmember compositions confirming mixing between magmatic and metamorphic fluids in the formation of the veins the quantification suggests that the hydrothermal sn quartz vein fluid contains 5 80 of an external metamorphic fluid component

the genesis of lct type granitic pegmatites as illustrated by lithium isotopes in micas abstract isotopic compositions in the monts d ambazac pegmatite field french massif central exhibit a narrow range of mica 7li values ranging from 3 6 to 3 4 the value obtained in biotite from the host saint sylvestre granite falls within this range 7li 1 5 lithium concentrations are consistent with the degree of magmatic evolution of each pegmatite type from 630 ppm in type ii up to 13 500 ppm in the more evolved type vi pegmatite although the rare element contents e g li cs and ta of the micas are consistent with pegmatite differentiation 7li are firstly independent of the degree of magmatic differentiation independent of pegmatite type and secondly independent of the content of li and other flux elements such as be and cs muscovite sampled in pegmatite v from the chabannes locality is the only pegmatite to exhibit a 7li variation from an intermediate unit 1 7 to an internal pegmatitic unit 3 4 the nature of this 7li variation suggests that there was extensive fractional crystallization during the pegmatite s consolidation the independence of 7li evolution from the degree of magmatic evolution and the presence of distinct major rare element bearing phases throughout the pegmatite field tend to confirm that the 7li values recorded in mica are inherited from crustal source rocks common to the granite and pegmatite forming melts we propose that the distinct pegmatite subtypes beryl columbite vs lepidolite petalite subtypes observed throughout the monts d ambazac pegmatite field reflect the diverse contributions of crustal protoliths the lack of evidence of surrounding alteration combined with the absence of increasing li content within the host granite tends to confirm that the 7li values obtained within this pegmatite field are primary and that no li diffusional process and or mixing driven li isotope fractionation has overprinted these isotopic compositions in light of these results the process of partial melting of protoliths enriched in rare element bearing phases e g mica and garnet seems to be more responsible for li isotope fractionation than li diffusion or fractional crystallization at the temperature of pegmatite consolidation finally we discuss the use of li isotopic compositions to identify the most highly evolved pegmatitic systems

unravelling the complex interaction between mantle and crustal magmas encoded in the lavas of san vincenzo tuscany italy part ii geochemical overview and modelling abstract this work reports a geochemical overview and modelling of the lavas erupted 4 4 ma ago at san vincenzo tuscan magmatic province tmp although these lavas cover a relatively small area 10 km2 they show very large geochemical variations caused by the interaction of mantle derived and crustal anatectic magmas the lavas consist of peraluminous rhyolites 87sr 86sr i up to 0 726 hosting primarily variably sized magmatic enclaves with shoshonite latite compositions 87sr 86sr i down to 0 708 new whole rock data for a large shoshonite enclave show high concentrations of lree lile and tetravalent hfse coupled with pentavalent hfse depletions and enrichments in compatible elements such as cr and co the chondrite normalised ree pattern is strongly fractionated and characterised by a negative eu anomaly eu eu 0 79 hybridisation and afc models suggest that the shoshonite enclave is the result of 12 rhyolite contamination of a mantle derived magma akin to the potassic trachybasalt shoshonite lavas of capraia island 4 6 ma tmp following an 18 5 assimilation of late triassic metasediments 13 evaporite and 5 5 carbonate and 56 fractionation of clinopyroxene 39 plagioclase 10 and biotite 7 each rhyolite sample is characterised by mineral scale isotopic disequilibria e g 87sr 86sr i 0 711 0 726 glass inclusions with large k2o na2o variations 1 1 3 4 and a poli thermobarometric history of crustal melt production at eutectic conditions a multi parametric approach accounting for k2o na2o 1 3 2 2 87sr 86sr i 0 713 0 725 sr 104 311 ppm and rb 294 403 ppm whole rock variations allowed us to divide the anatectic a rhyolites into five groups a1 a2 1 a2 2 a2 3 a3 group a1 shows the highest 87sr 86sr i ratios and the lowest values of sr k2o na2o and rb it is related to a2 1 and a3 rhyolites by positive k2o na2o rb and k2o na2o feo correlations these three rhyolite groups crop out in the south of san vincenzo and are interpreted to derive from mixing and ascent of peraluminous magma batches locally extracted at progressively higher temperatures from the heterogeneous paleozoic metamorphic basement the a2 3 rhyolites show the highest rb concentrations up to 403 ppm and inverse sr rb and k2o feo correlations suggesting reactions involving the incongruent melting of a biotite bearing crustal source which was previously depleted in low temperature melting components such as muscovite and albite most of the a2 3 rhyolites crop out in the northern svvc area and appear spatially and geochemically related to the a2 2 rhyolites by similarly low 87sr 86sr i ratios however a2 2 rhyolites show lower rb contents 296 316 ppm and geochemical correlations e g 87sr 86sr i rb sr rb mgo feo clearly indicating hybridisation mixing with the shoshonite mantle derived magma rb 109 122 ppm finally we propose a four stage model for the magma plumbing system of san vincenzo produced by the uprising of a mantle derived magma in a normally faulted and doubled crustal basement characterised by two sections of paleozoic siliciclastic rocks separated by late triassic evaporite carbonate sequences

