differentiation and crystallization conditions of basalts from the kerguelen large igneous province an experimental study phase relations of basalts from the kerguelen large igneous province have been investigated experimentally to understand the effect of temperature fo2 and fugacity of volatiles e g h2o and co2 on the differentiation path of lip basalts the starting rock samples were a tholeiitic basalt from the northern kerguelen plateau odp leg 183 site 1140 and mildly alkalic basalt evolved from the kerguelen archipelago mt crozier on the courbet peninsula representing different differentiation stages of basalts related to the kerguelen mantle plume the influence of temperature water and oxygen fugacity on phase stability and composition was investigated at 500 mpa and all experiments were fluid saturated crystallization experiments were performed at temperatures between 900 and 1 160 c under oxidizing log fo2 qfm 4 and reducing conditions log fo2 qfm in an internally heated gas pressure vessel equipped with a rapid quench device and a pt membrane for monitoring the fh2 in all experiments a significant influence of the fo2 on the composition and stability of the mg fe bearing mineral phases could be observed under reducing conditions the residual melts follow a tholeiitic differentiation trend in contrast melts have high mg mg2 mg2 fe2 and follow a calk alkalic differentiation trend at oxidizing conditions the comparison of the natural phenocryst assemblages with the experimental products allows us to constrain the differentiation and pre eruptive conditions of these magmas the pre eruptive temperature of the alkalic basalt was about 950 1 050 c the water content of the melt was below 2 5 wt h2o and strongly oxidizing conditions log fo2 qfm 2 were prevailing in the magma chamber prior to eruption the temperature of the tholeiitic melt was above 1 060 c with a water content below 2 wt h2o and a log fo2 qfm 1 early fractionation of clinopyroxene is a crucial step resulting in the generation of silica poor and alkali rich residual melts e g alkali basalt the enrichment of alkalis in residual melts is enhanced at high fo2 and low ah2o

flow directions in dikes from anisotropy of magnetic susceptibility data the bootstrap way one of the first applications of anisotropy of magnetic susceptibility ams was an attempt to determine flow directions from mafic dikes khan 1962 since the seminal work of knight and walker 1988 defining the expected behavior of ams in response to magma flow there has been increasing interest in using ams for this purpose here we present a quantitative method for interpretation of ams data from dikes using a parametric bootstrap first dikes must be sampled with at least five and preferrably more samples from within 10 cm of the dike margin the distributions of the eigenvalues and eigenvectors of the ams tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets we generate paradata sets by first selecting a sample at random then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site the bounds containing 95 of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest classification of dikes proceeds as follows sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable in addition sites with principal eigenvectors with angles 45 away from the dike margin inverse or with markedly different directions on either side of the dike scissored are excluded the remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors if neither side has principal eigenvectors distinct from the dike plane the dikes are classified as having lineation information only a study comprising 251 dikes from the troodos ophiolite has 151 sites with directional data 38 sites with lineations only 7 inverse sites 5 scissored sites and 55 sites not fitting into any other category the flow directions interpreted from the data were generally southerly toward a fossil transform zone

new time constraints on dyke swarms related to the parana etendeka magmatic province and subsequent south atlantic opening southeastern brazil abstract 40 ar 39 ar incremental heating analyses were carried out on tholeiitic dyke swarms of santa catalina florianopolis and rio de janeiro and alkaline dykes from the proximity of rio de janeiro city southeastern brazil most of the plagioclases from the tholeiitic dykes display disturbed age spectra with younger apparent ages most often clearly correlated with alteration phases however a calculated weighted mean age from the less disturbed released fractions of the ne oriented santa catarina dyke swarm gives an age of 129 4 0 3 ma which is contemporaneous with the ponta grossa dyke swarm pgds and at least the northern parana continental flood basalts cfb the rio de janeiro tholeiitic dykes display age spectra affected by alteration and probably excess argon but with low temperature mini plateau ages between 130 6 and 129 8 ma which are concordant with previous data obtained on the pgds and rio de janeiro dyke swarms by renne et al 1996a and turner et al 1994 these new geochronological data when added to the previous ones show that the initiation of gondwana s breakup in the region was characterized by a volcano tectonic system with a geometry more complicated than the rift rift rift system suggested by many authors the different rift arms of this structural feature were operating during a brief time period mainly from 131 5 to 129 ma and were contemporaneous at least with the northern part of the parana etendeka province pep single grain and bulk sample biotite from the alkaline mainly coast parallel dyke swarm near rio de janeiro city display plateau ages around 80 ma 81 8 1 8 and 78 8 2 8 ma and 70 ma 72 5 0 5 and 69 7 0 2 ma these magmatic intrusions refer to late cretaceous flexuring and vertical movements and rift basin formations almeida 1976 due the northward progressing rifting motions separating south america and africa during south atlantic ocean formation

experimental evidence for a protracted enrichment of tungsten in evolving granitic melts implications for scheelite mineralization the solubility of scheelite in evolved granitic magmas qz ab or an system with minor feototal tio2 and cao added was studied experimentally at 200 mpa 750 850 c and relatively oxidizing condition logfo2 nno 2 3 where nno is ni nio oxygen buffer water saturated granitic melts have been equilibrated with seeds of scheelite crystals the resulted wo3 contents in the melts vary only slightly from 0 21 to 0 32 wt wo3 over the investigated temperature and compositional range 0 7 to 1 4 wt cao but tends to increase with increasing temperature and decreasing cao concentration one important message conveyed from the study is that wo3 concentrations at scheelite saturation are more dependent on temperature in evolved ca poor melts than in ca rich melts natural granitic rocks associated with scheelite mineralization and associated melt inclusions hosted in quartz have much lower w contents than the experimental melts equilibrated with scheelite this implies that enrichment of tungsten w at magmatic stages is not sufficient to produce significant scheelite mineralization and confirms the important role of w mobilization by magmatic hydrothermal fluids in the formation of scheelite deposits

biotite stability in peraluminous granitic melts compositional dependence and application to the generation of two mica granites in the south bohemian batholith bohemian massif czech republic abstract crystallization experiments have been conducted in the system na2o k2o mgo feo al2o3 sio2 h2o with 4 normative corundum in order to constrain the stability of biotite as a function of water activity and the mg of biotite mg mg fetotal in equilibrium with peraluminous granitic melts the temperature at which biotite breakdown starts is strongly dependent on the mg of biotite at 500 mpa the temperature of biotite breakdown to form orthopyroxene increases from 750 c to 830 c as the mg of biotite increases from 0 4 to 0 5 considering that the system investigated is relevant for ca poor peraluminous biotite bearing rocks metapelites the biotite dehydration curves obtained are used to discuss the melting reactions and the temperatures that lead to the formation of two distinct types of two mica granites found in the south bohemian batholith specifically the eisgarn and destna granites the phase relationships were determined experimentally for the composition of these two granites in order to constrain the composition of the biotite in equilibrium with the melt in the protoliths we demonstrate that eisgarn granitic melts may have been generated at temperatures in the range 830 850 c from melting reactions involving biotite with a mg up to 0 5 as a reactant in contrast destna granitic melts cannot have been generated from dehydration melting reactions involving biotite

nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america the mountain ranges of arizona and adjacent california and nevada contain large areas underlain by proterozoic anorogenic granites comprising the southwesternmost portion of a transcontinental belt of 1 4 1 5 ga old anorogenic complexes that extends across north america northeast into labrador of these a two mica monazite bearing granitic suite resides in central and southeastern arizona as part of a peraluminous subprovince that is bordered on the south southern arizona to sonora and west western arizona and adjacent portions of california and nevada by marginally metaluminous granites bearing biotite sphene hornblende and fluorite all of these 1 4 ga granites are distinctly more potassic iron enriched relative to mg and depleted in ca mg and sr in contrast to typical orogenic granitoids in general the large ion lithophile element enriched composition is a consequence of limited melting of a water deficient crustal source at depths greater than 25 37 km for the peraluminous granites this contrast is less extreme perhaps resulting from a larger degree of melting as a consequence of a greater metasedimentary component and water in its crustal source the anorogenic granitic magmas intruded into the upper crust at depths of 8 17 km or shallower at temperatures up to 790 c the most dramatic variation in the crystallization intensive parameters resides in the oxygen fugacity which spans three orders of magnitude relative to other anorogenic suites all of the magmas crystallized at elevated levels of o2 as reflected in their assignment to the anorogenic magnetite series yet a regionally significant rise in primary o2 levels unmatched elsewhere in the transcontinental belt occurs for plutons in western arizona including the holy moses and hualapai granites the most extreme case is the hualapai granite whose biotite fe fe mg ratios drop due to high o2 to a low of 0 27 down from more typical levels of 0 54 to 0 75 such extreme variations in primary levels of oxygen fugacity must be an indirect imprint of regional changes of the level of oxidation of the lower crust the high fo2 holy moses and hualapai plutons have intruded near the regional boundary between the metaluminous and peraluminous granites and appear to be imaging a major change in the level of oxidation of the lower crust this boundary is also approximately equivalent to significant changes in the nd and pb isotopic compositions of these granites and the metamorphic and magmatic character of the older orogenic terrane on a global scale the crust forming orogenies ended by 1 6 ga ago and the continents entered a long lived era dominated by localized extension and transcontinental intrusion of anorogenic potassic rapakivi granite mafic dike swarms charnockite and anorthosite the absence of orogenic deformation implies that plate consumption became intraoceanic during this time the profuse and widespread nature of the igneous activity has no phanerozoic analogue and is considered to be unique to the proterozoic a crustal overturn model ties the magmatism to heating within a largely undepleted subcontinental mantle the eventual rise of mantle plumes and the transfer of heat into the youthful undifferentiated proterozoic crust subsequent melting and rise of potassic granitic magmas from the lower crust leads to considerable crustal reorganization a process that would continue until both the mantle and crust reached a stable configuration