geochronology and source of the rare metal pegmatite in the mufushan area of the jiangnan orogenic belt a case study of the giant renli nb ta deposit in hunan china abstract the jiangnan orogenic belt hosts many rare metal deposits that exhibit close ore forming ages and similar geological backgrounds and is an important li be nb ta concentration belt located in eastern china however the metallogenic mechanism and regularity of these rare metal deposits have not yet been reported in detail because of the lack of accurate geochronological data and systematic studies on the sources of metallogenic materials the giant renli rare metal deposit located at the south margin of the mufushan batholith in the central jiangnan orogenic belt is one of the few high grade granitic pegmatite type nb ta deposits in china columbite tantalite u pb dating has been developed as a more robust method when compared with the traditional dating methods to constrain the timing of rare metal mineralization six columbite grains obtained from the microcline albite pegmatite of the renli deposit exhibit a concordant laser ablation inductively coupled plasma mass spectrometry la icp ms u pb age of 133 0 2 6 ma which denotes the nb ta mineralization age the zircon grains from the columbite bearing microcline albite pegmatite subjected to different degrees of alteration by the hydrothermal fluids yield a concordant la icp ms u pb age of 131 2 2 4 ma which can be considered to be the age of newly magmatic hydrothermal growth and recrystallization under the transitional magmatic hydrothermal conditions observed in case of the renli deposit furthermore the zircon grains from the host biotite monzogranite yield concordant la icp ms u pb ages of 140 7 0 7 and 140 3 0 7 ma whereas the zircon grains from the host two mica monzogranite yield a concordant la icp ms u pb age of 138 3 0 3 ma denoting the intrusion timing of two types of host mufushan granites of the pegmatite dykes finally the zircon grains from two schist samples of the lengjiaxi group yield concordant la icp ms u pb ages of 825 9 5 6 and 834 4 5 3 ma constraining the maximum age for the termination of deposition these ages constitute a clear intrusion cooling hydrothermal time series with respect to the renli deposit zircon can retain the original magmatic isotopic composition and is extensively used to trace the petrogenetic processes because it is quickly crystallized from magma the mufushan granite and rare metal pegmatite exhibit consistent negative ehf t values mean ehf t values of 7 2 and 7 3 respectively and their two stage model ages exhibit similar distribution patterns when compared with that exhibited by the lengjiaxi group furthermore the mufushan granite shows the typical geochemical characteristics of the crustal derived granite thus the rare metal pegmatite and host granite of the renli deposit were concluded to be sourced from the partial melting of the lengjiaxi group metasedimentary strata the compressive torsional nature of the gajiang fault zone widespread occurrence of the s type granite and absence of basaltic rocks suggest that the central to eastern part of the jiangnan orogenic belt was in a subduction setting during the early cretaceous thus this economically important early cretaceous ca 133 129 ma rare metal metallogenic belt was formed owing to the development of the jiangnan orogenic belt and the thick lengjiaxi group metasedimentary strata provided the metallogenic material source

cloud creek structure central wyoming usa impact origin confirmed the circular cloud creek structure in central wyoming usa is buried beneath 1200 m of mesozoic sedimentary rocks and has a current diameter of 7 km the morphology morphometry of the structure as defined by borehole seismic and gravity data is similar to that of other buried terrestrial complex impact structures in sedimentary target rocks e g red wing creek in north dakota usa the structure has a fault bordered central peak with minimum diameter of 1 4 km composed predominantly of paleozoic carbonates thickened by thrust faulting and brecciation and is elevated some 520 m above equivalent strata beyond the outer rim of the structure there is a 1 6 km wide annular trough sloping away from the central peak maximum structural relief 300 m and terminated by a detached fault bounded rim anticline the youngest rocks within the structure are late triassic norian clastics and these are overlain unconformably by post impact middle jurassic bathonian sandstones and shales thus the formation of the cloud creek structure is dated chronostratigraphicly as 190 20 ma reported here for the first time are measurements of planar deformation features pdfs in shocked quartz grains in thin sections made from drill cuttings recovered in a borehole drilled at the southern perimeter of the central peak other less definitive microstructures consistent with impact occur in samples collected from boreholes drilled into the central peak and rim anticline the shock metamorphic evidence confirms an impact origin for the cloud creek structure

a granite gabbro complex from madagascar constraints on melting of the lower crust the ranomandry complex is a neoproterozoic nested intrusion from central madagascar composed of a gabbroic core within a coeval peraluminous granite ring intruding pelitic metasediments although xenocryst entrainment and magma mixing have both contributed to marginal phases of the granite the primary melt is characterised by steep lree hree ratios and negligible or slightly positive eu anomalies both isotopic and trace element systematics preclude an origin from either partial melting of the metapelitic country rock or from fractional crystallisation of the gabbroic magma however trace element modelling suggests an origin from the dehydration melting of a plagioclase poor garnet bearing metagreywacke at temperatures of 850 900 c and at lower crustal pressures 10 kbar melting of an enriched subcontinental mantle generated gabbroic magmas that caused advective heating and anatexis at the base of thickened continental crust during their ascent transport of the resulting granite magma was facilitated by the pre existing plumbing system that overcame thermal and mechanical problems associated with both diapirism and self propagating dykes as mechanisms for long distance transport of granite magmas the nested geometry of the intrusions is an indication of a structurally homogeneous lower crust that contains no pre existing shear zones or fault systems

primary crustal melt compositions insights into the controls mechanisms and timing of generation from kinetics experiments and melt inclusions abstract we explore the controls mechanisms and timing of generation of primary melts and their compositions and show that the novel studies of melt inclusions in migmatites can provide important insights into the processes of crustal anatexis of a particular rock partial melting in the source region of granites is dependent on five main processes i supply of heat ii mineral melt interface reactions associated with the detachment and supply of mineral components to the melt iii diffusion in the melt iv diffusion in minerals and v recrystallization of minerals as the kinetics of these several processes vary over several orders of magnitude it is essential to evaluate in nature which of these processes control the rate of melting the composition of melts and the extent to which residue melt chemical equilibrium is attained under different circumstances to shed light on these issues we combine data from experimental and melt inclusion studies first data from an extensive experimental program on the kinetics of melting of crustal protoliths and diffusion in granite melt are used to set up the necessary framework that describes how primary melt compositions are established during crustal anatexis then we use this reference frame and compare compositional trends from experiments with the composition of melt inclusions analyzed in particular migmatites we show that for the case of el hoyazo anatectic enclaves in lavas the composition of glassy melt inclusions provides important information on the nature and mechanisms of anatexis during the prograde suprasolidus history of these rocks including melting temperatures and reactions and extent of melt interconnection melt homogenization and melt residue equilibrium compositional trends in several of the rehomogenized melt inclusions in garnet from migmatites granulites in anatectic terranes are consistent with diffusion in melt controlled melting though trace element compositions of melt inclusions and coexisting minerals are necessary to provide further clues on the nature of anatexis in these particular rocks