high temperature low h2o silicic magmas of the yellowstone hotspot an experimental study of rhyolite from the bruneau jarbidge eruptive center central snake river plain usa the phase relations have been investigated experimentally at 200 and 500 mpa as a function of water activity for one of the least evolved indian batt rhyolite and of a more evolved rhyolite composition cougar point tuff xv from the 12 8 8 1 ma bruneau jarbidge eruptive center of the yellowstone hotspot particular priority was given to accurate determination of the water content of the quenched glasses using infrared spectroscopic techniques comparison of the composition of natural and experimentally synthesized phases confirms that high temperatures 900 c and extremely low melt water contents 1 5 wt h o are required to reproduce the natural mineral assemblages in melts containing 0 5 1 5 wt h o the liquidus phase is clinopyroxene excluding fe ti oxides which are strongly dependent on fo and the liquidus temperature of the more evolved cougar point tuff sample bjr 940 1000 c is at least 30 c lower than that of the indian batt rhyolite lava sample ibr2 970 1030 c for the composition bjr the comparison of the compositions of the natural and experimental glasses indicates a pre eruptive temperature of at least 900 c the composition of clinopyroxene and pigeonite pairs can be reproduced only for water contents below 1 5 wt h o at 900 c or lower water contents if the temperature is higher for the composition ibr2 a minimum temperature of 920 c is necessary to reproduce the main phases at 200 and 500 mpa at 200 mpa the pre eruptive water content of the melt is constrained in the range 0 7 1 3 wt at 950 c and 0 3 1 0 wt at 1000 c at 500 mpa the pre eruptive temperatures are slightly higher by 30 50 c for the same ranges of water concentration the experimental results are used to explore possible proxies to constrain the depth of magma storage the crystallization sequence of tectosilicates is strongly dependent on pressure between 200 and 500 mpa in addition the normative qtz ab or contents of glasses quenched from melts coexisting with quartz sanidine and plagioclase depend on pressure and melt water content assuming that the normative qtz and ab or content of such melts is mainly dependent on pressure and water activity respectively the combination of results from the phase equilibria and from the composition of glasses indicates that the depth of magma storage for the ibr2 and bjr compositions may be in the range 300 400 mpa 13 km and 200 300 mpa 10 km respectively

characterization and origin of aluminous a type granites from the lachlan fold belt southeastern australia the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are a distinctive group of igneous rocks on the basis of chemical and mineralogical criteria those granites that contain 65 72 sio2 can be distinguished from other types on the basis of higher abundances of fetotal fetotal mg high field strength elements trivalent rare earth elements ga and zn mineralogically they contain fe rich hydrous mafic minerals and primary ilmenite and hence are reduced relative to the ni nio buffer however the extremely felsic a type granites sio2 72 have the same chemical and mineralogical characteristics as felsic fractionated i type granites recent analyses indicate that the lachlan fold belt a type granites have sc f alkali element trace transition element and h2o contents similar to those of other unfractionated i type granites rb sr and nd sm isotopic compositions are highly variable probably reflecting source region heterogeneity the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are distinct from peralkaline rocks in terms of chemical composition petrography and field associations although these rocks have been grouped together as a single type in current classification schemes we propose that the metaluminous to weakly peraluminous a type granites such as those of the lachlan fold belt should be defined as aluminous a type granites and should not be grouped with peralkaline granites the lachlan fold belt aluminous a type granites have relatively high calculated zircon saturation temperatures we suggest that these granites were produced by high temperature partial melting of a felsic infracrustal source

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

magnetic studies of magma supply and sea floor metamorphism troodos ophiolite dikes abstract dikes of the eastern troodos ophiolite of cyprus intruded at slow ocean spreading axes with dips ranging up to 15 from vertical and with bimodal strikes now ne sw and n s due to post 88 ma sinistral microplate rotation varied dike orientations may represent local stress fields during dike crack propagation but do not influence the spatial distributions or orientation distributions of dikes magnetic fabrics nor of their palaeomagnetic signals anisotropy of magnetic susceptibility ams integrates mineral orientation distributions from each of 1289 specimens sampled from dikes at 356 sites over 400 km2 in the eastern troodos ophiolite of cyprus in 90 of dikes ams fabrics define a foliation kmax kint parallel to dike walls and a lineation kmax that varies regionally and systematically magma flow alignment of accessory magnetite controls the ams with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea floor metamorphism titanomagnetite has less influence on anisotropy occasionally intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility kmax still defines the magmatic flow axis such blended subfabrics of kinematically compatible mafic silicate and misaligned multidomain magnetite subfabrics are rare areas of steep magma flow kmax plunge 70 and of shallow magma flow alternate in a systematic and gradual spatial pattern foci of steep flow were spaced 4 km parallel to the spreading axes and 6 km perpendicular to the spreading axes ridge parallel separation of steep flow suggest the spacing of magma feeders to the dikes whereas ridge perpendicular spacing of 6 km at a spreading rate of 50 mm a implies the magma sources may have been active for 240 ka the magma feeders feeding dikes may have been 2 km in diameter stable paleomagnetic vectors in some cases verified by reversal tests are retained by magnetite and titanomagnetite in all specimens the stable components were isolated by three cycles of low temperature demagnetization ltd followed by 10 steps of incremental thermal demagnetization td 47 of primary a components 338 2 57 2 n 207 95 3 9 mean tub 397 8 c are overprinted by a b component 341 4 63 5 n 96 95 8 7 mean tub 182 11 c a and b components are ubiquitous and shared equally by the n s and ne sw striking dikes a component unblocking temperatures tub are zoned subparallel to the fossil spreading axis their spatial pattern is consistent with chemical remagnetization at some certain off axis distance determined by sea floor spreading a components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge axis thus their magnetizations are younger than those of the overlying volcanic sequence for which chrms are commonly reported as 274 33 88 ma

magnetic fabrics of the cretaceous dike swarms from sao paulo coastline se brazil its relationship with south atlantic ocean opening abstract magnetic fabric and rock magnetism studies were performed on 91 dikes from cretaceous diabase and lamprophyre dike swarms that outcrop side by side on the beaches of ne sao paulo state coastline the dikes crosscut archean and proterozoic poly metamorphic rocks of the costeiro complex their thicknesses range from a few centimeters to 2 m for lamprophyre and up to about 10 m for the diabase they trend predominantly n30 60 e with vertical dip magnetic fabrics were determined using anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm rock magnetism measurements reveal that magnetite grains in the range of 2 5 m are the magnetic mineral of both swarms for most dikes these grains are the carriers of bulk magnetic susceptibility but surprisingly are not responsible for the ams which is carried by fe bearing minerals as shown by aarm the main ams fabric recognized in the swarms is due to magma flow in which the kmax kint plane is parallel to the dike s plane and the magnetic foliation pole kmin is perpendicular to it the analysis of the kmax inclination showed that the dikes were fed by horizontal to vertical flows however for the majority of the dikes the ams and aarm tensors are not coaxial the aarm lineation aarmmax is oriented n30 60w approximately perpendicular to ams lineation kmax suggesting that magnetite grains were rotated approximately 90 anticlockwise from the dike plane the aarmmax orientation is similar to the direction of a fault system mainly in the santos marginal basin which was formed in the cretaceous rifting during the south atlantic opening therefore the aarm fabric is tectonic in origin and the comparison of ams and aarm fabrics suggests that lamprophyre and diabase dikes were emplaced in three distinct events in the earliest stages of the south atlantic opening

differentiation of tholeiitic basalt to a type granite in the sept iles layered intrusion canada the undeformed 564 ma sept iles layered intrusion quebec canada is a large igneous body of c 20 000km3 from the base to the top it consists of a layered series dominated by troctolite and gabbro an anorthositic upper border series and a dominantly granitic upper series the parent magma of the layered series is inferred to be an iron rich tholeiitic basalt 48 wt sio2 15wt feot whole rock compositions from the chilled margin dykes cross cutting the layered series and silicic rocks from the upper series display continuous major and trace element geochemical trends ranging from basalts to ferroan metaluminous a type granites 77 wt sio2 initial 143nd 144nd 0 51201 0 51207 and 87sr 86sr 0 70353 0 70548 indicate a juvenile mantle source and minimal contamination by old crust 1 2 during crystallization geochemical modeling using the melts thermodynamic calculator combined with equations predicting mineral melt equilibria from experiments on tholeiitic basalts indicate that basaltic to monzonitic melt compositions are in equilibrium with the troctolites and gabbros of the layered series fe ti oxides saturate early in the layered series after 14 fractionation of plagioclase olivine cumulates further fractionation of fe ti oxide bearing gabbros drives the residual liquids toward silica enrichment and iron depletion major and trace element modeling indicates that the a type granites from the upper series were produced by protracted fractional crystallization of an iron rich basaltic parent magma at a fraction of residual liquid of only 8 the observed relative volumes of mafic cumulates and silicic rocks in the intrusion are in agreement with the calculations most of the intermediate compositions correspond to magmatic mafic enclave bearing granitoids and display geochemical evidence of hybridization intermediate compositions produced by fractional crystallization are scarce and a daly gap occurs from 57 to 67 wt sio2 this gap could result either from the fractional crystallization process or from silicate liquid immiscibility during that compositional interval