the role of discontinuous magma inputs in felsic magma and ore generation for a long time granites have been considered as passive bodies ascending under intrinsic negative density and viscosity contrasts with their host rocks chemical variations within a granitic body resulted from in situ differentiation and crystal fractionation since the mid 1980s this global view has been significantly modified by i shifting melting from water saturated conditions to fluid absent reactions ii increasing the role played by the mantle during granite generation iii reassessing the rheology of partially molten rocks iv demonstrating stepwise segregation and ascent of magmas by analogue and numerical models v combining structural geophysical and geochemical studies to reveal the internal structures in granitic plutons it results that a granitic body is built up by a discontinuous accumulation of successive magma intrusions the discontinuous nature of magma emplacement has also significant consequences for its ability to generate ore the processes that lead to ore deposits are examined with a brief review of the magmatic and fluid phases that concentrate ore forming elements examples are taken from crustal derived granites and porphyry type deposits those are considered as the two end members of magmatic and hydrothermal ore deposits the source characteristics of the magma the emplacement mechanisms and magma mixing processes are the frame that controls the potential to carry base metals with the magma the distribution of elements is controlled by diffusion partition between minerals and melt solubility and redox conditions variations of those parameters are examined by considering their activation energy which controls the exponential dependence with temperature a characteristic length depending on the activation energy temperature variation and time is estimated for a characteristic time lag of 30 ka the intrusion of a magma into a magma chamber of similar composition hence temperature has few effects on diffusion partition coefficient and redox conditions because of a too low temperature contrast the intrusion of a mafic magma into a felsic one induces a variation of 300 c in both magmas the characteristic length of diffusion may vary by up to two orders of magnitude whereas the variation of partition coefficients is only one order of magnitude the redox conditions are about 2 5 log unit in the mafic magma but they can vary by 7 log units in the felsic magma hence a strong decrease in d values is observed in porphyry type deposits the effect is a removal of the elements with higher activation energy w sn zr from the mafic to the felsic magma deformation during the late stages of emplacement also controls ore formation

natural and experimental fluorine substitution in biotite implications for fluid rock thermochronometry and application to the serido belt northeastern brazil abstract we modeled non steady state fluid assisted diffusion in a sheet plane infinite cylinder and finite thin cylinder using analytical solutions of the diffusion equation and the experimental diffusivity of f in biotite diffusion experiments on natural biotite in the presence of hydrofluoric acid at 0 4 gpa and 650 700 and 750 c produced diffusive influx perpendicular to the c axis epma mapping and profiling allow us to define the following arrhenius expression of f diffusion in biotite d 7 04 m 2 s 10 4 exp 221 kj mol rt 1 the results yield the discrimination of a high t homogenization domain where cooling paths continuously reequilibrate f in biotite an intermediate t zonation domain where cooling paths record f in biotite diffusion profiles and a low t closure domain where f in biotite remains unchanged the zonation domain significantly expands with increasing cooling rates and decreasing diffusion time we systematically analyzed f in biotite with electron microprobe in samples collected along representative cross sections within the serido belt northeastern brazil its metasedimentary units reached upper amphibolite grade metamorphic conditions with constant f in biotite f 0 33 wt in grains from mica schist and paragneiss the ti in biotite geothermometer yielded nearly constant temperatures around 623 c the f in biotite regional background values remain unchanged at the contact with the major ediacaran f rich acari pluton it indicates the absence of magmatic fluid influx into the host rocks and is evidence for a fluid absent nature of melts by contrast the f in biotite of the metric mica schist enclaves within dykes and sills of cambrian pegmatitic granites was reequilibrated by interaction with exsolved fluids the ti in biotite geothermometer indicates temperatures around 642 c and our model suggests 100 kyr as a minimal duration for fluid rock interaction the preservation of intra grain f in biotite homogeneity requires fluid flux cessation before cooling or fluid present cooling rates below 100 c myr 1 higher cooling rates would generate f in biotite intra grain zonation at the contact with the pegmatitic granites the metasedimentary rocks show meter scale f in biotite gradients due to thermal profiles setting the fluid fh2o fhf gradient the preservation of such gradients is related to a relatively fast fluid flux episode compared to the overall duration of the elevated thermal profile the absence of intra grain zonation indicates that f in biotite re equilibration was fast relative to that of the fluid flux duration we found intra grain f in biotite zonation within an orthogneiss sample from the paleoproterozoic caico complex basement nearby the metasedimentary belt and intruded plutons the fit of the natural profile by modeled profiles for cooling rates below 10 c myr 1 obtained from available serido belt u pb and 40ar 39ar ages indicates that the orthogneiss cooled from 475 c it suggests the existence of a thermal gradient as the inner part of the belt cooled from 623 c

alkali metal and rare earth element evolution of rock forming minerals from the gatumba area pegmatites rwanda quantitative assessment of crystal melt fractionation in the regional zonation of pegmatite groups abstract this study presents a general model for the evaluation of rayleigh fractional crystallisation as the principal differentiation mechanism in the formation of regionally zoned common and rare element pegmatites the magmatic evolution of these systems from a granitic source is reconstructed by means of alkali element and rare earth element ree analyses of rock forming minerals feldspars micas and tourmaline which represent a whole sequence of regional pegmatite zonation the gatumba pegmatite field rwanda central africa is chosen as case study area because of its well developed regional zonation sequence the pegmatites are spatially and temporally related to peraluminous g4 granites 986 10 ma the regional zonation is developed around a g4 granite and the proximal pegmatites grade outwardly into biotite two mica and muscovite pegmatites rare element nb ta sn pegmatites occur most distal from the granite alkali metal fractionation trends in pegmatitic k feldspar rb 350 6000 ppm cs 2 160 ppm and muscovite rb 670 7000 ppm cs 10 150 ppm define a single and continuous trend which is modelled by rayleigh fractional crystallisation starting from a parental granite composition g4 composition k 3 1 wt rb 222 ppm cs 11 ppm the fractionation model shows moreover that the pegmatites adjacent to the parental pluton are the least fractionated and the distal pegmatites are the most fractionated biotite pegmatites form from 0 to 69 crystallisation two mica pegmatites from 69 to 92 and muscovite pegmatites from 92 crystallisation onwards the extreme rb and cs enrichment in rare element pegmatites requires at least 98 fractionation of the initial g4 granite composition mathematical derivation of the k rb versus cs relationship in k feldspars confirms rayleigh fractional crystallisation as the main differentiation process in the development of regional pegmatite zonation moreover 1 it demonstrates the continuity of the fractionation process from biotite pegmatites to rare element pegmatite and indicates a genetic link among them and 2 it allows a general evaluation of pegmatite fields in terms of system parameters i e the initial element concentration in the granitic melt and partition coefficients ree patterns of the rock forming minerals feldspars muscovite biotite and schorl show a distinct transition along the regional zonation they evolve from sloping fractionated to more flat patterns starting from the biotite up to the muscovite pegmatites minerals from the rare element pegmatites feldspars muscovite and elbaite show again more fractionated heavy ree depleted patterns the observed evolution in the ree patterns corresponds to early crystallisation of light ree enriched monazite and a late crystallisation of mainly cogenetic heavy ree enriched phases such as apatite columbite group minerals and beryl modelling of rayleigh fractionation starting from an initial parental granite g4 composition lan ybn 10 and ree 83 shows that this evolution in mineral ree pattern is the result of fractional crystallisation of the pegmatitic melt and precipitation of ree incorporating minerals such as monazite and apatite consequently trace element modelling indicates that rayleigh fractional crystallisation governs the mineralogical and geochemical evolution from a granite source to common and eventually rare element pegmatites this mechanism shows that granitic pegmatites are extremely fractionated residual melts which are genetically and directly connected to a main granite body