methane origin and oxygen fugacity evolution of the baogutu reduced porphyry cu deposit in the west junggar terrain china most porphyry copper deposits worldwide contain magnetite hematite and anhydrite in equilibrium with hypogene copper iron sulfides chalcopyrite bornite and have fluid inclusions with co2 ch4 that are indicative of high fo2 in contrast the baogutu porphyry cu deposit in the west junggar terrain xinjiang china lacks hematite and anhydrite contains abundant pyrrhotite and ilmenite in equilibrium with copper iron sulfides chalcopyrite and has fluid inclusions with ch4 co2 that are indicative of low fo2 the mineralized intrusive phases at baogutu include the main stage diorite stock and minor late stage diorite porphyry dikes the main stage stock underwent fractional crystallization and country rock assimilation contamination and consists of dominant diorite and minor gabbro and tonalite porphyry the country rocks contain organic carbons 0 21 0 79 wt the 13cvpdb values of the whole rocks 23 1 to 25 8 in the wall rocks suggest a sedimentary organic carbon source the 13cvpdb values of ch4 28 2 to 36 0 and co2 6 8 to 20 0 in fluid inclusions require an organic source of external carbon and equilibration of their 13cco2 ch4 values 8 2 25 0 at elevated temperatures 294 830 c suggesting a significant contribution of thermogenic ch4 mineral composition data on the main stage intrusions such as clinopyroxene hornblende biotite magnetite ilmenite sphene apatite and pyrrhotite suggest that the primary magma at baogutu was oxidized and became reduced after emplacement by contamination with country rocks mineral compositions and fluid inclusion gas compositions suggest that the redox state of the system evolved from logfo2 fmq 1 in the magma stage to logfo2 fmq in the hydrothermal stage though oxidized magma was emplaced initially assimilation contamination of carbonaceous country rocks decreased its fo2 such that exsolved fluids contained abundant ch4 and deposited a reduced assemblage of minerals

water in granite and pegmatite forming melts article i nfo in this paper we have compiled water concentration data obtained from melt inclusion in minerals mostly quartz and topaz from granites and pegmatites a small number of data come from analysis of natural glasses about 4600 melt inclusions are from granites and about 2100 melt inclusions are from pegmatites another 4159 values are from melt inclusions and glasses of rocks with more than 66 g g sio2 are added belonging to a recent compilation the total of 10 859 melt inclusions provides a statistically significant basis from which to derive some conclusions generally the water concentration in granite melt inclusion is b10 g g with three maxima at 4 0 1 3 5 9 1 2 and 8 1 1 1 g g water for pegmatites a larger range of water contents was found from 2 to more than 50 g g water with two distinct maxima at 10 5 8 1 and 26 2 14 3 g g we show that the great spread and the generally high water content of the pegmatite forming melts as sampled by melt inclusions are the result of melt melt immiscibility pro cesses connected with the formation of conjugate melt fraction with contrasting physical and chemical char acteristics which have also a major influence on the element partitioning between such fractions in this paper we also discuss viscosity as an important transport property of natural silicate melts and we show that due to high primary water concentration in the pegmatite forming melts their viscosity is very low over a wide range of temperatures in the case of water rich peralkaline melt fractions the viscosity can decrease still further together these suggest that liquidus undercooling as the major cause of pegmatite textures is redundant furthermore we demonstrate that the effect of boron carbonates and bicarbonates on pegmatite melts these two last mostly ignored in the past have a large influence on the melt structure and they enable pegmatite formation in the absence of the usual discussed fluxing elements such as f and p

late carboniferous high end t ehf t granitoids enclaves and dikes in western junggar nw china ridge subduction related magmatism and crustal growth abstract we report results of petrologic geochronological and geochemical investigation of the late carboniferous diorites granodiorites amphibole am bearing granites and associated dioritic and monzonitic enclaves and mafic and granitic dikes in the keramay area of the western junggar region of central asian orogenic belt caob zircon u pb dating suggests that they were generated in the late carboniferous 316 304 ma the diorite and granodiorite compositions extend over a wide range of sio2 53 70 wt sr 240 602 ppm and mg 41 58 values and are characterized by moderately fractionated rare earth element ree patterns nb ta depletion and relatively low y and yb contents the mafic dikes consist of dolerites diorite porphyries and minor granodiorite porphyries and have variable sio2 51 59 wt and high mg cr and ni values with the exception of two samples with relatively high heavy ree hree contents the mafic dikes exhibit trace element characteristics similar to diorites and granodiorites the am bearing granites and a granite porphyry dike sample have high levels of sio2 73 77 wt hrees e g yb 3 46 15 7 ppm and low mg cr and ni contents along with clearly negative eu ba and sr anomalies similar to typical a type granites all granitoids enclaves and dikes in this region have high positive end t 7 13 to 9 74 and zircon ehf t 10 to 16 values and moderate initial 87sr 87sr ratios 0 7004 0 7049 mineral composition data suggest that the parental magmas for mafic dikes are similar to cenozoic sanukitoids in the setouchi arc area japan and were possibly generated under water rich and high oxygen fugacity nno 1 5 to nno 2 7 conditions they most likely originated from partial melting of a mantle source variably modified by subducted oceanic crust derived melts and minor fluids and subsequently underwent fractional crystallization the diorites and granodiorites were possibly generated by magma mixing between enriched lithospheric mantle and juvenile lower crust derived magmas coupled with minor crystal fractionation the am bearing granites and granite porphyry dike were produced by partial melting of juvenile crustal materials at shallow crustal levels taking into account widespread contemporaneous magmatism including morb type basalts and slab derived adakites in western junggar we suggest that the keramay intrusive rocks were generated in a special arc setting related to ridge subduction and resultant slab window which played an important role in the crustal growth of the caob

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

mineralogy geochemistry and petrology of neoarchean ferroan to magnesian granites of carajas province amazonian craton the origin of hydrated granites associated with charnockites abstract 2 75 to 2 73 ga old granitic intrusions associated with coeval charnockitic rocks were emplaced in the northern domains of the carajas province the vila jussara suite was recognized recently and its geologic mineralogical and geochemical characteristics are presented in this paper and compared with the data available in the literature on the estrela complex serra do rabo and igarape gelado granites and planalto suite monzogranites and syenogranites are dominant in most units but in the igarape gelado granite and vila jussara suite granodiorites and tonalites are also relatively abundant the main mafic phases in all these granitoids are amphibole potassian hastingsite with subordinate magnesiohastingsite 0 95 fe fe mg 0 47 and biotite 0 88 fe fe mg 0 52 with rare occurrence of clinopyroxene relict crystals magmatic epidote of pistacite component from 26 to 31 occurs only in the vila jussara suite the rocks of estrela serra do rabo and planalto units contain only ilmenite generally with coronas of titanite or ilmenite associated with magnetite and are essentially reduced ferroan granites whole rock feot feot mgo 0 89 that evolved at low f o 2 fmq 0 5 the igarape gelado granite needs additional studies but it is formed apparently by reduced ferroan and oxidized ferroan granites the vila jussara suite is also composed of reduced ferroan granites that are similar to those of the other granite units however it has additionally oxidized ferroan and magnesian granitoids which contain magnetite ilmenite as the main fe ti oxide phase and evolved respectively at moderate f o 2 nno 0 5 and at comparatively higher f o 2 nno to nno 1 the magmas of these granites were formed at temperatures 900 c and derived from a heterogeneous dominantly reduced and tholeiitic granulitic crust locally varying to oxidized and calc alkaline in composition plagioclase orthopyroxene clinopyroxene and fe ti oxides were the main phases in the melt residues and the liquids were enriched in ba zr y and ree except for eu but not in sr compared to their sources the magmas should have relatively high water content 4 wt the magmatic evolution was strongly influenced by partial melting processes the different varieties of the vila jussara suite are not linked by fractional crystallization and derived from distinct magmas the neoarchean granites of the carajas province were emplaced in a collisional tectonic setting they are syntectonic granites that were deformed and sometimes intensely recrystallized during their emplacement and subsequent subsolidus cooling the magmas were generated in the crust at pressures of 800 to 1000 mpa and emplaced at pressures of 200 to 500 mpa the studied granitoids are similar geochemically to the ferroan and magnesian groups of charnockites the magnesian variety of the vila jussara suite is similar to the pyroxene free granitoids of the neoarchean matok pluton of the collisional limpopo belt the neoarchean granites of carajas are hydrated granites emplaced at comparatively higher crustal levels when compared with the matok pluton