experimental silicate mineral melt partition coefficients for beryllium and the crustal be cycle from migmatite to pegmatite partition coefficients dbemineral melt for beryllium between hydrous granitic melt and alkali feldspars plagioclase feldspars quartz dark mica and white mica were determined by experiment at 200 mpa h2o as a function of temperature 650 900 c activity of be in melt trace levels to beryl saturation bulk composition and thermal run direction at trace levels be is compatible in plagioclase of an31 1 84 at 700 c and muscovite 1 35 at 700 c but incompatible in biotite 0 39 0 54 from 650 800 c alkali feldspar 0 38 0 19 from 680 850 c quartz 0 24 at 800 c and albite 0 10 at 750 c the partition coefficients are different at saturation of the melt in beryl lower in the case of plagioclase of an31 0 89 at 700 c muscovite 0 87 at 700 c biotite 0 18 0 08 from 675 800 c alkali feldspar 0 18 0 14 from 680 700 c and quartz 0 17 0 08 from 750 800 c but higher in the case of albite 0 37 at 750 c with other data sources these new partition coefficients were utilized to track first the distribution of be between aluminous quartzofeldspathic source rocks and their anatectic melts and second the dispersion or concentration of be in melt through igneous crystal fractionation of different magma types e g s type i type up to beryl saturated granitic pegmatites and finally into their hydrothermal aureoles among the rock forming minerals cordierite calcic oligoclase and muscovite in this order control the fate of be because of the compatibility of be in these phases in general beryl bearing pegmatites can arise only after extended crystal fractionation of large magma batches to f fraction of melt remaining 0 05 granitic magmas that originate from cordierite bearing protoliths or that contain large modal quantities of calcic oligoclase will not achieve beryl saturation at any point in their evolution

beryllium in silicic magmas and the origin of beryl bearing pegmatites this chapter addresses the portion of the geochemical cycle of beryllium that entails its transfer through the continental crust by the generation and migration of silicic magmas which will be referred to loosely as granitic but will include all of the textural variants of rocks derived from the granitic bulk compositions within the granite system we will examine the distribution of be between anatectic melts and residual crystals and we will track the accumulation of be in melt via crystal fractionation culminating in the saturation of pegmatite forming melts in the mineral beryl beryl is unquestionably the most common manifestation of be enrichment and the most abundant of the phases that contain essential beryllium see cern this volume to explain the presence of beryl in pegmatites we pose four questions 1 how much be does it take to saturate a granitic pegmatite forming melt in beryl 2 how much be is likely to be incorporated in silicic partial melts at their source 3 consequently how much fractionation of a typical granitic magma must occur to achieve beryl saturation in pegmatitic derivatives or is beryl saturation possible without extended magmatic fractionation 4 and should all granitic magmas be expected to achieve beryl saturation eventually or are there special attributes of the source material or crystalline phases in the magma that predetermine the likelihood of forming beryl in pegmatitic differentiates to address these questions we start with a review of beryl saturation in pegmatites we then examine the likely abundance of be derived by anatexis at the sources of granitic magmas with the two ends of the system bracketed we can then subject be fractionation to conventional modeling methods to ascertain how much fractionation must take place to bring granitic melts to beryl saturation as a review this chapter relies as much as possible on readily available publications

phase relations of a f enriched peraluminous granite an experimental study of the kymi topaz granite stock southern finland phase relations of f rich kymi equigranular topaz granite have been investigated experimentally at 100 500 mpa as a function of water activity and f content fluorite and topaz can crystallize as liquidus phases in f rich peraluminous systems but the f content of the melt should exceed 2 5 3 0 wt for the crystallization of topaz in peraluminous f bearing melts containing more than 1 wt f topaz and muscovite are expected to be the first f bearing phases to crystallize at high pressure whereas fluorite and topaz should crystallize first at low pressure following reaction models the saturation of fluorite and topaz caal2si2o8 plagioclase 2 alf3 melt caf2 fluorite 2al2sio4f2 topaz the obtained partition coefficient for f between biotite and glass d f bt glass ranges from 1 89 to 0 80 average value 1 29 and can be used as an empirical fluormeter to determine the f content of coexisting melts combined petrological and experimental studies of the kymi equigranular topaz granite indicate that plagioclase was the liquidus phase at nearly water saturated fluid saturated conditions and that the f content of the melt was at least 2 wt the mean f content of natural biotite 3 92 wt suggests that the late stage crystallization of biotite occurred in melts containing about 3 wt f the early crystallization of biotite and the presence of muscovite in our experiments at 200 mpa contrasts with the late stage crystallization of biotite and the absence of muscovite in the natural assemblage indicating that crystallization pressure may have been lower than 200 mpa for the granite