phase equilibrium constraints on pre eruptive conditions of recent felsic explosive volcanism at pantelleria island italy we present experimental phase equilibria carried out on a pantelleritic bulk rock composition with a peralkalinity index pi molar na2o k2o al2o3 1 4 which is representative of the most energetic pumice fall eruption of the resurgent post caldera volcanism on pantelleria island for the explored conditions p 25 150 mpa t 680 800 c h2omelt up to 6 wt fo2 nno where nno is nickel nickel oxide buffer clinopyroxene is the liquidus phase followed by alkali feldspar and then quartz the crystallization of amphibole is limited to temperatures below 700 c aenigmatite crystallizes near the liquidus for p 100 mpa when clinopyroxene is the sole liquidus phase the coexisting melt is characterized by a decrease in peralkalinity index to 1 2 if compared with the starting material approaching comendite composition in contrast when alkali feldspar joins the liquidus phase assemblage residual melts become extremely peralkaline pi 3 2 with feo contents in excess of 14 wt comparison of our experiments with natural phase abundances and compositions constrains the depth of the fastuca pantellerite reservoir to be around 5 1 km or p 120 20 mpa at t 730 10 c with h2omelt around 4 wt and fo2 1 5 log unit below nno such conditions correspond to the dominant phase assemblage of feldspar clinopyroxene and aenigmatite amphibole occurrence requires both higher melt water contents and lower temperatures up to 6 5 wt at 680 c and may reflect provenance from a deeper and wetter portion of the tapped reservoir in contrast quartz is more probably related to crystallization during magma ascent which explains the lower melt water content of its melt inclusions overall the results confirm that pantellerites may be both water rich and reduced magmas these characteristics imply that any coexisting fluid phase in the reservoir will be dominated by water and hydrogen species the latter amounting to a few mole per cent magma unrest at pantelleria may thus be heralded by emissions of hydrous and reduced gases i e co2 poor and h2 rich if the present day reservoir is felsic in composition

phase equilibria of the lyngdal granodiorite norway implications for the origin of metaluminous ferroan granitoids the proterozoic 950 ma lyngdal granodiorite of southern norway belongs to a series of hornblende biotite metaluminous ferroan granitoids hbg suite coeval with the post collisional rogaland anorthosite mangerite charnockite amc suite this granitoid massif shares many geochemical characteristics with rapakivi granitoids yet granodiorites dominate over granites to constrain both crystallization p t fo2 h 2o in melt and magma generation conditions we performed crystallization experiments on two samples of the lyngdal granodiorite with 60 and 65 wt sio2 at 4 2 kbar mainly at fo2 of nno nickel nickel oxide to nno 1 and under fluid saturated conditions with various h2o co2 ratios for each temperature comparison between experimental phase equilibria and the mineral assemblage in the lyngdal granodiorite indicates that it crystallized between 4 and 2 kbar from a magma with 5 6 wt h2 oa t an fo2 of nno to nno 1 these oxidized and wet conditions sharply contrast with the dry and reduced conditions inferred for the petrogenesis of the amc suite and many other rapakivi granites worldwide the high liquidus temperature and h2o content of the lyngdal granodiorite imply that it is not a primary magma produced by the partial melting of the crust but is derived by the fractionation of a mafic magma lyngdal type magmas appear to have volcanic equivalents in the geological record in particular our results show that oxidized high silica rhyolites such as the bishop tuff could be derived via fractionation of oxidized intermediate magmas and do not necessarily represent primary crustal melts this study underlines the great variability of crystallization conditions from anhydrous to hydrous and reduced to oxidized and petrogenetic processes among the metaluminous ferroan magmas of intermediate compositions granodiorites quartz mangerites quartz latites suggesting that there is not a single model to explain these rocks

magnetic and plagioclase linear fabric discrepancy in dykes a new way to define the flow vector using magnetic foliation in basaltic dykes the magnetic lineation k1 maximum magnetic susceptibility axis is generally taken to indicate the flow direction during solidification of the magma this assumption was tested in tertiary basaltic dykes from greenland displaying independent evidence of subhorizontal flow the digital processing of microphotographs from thin sections cut in k1 k2 planes yields the preferred linear orientation of plagioclase which apparently marks the magma flow lineation in up to 60 of cases the angular separation between k1 and the assumed flow direction is greater than 45 this suggests that the uncorroborated use of magnetic lineations in dykes is risky a simple geometrical method is proposed to infer the flow vector from ams in dykes based solely on magnetic foliations

petrogenesis and geodynamic significance of the 850 ma dongling a type granites in south china abstract the ongoing controversy on the timing of amalgamation of the yangtze and cathaysia blocks either prior to 890 ma or later than 830 ma impedes a proper understanding of geological evolution of the south china block and its role in the breakup of rodinia supercontinent in this study we report 850 ma la icp ms zircon u pb ages and whole rock geochemistry of the dongling granites from the southeastern yangtze block that did not receive much geoscientific attention as compared to the better studied 830 760 ma sedimentary and igneous rocks in the south china block the studied dongling granites have high k2o na2o 7 44 9 09 wt and low mgo 0 07 0 54 wt and cao 0 16 1 21 wt contents their high feot feot mgo 0 82 0 94 and ga al 2 6 values and hfse abundance zr nb ce y 443 965 ppm allow their grouping as a type granite in the chondrite normalized diagrams these granites display right inclined ree patterns and strong to moderate negative eu anomalies eu eu 0 32 0 70 the dongling granites have variable and negative whole rock end t 14 6 to 8 6 and zircon ehf t 12 8 to 4 4 with paleoproterozoic two stage nd 2 20 2 69 ga and hf model ages 2 01 2 41 ga zircon saturation parameters tzr suggest 850 c temperature and other geochemical characteristics reveal low melting pressure 350 to

fractionation vs magma mixing in the wangrah suite a type granites lachlan fold belt australia experimental constraints abstract the wangrah suite granites lachlan fold belt australia reflect different stages of differentiation in the magmatic history of an a type plutonic suite in this study we use experimentally determined phase equilibria of four natural a type granitic compositions of the wangrah suite to constrain phases and phase compositions involved in fractionation processes each composition represents a distinct granite intrusion in the wangrah suite the intrusions are the danswell creek dcg wangrah wg eastwood eg and dunskeig granite dg ordered from most mafic to most felsic by increasing sio2 and decreasing feototal experimental investigation show that the initial water content in melts from dcg is between 2 3 wt h2o if the dcg is viewed as the parental magma for the wangrah suite then 1 fractionation of magnetite orthopyroxene and plagioclase 20 wt of the dcg composition leads to compositions similar to that of the eg 2 further fractionation of plagioclase quartz k feldspar and biotite 40 wt from the eg composition leads to the dg composition these fractionation steps can occur nearly isobarically and are confirmed by bulk rock ba sr rb and zr concentrations in contrast the generation of the most abundant wg composition cannot be explained by fractional crystallisation from the dcg at isobaric conditions because of the high k2o content of this granite magma mixing could be the process to explain the chemical distinctiveness of the wangrah granite from all the other granites of the wangrah suite

geochronology mineralogy and geochemistry of alkali feldspar granite and albite granite association from the changyi area of jiao liao ji belt implications for paleoproterozoic rifting of eastern north china craton abstract paleoproterozoic granites divided into alkali feldspar granite and albite granite were newly discovered in the changyi area of the jiao liao ji belt from the eastern block of the north china craton la icp ms zircon u pb dating shows that the alkali feldspar granite was emplaced at 2193 ma whereas the albite granite intruded at 2171 ma these granites have high sio2 na2o k2o zr nb ga y and hree contents and show remarkably low cao eu ba and sr concentrations strongly suggesting their a type affinities zircon saturation thermometry and alt in biotite geobarometer indicate that the alkali feldspar granite crystallized at temperatures of 823 856 c averaging at 839 c and pressures of 1 54 2 73 kbar averaging at 2 05 kbar while the albite granite was cooled at the temperatures of 851 853 c and the pressures of 2 34 2 64 kbar averaging at 2 45 kbar combined with such high temperatures and low pressures the crustal like nd hf isotopic compositions nd t 5 3 to 4 4 hf t 7 0 to 3 8 support their derivation from the shallow melting of archean crustal rocks under high heat flow however the contrasted k2o and na2o compositions between the alkali feldspar granite k2o 4 76 5 14 wt na2o 4 76 5 14 wt and the albite granite k2o 0 29 0 44 wt na2o 6 15 6 56 wt imply that they may have been derived from distinct source rocks or have undergone different magmatic processes genetic modeling suggests that the alkali feldspar granite was originated from melting of granodioritic rocks the albite granite however was most likely derived from the reworking of plagioclase rich layers after the extraction of a type melts coupled with other studies on sedimentary and volcanic successions in the jiao liao ji belt the unusual association of the a type alkali feldspar granite and albite granite indicates that the southern segment of the jiao liao ji belt experienced continental rifting during 2 1 2 2 ga

anisotropies of partial anhysteretic remanence and susceptibility in compacted black shales grainsize and composition dependent magnetic fabric we have quantified the magnetic fabric of the highly compacted heebner shale pennsylvanian of kansas in terms of the anisotropies of partial anhysteretic remanent magnetization parm and low field susceptibility hysteresis loops exhibit almost purely paramagnetic behavior and the ratio of high field linear susceptibility to low field susceptibility is very close to one the susceptibility anisotropy therefore reflects the fabric of the paramagnetic matrix while the remanent anisotropies are due to the preferred orientations of ferrimagnetic grains both the paramagnetic and ferrimagnetic fabrics are dominated by a horizontal foliation with a weak northeasterly lineation but the parm anisotropies are generally much larger than the susceptibility anisotropy the degree of foliation measured with parm s decreases markedly with increasing coercivities indicating stronger foliation in the coarser grainsize fraction in contrast the parm mean lineations are grainsize independent suggesting a post compactional origin related to tectonic compression the susceptibility lineation appears to have a slightly different azimuth and may therefore be due to paleocurrents