lithium isotope fractionation during fluid exsolution implications for li mineralization of the bailongshan pegmatites in the west kunlun nw tibet abstract granitic pegmatites are considered significant host rocks for the rare metal deposits however the enrichment mechanisms for the rare metal elements e g li be nb and ta remain debated here we report the whole rock and mineral e g muscovite and spodumene major trace elements and li isotopic compositions of the bailongshan granitic pegmatites in the west kunlun nw tibet to address this issue the bailongshan pegmatites hosting a super large li rb polymetallic deposit are composed of li poor and li rich pegmatites the li poor pegmatites display high 7li values in the whole rock 2 30 4 94 and muscovite 2 22 7 55 in contrast the li rich pegmatites show low 7li values in the whole rock 1 89 to 0 35 muscovite 3 39 to 4 49 and spodumene 2 83 to 1 87 these results together with the variations of the elements suggest that the li poor and li rich pegmatites were produced by the melt fluid separation during the late stage of granitic magma evolution this means that the li poor pegmatites were formed in a h2o poor silicate rich melt system while the li rich pegmatites were generated in a h2o rich silicate poor melt supercritical fluid system the melt fluid separation can lead to significant li isotope fractionation with 7li enriched in the strongly bonded residual silicate melt and 6li preferring to weaker hydrated bonds in the fluid alternatively the li rich pegmatites enriched in 6li could have been induced by preferential accumulation of the faster diffusing 6li in the supercritical or near critical fluid the li rich pegmatites experienced extensive crystal fluid interaction in a relatively closed magmatic hydrothermal system which resulted in the leaching and reprecipitation of ore forming elements but produced restricted li isotope fractionation the exsolved supercritical fluid is more enriched in alkali fluxing components li rb cs p and f by the continuous interaction with the residual silicate melt therefore the supercritical fluid exsolution plays an important role in the li isotope fractionation and rare metal mineralization

primary melt inclusions in andalusite from anatectic graphitic metapelites implications for the position of the al2sio5 triple point anatectic crustal xenoliths in the miocene volcanic rocks of mazarron southeast spain contain andalusite with melt inclu sions an unprecedented finding microstructures indicate that the melt inclusions were trapped during andalusite growth the vola tile content of the peraluminous inclusions is too low cl 0 4 wt f 0 3 wt p 1500 ppm b 850 ppm to have caused a significant reduction of the wet solidus temperature moreover the presence of graphite as observed during partial melting is expected to have raised the temperature of the wet solidus melting temperatures for the inclusions obtained from quartz albite or thoclase haplogranite system 680 790 8c zr 620 705 8c and light rare earth element 615 725 8c thermometry indicate that the stability of andalusite 1 melt is incompatible with the position of the most commonly used andalusite 5 sillimanite equilibrium and that the al2sio5 triple point must be placed at higher tem peratures and pressures

lead contents of s type granites and their petrogenetic significance an evaluation of pb and ba contents in s type granites can provide important information on the processes of crustal partial melting primary low t s type granites which form mainly by fluid absent muscovite melting may acquire a significant enrichment in pb when compared to higher t s type granites for a given ba content we consider the following factors are responsible for this enrichment muscovite is a major carrier of pb in amphibolite facies metapelites and thus large quantities of pb can be liberated upon its breakdown the typical restite assemblage of qz bt sil pl grt kfsp that forms during low t fluid absent muscovite melting can take up only minor amounts of this pb this is because the crystal melt pb distribution coefficients for these restite minerals are low to very low only k feldspar is moderately compatible for pb with a crystal melt distribution coefficient of 3 but its modal content in restites is usually low at the same time the restite assemblage will retain much ba owing to the very high ba uptake in both biotite and k feldspar which is an order of magnitude higher than for pb thus during a low t anatectic event involving a low degree of crustal melting pb as an incompatible element can become strongly enriched in the partial melt relative to ba and also relative to source rock values in the case of higher t anatexis and larger partial melt amounts the pb becomes less enriched and the ba less depleted or even enriched relative to source rock values during fractional crystallization of a s type granite magma ba behaves strongly compatibly and pb weakly compatibly the concentrations of both elements decrease along the liquid line of decent owing to this sympathetic fractionation behavior the primary source related pb ba fingerprint with weak or strong pb enrichment remains in evolved s type granites this facilitates a distinction between primary low t s type granites which are related to muscovite melting and secondary low t s type granites that evolve through fractional crystallization from a higher t parental magma we show in this paper that a simple logarithmic pb versus ba diagram can be a valuable aid for interpreting the petrogenesis of s type granite suites

petrogenesis of late mesozoic high ba sr granites in the qiushuwan cumo orefield implications for the distribution of porphyry mo mineralization in the east qinling area of central china abstract the qinling dabie orogenic belt of china is well known for its large mo resources there have been many studies of the genesis of the mo deposits but the reasons for the intensive mineralization during the late mesozoic and the spatially uneven distribution of the resulting ore deposits are not fully understood deposits with reserves of 6 mt of mo occur predominantly in the southern margin of the north china craton ncc and are mostly late mesozoic porphyry deposits in comparison a few relatively insignificant and cu dominated porphyry ore deposits have been identified in the north and south qinling terranes this study presents new whole rock geochemical isotopic and zircon u pb age data for ore bearing granites associated with the qiushuwan cu mo deposit the largest porphyry deposit in the north qinling nq terrane which provide insights into the asymmetric distribution of mo mineralization in this region the rocks display high k calc alkaline to shoshonitic affinities and are metaluminous to weakly peraluminous with high ba and sr contents low rb nb and ta contents and depletion in heavy rare earth elements and y which are features typical of high ba sr granites zircon u pb dating indicates that the granites crystallized at ca 145 ma their narrow ranges of whole rock initial 87sr 86sr ratios 0 7056 0 7069 and end t 7 4 to 9 7 zircon ehf t 2 49 to 5 13 and 18o 5 58 to 6 49 values together with the presence of significant nd hf isotopic decoupling hf 7 61 to 11 3 indicate that the parent magma was derived from partial melting of enriched subcontinental lithospheric mantle with minor assimilation of the lower crust in contrast late mesozoic granitic rocks in the southern margin of the ncc including both mineralized and barren intrusions are geochemically associated with the crust of the northern margin of the yangtze block yb the yb contains mo rich shales that are thought to have been subducted beneath the southern margin of the ncc during the triassic melting of these mo rich black shales would have mobilized mo a process that enabled the formation of the extensive mo mineralization associated with the late mesozoic granites of the southern margin of the ncc this indicates that the extensive mo mineralization within the southern margin of the ncc is genetically related to a mo rich source that was generated by the stagnation of subducted yb continental crust beneath the southern margin of the ncc in comparison the less significant mo mineralization within the east qinling orogenic belt might reflect the variable removal of the subducted yb continental slab during post collisional intracontinental orogenesis

ra partitioning between phlogopite and silicate melt and 226ra ba 230th ba isochrons in this study we experimentally determine phlogopite melt partition coefficients of ra and other trace elements in a lamproitic system this work was achieved using an analytical technique la icp ms with low detection limits 0 01 fg permitting the measurement of the very low ra concentrations feasible in experiments 1 ppb d ra phlogopite melt was determined to 2 28 0 44 and 2 84 0 47 in two experiments the ratio d ra d ba is around 1 6 the compatibility of ra in phlogopite results from an ionic radius being close to the apex of the lattice strain parabola for earth alkalis in the large xii coordinated interlayer site of phlogopite a re evaluation of d ra and d ra d ba for magmatic minerals containing appreciable ra yields d ra mineral melt ranging from 2 6 for phlogopite down to 2 3 10 5 for pyroxenes and d ra d ba mineral melt from 4 for leucite to 2 10 2 for orthopyroxene the influence of melt composition on d ra d ba is less than 10 all investigated minerals have different d ra d ba strongly fractionating ra from ba thus for magmatic systems 226 ra ba in the various minerals is not constant these minerals do not form a straight line in the 226 ra ba 230 th ba system at the time of crystallization and thus there is no 226 ra ba 230 th ba isochron at t 0 226ra 230th ba mineral dating is thus applicable only to model ages calculated from mineral glass pairs with known d ra

petrogenesis of migmatites in maine usa possible source of peraluminous leucogranite in plutons high t low p facies series conditions during deformation within a devonian crustal scale shear zone system defined by kilometer scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain at upper amphibolite facies grade metapelites are partially melted the onset of which is introduction