water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb evidence for the origin of silicic magmas in the taupo volcanic zone new zealand and other occurrences

anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust abstract the sept iles layered intrusion quebec canada is dominated by a basal layered series made up of troctolites and gabbros and by anorthosites occurring 1 at the roof of the magma chamber 100 500 m thick and 2 as cm to m size blocks in gabbros of the layered series anorthosite rocks are made up of plagioclase with minor clinopyroxene olivine and fe ti oxide minerals plagioclase displays a very restricted range of compositions for major elements an 68 an 60 trace elements sr 1023 1071 ppm ba 132 172 ppm and sr isotopic ratios 87 sr 86 sr i 0 70356 0 70379 this compositional range is identical to that observed in troctolites the most primitive cumulates of the layered series whereas plagioclase in layered gabbros is more evolved an 60 an 38 the origin of sept iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts the density of the feo rich tholeiitic basalt parent magma first increased from 2 70 to 2 75 g cm 3 during early fractionation of troctolites and then decreased continuously to 2 16 g cm 3 with fractionation of fe ti oxide bearing gabbros plagioclase an 69 an 60 was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite with further differentiation plagioclase 60 became negatively buoyant and anorthosite stopped forming blocks of anorthosite autoliths even fell downward to the basal cumulate pile the presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and or minor plagioclase nucleation within the main magma body dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid the processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust with gabbro noritic rocks at the base and anorthositic rocks at the top we propose that the lunar crust mainly crystallized bottom up this basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian suite without kreep contamination while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust

petrogenesis of coexisting high silica aluminous and peralkaline rhyolites from yunshan yongtai southeastern china abstract the late cretaceous bimodal yunshan yongtai volcanics in fujian province contain peralkaline rhyolites the only presence of such rhyolites in southeastern china whole rock and mineral chemical compositions are analyzed for the coexisting aluminous metaluminous to weakly peraluminous peralkaline high silica rhyolites from the yunshan volcanics they are sparsely porphyritic and contain k feldspar ferromagnesian minerals quartz magnetite and titanomagnetite phenocrysts as well as accessory minerals such as fayalite chevkinite apatite and zircon the mineral assemblage indicates an oxidizing pre eruption condition these rhyolites exhibit diagnostic geochemical features of a type granites such as elevated 10 4 ga al mostly greater than 2 6 and feo t feo t mgo ratios enrichment in high field strength elements hfse such as zr 400 ppm and nb and strong depletion in al 2 o 3 2 o poor meta granitic igneous rocks in the deep crust and cannot be fractionated from the coeval yunshan mafic magmas geochemical variations of major and trace elements indicate the possible fractionation of k feldspar calcium rich pyroxene fe ti oxides and minor chevkinite during the magma evolution in peralkaline rhyolites we found that the pre existing fe ti oxide and hedenbergite phenocrysts had been transformed into aegirine oxide and aegirine oxide fluorite assemblages respectively these mineral assemblages are the products of the subsolidus reaction of pre existing phenocrysts and extraneous na f rich fluids such na f rich fluids may be derived from the degassing of the subvolcanic rocks the reactions indicate that the yunshan peralkaline rhyolites could be generated through the reaction of highly fractionated aluminous silica magmas and na f rich fluids

a temporal record of magma accumulation and evolution beneath nevado de toluca mexico preserved in plagioclase phenocrysts plagioclase crystals from the 8 km 10 5 ka upper toluca pumice utp eruption from nevado detoluca provide a detailed temporal record of pre eruptive magmatic processes the crystals display a range of textures and major and trace element concentrations a distinct feature of the crystals is the presence of several sharp increases in mgo feo andtio2 which occur at some of the numerous resorption horizons and coincide with increases in xan and in some cases ce and la these abrupt compositional peaks and the associated textural variations reflect recharge events of more mafic melt three distinct recharge events can be recognized the general compositional trends suggest that crystallization occurred within a common chamber however the crystal cores display a range of textures and a wide range of compositions suggesting that they are relics of earlier crystallization episodes within the upper crust the temporal record of sr and ba melt contents recorded by the crystals calculated using partition coefficients fluctuates significantly within single crystals overall two trends of sr ba variation in the melt are apparent the first trend involves decreasing sr and slightly increasing ba consistent with plagioclase crystallization the second trend involves an increase in both sr and ba that is not consistent with crystallization of plagioclase alone this trend is ascribed to chemical variations within the deeper magma reservoirs from which the various magma batches entering the utp shallow reservoir were ultimately sourced the magmatic system under nevado de toluca was open and received intermittent but relatively small pulses of magma from a deeper source dacite was the predominant recharge composition although some pulses were distinctly more mafic and hotter these processes led to the accumulation of a large volume of dacitic magma in the upper crust one of the more mafic pulses appears to have triggered the eruption

internal structure and emplacement mechanism of composite plutons evidence from mt kinabalu borneo the internal structure and emplacement mechanisms of composite plutons are investigated using new field data from the composite late miocene granitic intrusion of mt kinabalu in northern borneo the pluton was emplaced in the upper to middle crust in the late miocene at the contact between the ultramafic basement and sedimentary cover rocks structural data indicate that emplacement occurred during regional nnw sse oriented extension challenging tectonic models that involve contemporaneous regional compression the six major units forming the pluton were accommodated by upward flexure of the cover rocks with most magma pulses emplaced successively beneath their predecessors however the irregular 3d internal structure of the pluton also reflects preferential emplacement of successive units along the granite country rock contact of previous units in preference to the basement cover rock contact exploited by the initial units this study highlights the complex emplacement mechanisms and internal structure of composite intrusions and assesses how they differ from models of tabular emplacement

precursory seismic quiescence before the mw 8 3 tokachi oki japan earthquake on 26 september 2003 revealed by a re examined earthquake catalog 1 the 2003 tokachi oki earthquake mw 8 3 occurred on 26 september 2003 off the pacific coast of hokkaido japan in the present study an earthquake catalog is used that lists 2 000 earthquakes with m 3 3 all of the earthquake waveforms were recorded by the institute of seismology and volcanology hokkaido university in the present study these waveforms are manually re examined and hypocenters and magnitudes are re calculated a detailed analysis of the re determined earthquake catalog between 1994 and 2003 using a gridding technique zmap shows that the 2003 tokachi oki earthquake is preceded by two neighboring seismic quiescence anomalies that start around the beginning of 1999 and last about 5 years until the main shock occurs these quiescence anomalies are located around the asperity ruptured by the main shock and the z values are 3 9 and 4 0 for a time window of tw 4 years using a sample size of n 100 earthquakes the detected seismic quiescences can be interpreted as being caused by a decrease of 50 in the stressing rate based on dieterich s theory it is proposed that a quasi static pre slip occurs at the northeastern edge of the asperity ruptured by the main shock and lasts for five years until the main shock occurs by calculating the change in the coulomb failure stress cfs it is found that negative cfs areas are consistent with the two quiescence anomalies and a positive cfs area corresponds to the hypocenter of the main shock indicating that the quasi static pre slip model is a plausible one

factors affecting the relative importance of shape and distribution anisotropy in rocks theory and experiments abstract the effect of magnetic interactions has been commonly neglected in the interpretation of the anisotropy of magnetic susceptibility ams of igneous rocks despite the likely occurrence of magnetic interactions due to their relatively high content of ferromagnetic minerals current models of magnetic interactions apply to highly simplified systems and are not directly applicable to natural rocks on this work a model that includes variables previously neglected is used to assess the relative importance of magnetically interactive and non interactive fractions in a rock results of the model show that a variable proportion of magnetically interactive grains the relative values of the bulk susceptibility degree of anisotropy and orientation of the anisotropies characteristic for each fraction all play an important role in controlling the rock ams experimental evidence illustrating the importance of the distribution of the ferromagnetic grains within a specimen is also reported and supports the model predictions both theory and experiments indicate that if a rock has unequant magnetic grains and clusters of magnetic grains the contribution to the ams of the rock will depend on the relative values of all the model variables and not only on the occurrence of magnetic interactions as previously presumed

petrological constraints on crystallization conditions of mesoarchean sanukitoid rocks southeastern amazonian craton brazil we report petrological and geochemical data for the 2 87 ga rio maria sanukitoid granodiorite and associated rocks from mesoarchean granite greenstone terranes of the eastern amazonian craton brazil the dominant rocks have granodiorite to subordinate monzogranitic compositions with minor proportions of intermediate quartz diorites or quartz monzodiorites in addition to mafic endmembers occurring as layered rocks or as enclaves the mineral assemblage is dominated by amphibole plagioclase biotite and epidote minerals all of inferred magmatic origin pyroxenes being notably absent textural and compositional criteria indicate that amphibole is a principal mineral on the liquidus of all the rio maria rocks crystallization conditions have been derived from a comparison between natural phase assemblages proportions and compositions and experimental studies carried out on similar magma compositions the comparison shows that the parental magmas were water rich with more than 7 wt dissolved h2o with crystallization temperatures in the range 950 680 c the mg mg fe ratios of both amphibole and biotite indicate fo2 conditions in the range nno 0 5 to nno 2 5 where nno is nickel nickel oxide buffer therefore pointing to both water rich and oxidizing conditions for sanukitoid magmas amphibole compositions indicate emplacement at around 200 mpa and record a high pressure stage of magma crystallization around 600 900 mpa sanukitoid magmas share two of the principal characteristics of modern arc magmas elevated redox state and volatile contents which suggest that they may have formed in a geodynamic environment broadly similar to present day subduction zones

magma storage conditions of large plinian eruptions of santorini volcano greece the intensive variables of dacitic rhyodacitic magmas prior to four large plinian eruptions of santorini volcano over the last 200 kyr minoan cape riva lower pumice 2 and lower pumice 1 were determined by combining crystallization experiments with study of the natural products including the volatile contents of melt inclusions trapped in phenocrysts phase equilibria of the silicic magmas were determined at pressures of 1 2 and 4 kbar temperatures of 850 900 c fluid h2o co2 saturation xh2o molar h2o h2o co2 between 0 6 and 1 melt h2o contents of 2 10 wt and redox conditions of fmq fayalite magnetite quartz buffer or nno 1 where nno is ni nio buffer experiments were generally successful in reproducing the phenocryst assemblage of the natural products the phase relationships vary significantly among the investigated compositions revealing a sensitivity to small variations in whole rock compositions our results show that the pre eruptive storage conditions of the four silicic magmas were all very similar the magmas were stored at t 850 900 c and p 2 kbar under moderately reduced conditions nno 0 9 to 0 1 and were poor in fluorine 500 800 ppm and sulphur 100 ppm but rich in water and chlorine 5 6 wt and 2500 3500 ppm respectively in all cases the melts were slightly undersaturated with respect to h2o but most probably saturated with respect to h2o cl co2 and a brine the santorini magma plumbing system appears to be dominated by a large long lived 200 kyr predominantly silicic magma storage region situated at 8 km depth from which crystal poor melt batches were extracted during the largest caldera forming eruptions of the volcanic system

fractionation of metaluminous a type granites an experimental study of the wangrah suite lachlan fold belt australia abstract crystallisation experiments were performed on two metaluminous a type granite compositions ab412 and ab422 from the wangrah suite a representative a type granite suite from the lachlan fold belt south eastern australia ab412 is more mafic than ab422 and zircon saturation temperatures are 897 and 843 c respectively experiments were performed at 200 mpa f o 2 between nno and nno 1 05 in a temperature range between 700 and 900 c for various melt h 2 o contents experimental results were compared to the natural phase compositions to constrain the water content of the melt and the phases involved in fractionation processes in the wangrah suite with decreasing temperature fe ti oxide orthopyroxene and plagioclase are the first phases to crystallise followed by biotite the orthopyroxene stability is mainly restricted to temperatures above 750 c hornblende is only stable in the less evolved composition if the water content of the melt is at least 4 5 wt h 2 o the initial water content in the more mafic melt ab412 was between 2 and 3 wt h 2 o although orthopyroxene is not observed in the natural assemblages of the wangrah suite it has influenced the early fractionation of the wangrah suite granites mass balance calculations show that composition ab422 can be obtained after the crystallisation of 4 7 wt iron rich orthopyroxene plus 8 5 wt plagioclase from ab412

geochemistry zircon ages and whole rock nd isotopic systematics for palaeoproterozoic a type granitoids in the northern part of the delhi belt rajasthan nw india implications for late palaeoproterozoic crustal evolution of the aravalli craton determination of zircon ages as well as geochemical and sm nd isotope systematics of granitoids in the khetri copper belt of the aravalli mountains nw india constrain the late palaeoproterozoic crustal evolution of the aravalli craton the plutons are typical a type within plate granites derived from melts generated in an extensional tectonic environment they display ree and multi element patterns characterized by steep lree enriched and almost flat hree profiles and distinct negative anomalies for sr p and ti initial e nd values range from 1 3 to 6 2 and correspond to crustal sources with mean crustal residence ages of 2 5 to 2 1 ga a lower mafic crustal anatectic origin is envisaged for these granitoids and the heterogeneous e nd t values are inferred to have been acquired from the magma source region zircon pb pb evaporation and u pb ages indicate widespread rift related a type magmatism at 1711 1660 ma in the northern delhi belt and also suggest a discrete older magmatic event at around 1800 ma the emplacement ages of the compositionally distinct a type granitoid plutons and virtually coeval granulite metamorphism and exhumation in another segment of the aravalli mountains further signify that part of the aravalli crust evolved during a widespread extensional event in late palaeoproterozoic time

the magmatic feeding system of el reventador volcano sub andean zone ecuador constrained by texture mineralogy and thermobarometry of the 2002 erupted products abstract after a 26 years long quiescence el reventador an active volcano of the rear arc zone of ecuador entered a new eruptive cycle which lasted from 3 november to mid december 2002 the initial sub plinian activity vei 4 with andesite pyroclastic falls and flows shifted on 6 and 21 november to an effusive stage characterized by the emission of two lava flows andesite to low silica andesite lava 1 and basaltic andesite lava 2 containing abundant gabbro cumulates the erupted products are medium to high k calc alkaline and were investigated with respect to major element oxides mineral chemistry texture and thermobarometry inferred pre eruptive magmatic processes are dominated by the intrusion of a high t mafic magma possibly up to 1165 15 c into an andesite reservoir acting as magma mixing and trigger for the eruption before this refilling the andesite magma chamber was characterized by water content of 5 3 1 0 high oxygen fugacity nno 2 and temperatures in the upper and lower part of the reservoir of 850 and 952 65 c respectively accurate amphibole based barometry constrains the magma chamber depth between 8 2 and 11 3 km 2 2 km the 6 october 2002 seismic swarm hypocenters from 10 to 11 km preceding el reventador eruption supports the intrusion of magmas at these depths the widespread occurrence of disequilibrium features in most of the andesites e g complex mineral zoning and phase overgrowths indicates that convective self mixing have been operating together with fractional crystallization inferred from the cognate gabbro cumulates before the injection of the basic magma which then gave rise to basaltic andesite and low silica andesite hybrid layers magma mixing in the shallow chamber is inferred from the anomalous sio 2 al 2 o 3 whole rock pattern and strong olivine disequilibria both lavas show three types of amphibole breakdown rims mainly due to heating mixing processes and or relatively slow syn eruptive ascent rate decompression of the magmas the lack of any disequilibrium textures in the pumices of the 3 november fall deposit suggest that pre eruptive mixing did not occur in the roof zone of the chamber a model of the subvolcanic feeding system of el reventador consistent with the intrusion of a low al 2 o 3 crystal rich basic magma into an already self mixed andesite shallow reservoir is here proposed it is also inferred that before entering the shallow chamber the basaltic magma underwent a polybaric crystallization at deeper crustal levels

high magnesian granitoids in the precambrian continental crust implication for the continuum between ferro potassic and magnesio potassic rock suites abstract ferro potassic fe k to magnesio potassic mg k hornblende biotite granitoids hbgs with a wide range of sio2 from 52 to 77 wt are typical of precambrian continental crust and are generally related to late and post collisional orogenic events despite their broadly uniform mineralogy and tectonic environment the chemical compositions of these granites suggest different sources from lower crust for the fe k suites to enriched mantle for the mg k counterparts here we propose a classification of the magnesian rocks into moderate mg high mg and ultra mg with corresponding examples from various parts of the world such as the closepet type sanukitoids and their high mg counterparts of the elan type the elan type granitoids are characterized by the highest mgo cr ni end t and lowest tio2 feot feot mgo contents among all the precambrian hbgs and are hence close to a true end member the precambrian hbgs show common features such as a wide range of sio2 contents enrichment in lile k ba sr lree nb ti negative primitive mantle normalized anomalies and weak negative or no eu anomalies in intermediate suites the mg of magmatic and cumulus relict amphiboles displays a compositional array of ultra mg to fe granites and shows positive correlation with the whole rock mg continuum between the compositions of mg k and fe k granitoids is reflected in a gradual decrease in mg cr ni of the rocks and mg of the mafic silicates increase in ti p zr y zn u total ree and decrease of the nd initial isotopic composition together with a change in the paragenesis of fe ti oxides we interpret these features to result from mixing of metasomatized mantle and crustal magmas and their homogenization at the lower crust level the lower crustal source component increases from the mg k to fe k series during their passage to higher crustal levels the magmas underwent fractional crystallization and crustal contamination leading to the wide range in silica content

maximum and minimum water contents of granitic melts generated in the crust a reevaluation and implications abstract recent results of studies aimed to determine the water solubilities and the liquidus temperatures in synthetic and natural granitic systems are used to constrain the minimum water contents of granitic melts generated in the crust as a function of pressure temperature melt composition and water activity it is concluded that melts generated in the crust at high temperature and low water activities such as a type granites contain higher water contents than previously assumed up to 20 at 800 900 c and 3 7 kbar as a consequence the melt fraction produced at a given amount of available water can be up to 20 lower the minimum water content of melts generated by partial melting of crustal protoliths especially quartzofeldspathic rocks can be predicted adequately from the liquidus curves in granitic systems this water content can be higher if the melt composition departs from a minimum or eutectic quartzofeldspathic composition melts generated from quartzofeldspathic rocks at low temperature 750 c and below are necessarily water rich 5 wt h 2 o and have approximately the same viscosity or even slightly lower viscosities than melts generated at higher temperatures viscosities in the range 10 4 3 10 5 4 pa s recent experimental data also show that the value of the positive d p d t slope of the liquidus curves for fixed water contents above 800 c is lower than predicted by previous models thus adiabatic decompression produces a higher amount of melt than previously assumed at high temperature the formation of melt by adiabatic decompression can be up to 50 higher than assumed

quantifying frozen melt in crustal rocks a new melt o meter based on zircon rim volumes abstract quantifying the distribution of granitic melt at all scales in mid to lower crustal migmatitic terranes is critical to understand crustal melting processes chemical differentiation of the crust and its rheological behavior during deformation we propose a new method to determine the fraction of frozen granitic melt on a hand specimen scale based on the relative volumes of newly precipitated to total zircon fpz fraction of newly precipitated zircon as obtained by image analysis on dated zircon cores and rims using the calculated zr solubility zr sat in the melt at the inferred melting temperature and the zr concentration in the bulk sample zr bulk the fraction of melt fmelt can be determined through fmelt fpz zr bulk zr sat the such obtained fmelt corresponds to the melt fraction in the hand specimen at the time the system closed for melt mobility thermodynamic modelling further allows estimation of h2o contents required to maintain the melt fraction obtained from the melt o meter in a molten stage the applicability of this method has been tested on eight migmatitic samples with peak temperatures between 725 and 925 c most of the lower temperature migmatites 800 c retained fmelt of 0 35 0 50 0 07 0 10 at these melt fractions melt extraction and melt migration from and within the source should be efficient consequently these samples are likely open system migmatites affected by melt accumulation or depletion processes the melt o meter requires that the rock types under consideration produced a granitic melt that remained zircon saturated and is therefore restricted to migmatitic meta sediments and meta granitoids when applied carefully this melt o meter offers a new and powerful tool to not only quantify melt distribution but also evaluate the extent of melt mobility in migmatites

reduced rapakivi type granites the tholeiite connection reduced rapakivi type granites are the most iron enriched and reduced i e least oxidized of the anorogenic granite association the low oxygen fugacity and chemical composition of these granites severely limit their sources in this paper we argue that reduced rapakivi type granites and their eruptive equivalents high potassium fayalite rhyolites are derived from mafic sources because tholeiitic magmas and their derivatives have the required low oxygen fugacity reduced rapakivi type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents they form in extensional environments where the asthenosphere is present at shallow depths we envision three stages in the origin of these rocks 1 tholeiitic melts are emplaced at the base of the crust 2 continued introduction of heat partially remelts these tholeiitic rocks and 3 the hot dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites partial melting of felsic continental crust may accompany the intrusion of rapakivi type magmas thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite

plastic behavior of magnetite and high strains obtained from magnetic fabrics in the parry sound shear zone ontario grenville province abstract magnetic anisotropy and quantitative petrology of a transition from relatively undeformed metaanorthosite protolith to highly deformed ultramylonite in the parry sound shear zone provide information on magnetite as a kinematic indicator rock magnetic experiments show that anisotropy of magnetic susceptibility ams is controlled by magnetite in the mylonites and ultramylonites and by paramagnetic minerals hornblende biotite ilmenite in the protolith geothermometry in the ultramylonite indicates a temperature of 630 50 c during deformation a deformation mechanism map calculated from published experimental work for 630 c indicates that magnetite in the parry sound shear zone deformed plastically rather than via rigid body rotation the ams orientations accurately track a foliation trajectory in a portion of the shear zone which allows the utilization of orientation based strain models to determine shear strains shear strains of 1 9 were obtained and an empirical correlation of strain with the ellipsoid shape resulted in the logarithmic relationships mmax 0 15 ln x and mmin 0 13 ln z where mi ki kmean describes the principal ams axes or k max k min x z 0 14 these ams strain correlations were subsequently applied to mylonites and ultramylonites elsewhere in the shear zone predicting shear strains as high as 13 these observations also reveal that ams fabrics do not saturate at high strains as would be expected for rigid body rotation due to the plastic behavior of magnetite during deformation

porphyry cu mo stockwork formation by dynamic transient hydrothermal pulses mineralogic insights from the deposit at butte montana the systematic distribution of vein and alteration mineral assemblages in porphyry cu deposits largely arises from changes in the temperature and pressure of fluids that traversed fractures throughout the hydrothermal system magmatic and hydrothermal minerals record the complex history of the fluctuating temperature and pressure regime as hydrothermal fluids transfer heat from their magmatic source to cold wall rock in response to lithostatic to hydrostatic pressure variations we examine the thermal profile of the porphyry cu mo deposit in butte montana by determining formation temperatures for magmatic and hydrothermal samples representing different time frames and depths within the deposit in context with sample pressure estimates we use three independent mineral thermobarometers ti in quartz zr in rutile and x mg ti in biotite from which we estimate that final dike injection temperature and hence the initial magmatic hydrothermal fluid temperature was 700 c while the ambient host rock temperature was 450 to 500 c we find a magmatic hydrothermal continuum represented in hydrothermal veins ranging from 710 to

petrogenesis of the paleoproterozoic rapakivi a type granites of the archean carajas metallogenic province brazil three paleoproterozoic a type rapakivi granite suites jamon serra dos carajas and velho guilherme are found in the carajas metallogenic province eastern amazonian craton liquidus temperatures in the 900 870 c range characterize the jamon suite those for serra dos carajas and velho guilherme are somewhat lower pressures of emplacement decrease from jamon 3 2 0 7 kbar through serra dos carajas 2 0 1 0 kbar to velho guilherme 1 0 0 5 kbar oxidizing conditions nno 0 5 characterized the crystallization of the jamon magma the velho guilherme magmas were reducing marginally below fmq and the serra dos carajas magmas were intermediate between the two in this respect the three granite suites have archean tdm model ages and strongly negative nd values 12 to 8 at 1880 ma and they were derived from archean crust the jamon granite suite may have been derived from a quartz dioritic source and the velho guilherme granites from k feldspar bearing granitoid rocks with some sedimentary input the serra dos carajas granites either had a somewhat more mafic source than velho guilherme or were derived by a larger degree of melting underplating of mafic magma was probably the heat source for the melting the petrological and geochemical characteristics of the carajas granite suites imply considerable compositional variation in the archean of the eastern amazonian craton the oxidized jamon suite granites are similar to the mesoproterozoic magnetite series granites of laurentia and they were derived from archean igneous sources that were more oxidized than the sources of the fennoscandian rapakivi granites the serra dos carajas and velho guilherme granites approach the classic reduced rapakivi series of fennoscandia and laurentia no counterparts of the mesoproterozoic two mica granites of laurentia have been found however following the model of hoffman hoffman p 1989 speculations on laurentia s first gigayear 2 0 to 1 0 ga geology 17 135 138 the origin of the 1 88 ga carajas granites is related to a mantle superswell beneath the trans amazonian supercontinent this caused breakup of the continent and was associated with magmatic underplating and resultant crustal melting and generation of a type granite magmas the paleoproterozoic continent that included the archean and trans amazonian domains of the amazonian craton was assembled at 2 0 ga its disruption was initiated at 1 88 ga at least 200 ma earlier than in laurentia and fennoscandia the carajas granites were related to the breakup of the supercontinent not to subduction processes

petrogenesis of early silurian granitoids in the longshoushan area and their implications for the extensional environment of the north qilian orogenic belt china abstract analyses of the zircon u pb age mineralogical chemistry and whole rock geochemistry of the jiling pluton along the southern margin of the alxa block were conducted the results of these analyses will provide crucial information for investigations of paleozoic crust mantle interactions in the north qilian orogenic belt nqob and explorations of the transition age from a compressional to extensional environment during the collision between the alxa block and the qilian qaidam block the zircon dating results reveal that the jiling pluton is composed of early silurian granitoids 435 442 ma with contemporaneous mafic microgranular enclaves mmes most granitoids contain amphibole and titanite and have a cnk values

oxidized magnetite series rapakivi type granites of carajas brazil implications for classification and petrogenesis of a type granites abstract the varying geochemical and petrogenetic nature of a type granites is a controversial issue the oxidized magnetite series a type granites defined by anderson and bender anderson j l bender e e 1989 nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america lithos 23 19 52 are the most problematic as they do not strictly follow the original definition of a type granites and approach calc alkaline and i type granites in some aspects the oxidized jamon suite a type granites of the carajas province of the amazonian craton are compared with the magnetite series granites of laurentia and other representative a type granites including finnish rapakivi and lachlan fold belt a type granites as well as with calc alkaline i type orogenic granites the geochemistry and petrogenesis of different groups of a types granites are discussed with an emphasis on oxidized a type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis oxidized a type granites are clearly distinguished from calc alkaline cordilleran granites not only regarding trace element composition as previously demonstrated but also in their major element geochemistry oxidized a type granites have high whole rock feo t feo t mgo tio 2 mgo and k 2 o na 2 o and low al 2 o 3 and cao compared to calc alkaline granites the contrast of al 2 o 3 contents in these two granite groups is remarkable the cao feo t mgo tio 2 vs cao al 2 o 3 and cao feo t mgo tio 2 vs al 2 o 3 diagrams are proposed to distinguish a type and calc alkaline granites whole rock feo t feo t mgo and the feo t feo t mgo vs al 2 o 3 and feo t feo t mgo vs al 2 o 3 k 2 o na 2 o diagrams are suggested for discrimination of oxidized and reduced a type granites experimental data indicate that besides pressure the nature of a type granites is dependent of o 2 conditions and the water content of magma sources oxidized a type magmas are considered to be derived from melts with appreciable water contents 4 wt originating from lower crustal quartz feldspathic igneous sources under oxidizing conditions and which had clinopyroxene as an important residual phase reduced a type granites may be derived from quartz feldspathic igneous sources with a metasedimentary component or alternatively from differentiated tholeiitic sources the imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct a type granite groups assuming conditions near the nno buffer as a minimum for oxidized granites magnetite bearing granites formed near fmq buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced a type granites and respectively magnetite series and ilmenite series granites is not always observed

late ediacaran 605 580 ma post collisional alkaline magmatism in the arabian nubian shield a case study of serbal ring shaped intrusion southern sinai egypt abstract the serbal pluton is a late neoproterozoic 605 580 ma post collisional a type granites in southern sinai egypt northernmost arabian nubian shield ans it is characterized by discontinuous ring shaped outcrops dislocated by later faulting the pluton intrudes late neoproterozoic metamorphic and high k calc alkaline rocks the serbal pluton mostly comprises an outer zone of alkali feldspar granite surrounding a core of peralkaline granite gradational and sharp contacts in the serbal granites suggest that they were emplaced with a very short time interval still before complete crystallization of the earlier batch serbal granites are highly evolved 75 98 78 52 wt sio2 and display the typical geochemical characteristics of post collisional a type granites namely high sio2 na2o k2o feo mgo ga al zr nb ga and y and low cao mgo ba and sr they are rich in ree and have extreme eu negative anomalies eu eu 0 01 0 23 the chemical characteristics indicate that the peralkaline granite shares many features of granites with the tetrad ree effect the serbal pluton evolved through fractional crystallization of a parental magma derived through partial melting of a juvenile crustal protolith that had been extracted from a source having mantle geochemical and isotopic characteristics the crystallization temperatures using fe ti oxides of the serbal granites point to their formation at high temperatures up to 650 850 c at a shallow depth of emplacement

an experimental study of a lower proterozoic a type granite from theeastern amazonian craton brazil the jamon granite is representative of the lower proterozoic 1 88 introduction ga oxidized a type granites of the eastern part of the amazonian proterozoic anorogenic granites are found in several craton the dominant facies is a metaluminous to slightly percratonic areas such as in the fennoscandian shield aluminous hornblende biotite monzogranite with k2o na2o be nurmi haapala 1986 haapala ramo 1990 ramo tween 0 8 and 1 5 and feot feot mgo between 0 8 and haapala 1991 in north america bridgwater 0 9 in contrast to many other a type granites the jamon granite windley 1973 barker et al 1975 van schmus et al is characterized by early hornblende crystallization and the presence 1987 anderson bender 1989 gower et al 1990 of magnetite crystallization experiments were performed on glass emslie 1991 emslie stirling 1993 and australia at 300 mpa between 700 and 900 c for various melt h2o wyborn et al 1992 pollard mitchell 1995 they contents and for both oxidizing and reducing fo2 nno nickel are widespread in several tectonic provinces of the amnickel oxide 2 5 and nno 1 5 on average for nno azonian craton which is now recognized as one of 2 5 and under h2o rich conditions ilmenite clinopyroxene the world s major provinces of anorogenic magmatism magnetite and hornblende are near liquidus phases followed by dall agnol et al 1994 1999a bettencourt et al 1995 plagioclase the orthopyroxene stability field is restricted to high anorogenic amazonian granites have ages ranging from temperatures and h2o contents in the melt 5 wt in contrast 1 9 to 1 0 ga machado et al 1991 dall agnol et al for nno 1 5 magnetite and titanite are absent and orthopyroxene 1994 bettencourt et al 1995 and intrude either ar never observed in the granite clinopyroxene and ilmenite are the chaean or early to middle proterozoic sequences they liquidus phases conditions of crystallization of amphibole magare petrologically and geochemically diverse with rock netite and plagioclase constrain the initial melt h2o content to types similar to those found in rapakivi granites from the between 4 5 and 6 5 wt plagioclase cores crystallized from fennoscandian shield haapala ramo 1992 ramo 870 to 720 c clinopyroxene amphibole and biotite fe fe haapala 1995 the amazonian anorogenic granites mg values suggest fo2 around nno 0 5 during crystallization are classified as a type dall agnol et al 1994 1999a of the granite the demonstration of relatively hydrous conditions 1999b many granite complexes are mineralized sn w and oxidizing fo2 for the jamon granite stresses the diversity of ree zr f nb be ta and the tin deposits associated a type magmas in terms of h2o contents and redox states the with these granites are largely responsible for brazil jamon granite was most probably generated from oxidized archaean igneous rocks of mafic intermediate composition becoming a major tin producer in the last 20 years given the importance of the province large time span covered variety of rock types and the significant mineral resources associated there is clearly a need for a better

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

petrogenesis of the 1 43 ga sherman batholith se wyoming usa a reduced rapakivi type anorogenic granite the 1 43 ga sherman batholith southeastern wyoming usa origin that are found throughout the proterozoic anorogenic granite provinces of the southwestern usa adirshows extreme a type petrochemical characteristics compared with other mid proterozoic granite batholiths of north america it ondacks eastern canada southern greenland and the fennoscandian shield anderson 1983 phanerozoic consists of 1 the sherman granite a coarse grained biotite hornblende granite that locally contains fayalite and pyroxenes 2 examples include paleozoic granites of southeastern australia collins et al 1982 and of corsica poitrasson et the lincoln granite a medium grained biotite granite 3 a porphyritic biotite hornblende granite that probably formed by interaction al 1995 and the mesozoic nigerian younger granites jacobsen et al 1958 and the white mountain magma of granitic and mafic magmas and 4 iron enriched mafic dikes and pods the ilmenite series metaluminous sherman granite series foland allen 1991 anorogenic granite refers to plutons that are not associated with compressional exhibits extreme values of feo feo mgo and is rich in structures and that were emplaced long after any known k ree nb and y it crystallized at temperatures exceeding orogenic event anorogenic or a type granites loiselle 900 c and a pressure of 2 5 kbar with water activity of 0 7 wones 1979 are characterized by low h2o and o2 and dlog fo2 of 0 1 to 0 5 the lincoln granite which is fugacities along with high feo feo mgo k2o peraluminous and has less extreme a type geochemical characteristics na2o and k2o contents a type granites typically are rich crystallized at temperatures as low as 750 c and dlog fo2 of in incompatible elements including rare earth elements around 0 5 units above fmq fayalite magnetite quartz the ree zr nb and ta but poor in co sc cr ni ba rocks of the sherman batholith are chemically equivalent to lavas sr and eu minimum magma liquidus temperatures are from the yellowstone hotspot like the yellowstone magmas the 900 1000 c clemens et al 1986 creaser white sherman batholith probably originated by partial melting of under1991 some workers suggest these rocks are derived plated mantle derived mafic rocks from melting of tonalitic or more felsic crust anderson cullers 1978 collins et al 1982 clemens et al 1986 creaser et al 1991 whereas others relate the granites

earth s earliest granitoids are crystal rich magma reservoirs tapped by silicic eruptions granitoids of the tonalite trondhjemite granodiorite ttg series dominate earth s earliest continental crust the geochemical diversity of ttgs is ascribed to several possible geodynamic settings of magma formation from low pressure differentiation of oceanic plateaus to high pressure melting of mafic crust at convergent plate margins these interpretations implicitly assume that the bulk rock compositions of ttgs did not change from magma generation in the source to complete crystallization however crystal liquid segregation influences the geochemistry of felsic magmas as shown by the textural and chemical complementarity between coeval plutons and silicic volcanic rocks in the phanerozoic eon we demonstrate here that paleoarchean ca 3 456 million years old ttg plutons from south africa do not represent liquids but fossil crystal rich magma reservoirs left behind by the eruption of silicic volcanic rocks being possibly coeval at the million year scale as constrained by high precision uranium lead geochronology the chemical signature of the dominant trondhjemites conventionally interpreted as melts generated by high pressure melting of basalts reflects the combined accumulation of plagioclase phenocrysts and loss of interstitial liquid that erupted as silicic volcanic rocks our results indicate that the entire compositional diversity of ttgs could derive from the upper crustal differentiation of a single tonalitic magma formed by basalt melting and or crystallization at 40 km depth these results call for a unifying model of hadean archean continent nucleation by intracrustal production of ttg magmas the chemical diversity of earth s early continental building blocks can be explained by differentiation of a single melt without complex geodynamic settings according to petrological and geochemical analysis of samples from south africa