textural and chemical constraints on the formation of disseminated granite hosted w ta nb mineralization at the dajishan deposit nanling range southeastern china the dajishan deposit located in the nanling range of southeastern china is the largest wolframite deposit in the world approximately two thirds of the tungsten resource is contained in large quartz wolframite k feldspar muscovite scheelite sulfide beryl veins but the remainder occurs as wolframite and lesser scheelite disseminated through the nb ta mineralized no 69 granite which is characterized by abundant snowball quartz crystals disseminated granite hosted wolframite mostly occurs as a replacement after lath shaped muscovite although an interstitial variety is present in some samples the major element chemistry of the disseminated and vein hosted wolframite is similar however the former has one to two orders of magnitude higher concentrations of high field strength elements hfse including nb and ta disseminated muscovite in the no 69 granite is fe and mg rich replacement rims on these muscovite crystals are enriched in ta sn and cs but more depleted in nb related to the original phase vein hosted muscovite is characterized by lower nb ta rb and cs contents than the granite hosted muscovite although sn and w concentrations are comparable columbite mn is the principal nb ta ore mineral and occurs disseminated through the no 69 granite where it is hosted by snowball quartz crystals some of the columbite has been altered and overgrown by wodginite the restriction of these altered columbite crystals to within snowball quartz along with the ta and sn enrichment of the alteration rims on muscovite tie the formation of snowball quartz to a ta sn metasomatic event high ge ti and al ti ratios in the snowball quartz relative to interstitial quartz are also consistent with a late stage of formation for the former variety the disseminated granite hosted w mineralization formed through fluid rock interaction the fluids that precipitated the disseminated wolframite were most likely those that formed the veins but were modified through reaction with the hfse enriched no 69 granite nb ta mineralization however is represented by magmatic columbite and by a later metasomatic stage of wodginite the low solubility of ta in aqueous fluids combined with the evidence for ta sn metasomatism suggests that the ta sn metasomatic event was caused by hydrosilicate liquids a transitional fluid that lies compositionally between silicate magmas and hydrothermal fluids whole rock and mineral chemistry both suggest that deep level fractional crystallization of the no 69 granite was required to elevate ta and nb concentrations to allow saturation of columbite

the transition from peraluminous to peralkaline granitic melts evidence from melt inclusions and accessory minerals fractional crystallization of peraluminous f and h2o rich granite magmas progressively enriches the remaining melt with volatiles we show that at saturation the melt may separate into two immiscible conjugate melt fractions one of the fractions shows increasing peraluminosity and the other increasing peralkalinity these melt fractions also fractionate the incompatible elements to significantly different degrees coexisting melt fractions have differing chemical and physical properties and due to their high density and viscosity contrasts they will tend to separate readily from each other once separated each melt fraction evolves independently in response to changing t p x conditions and further immiscibility events may occur each generating its own conjugate pair of melt fractions the strongly peralkaline melt fractions in particular are very reactive and commonly react until equilibrium is attained consequently the peralkaline melt fraction is commonly preserved only in the isolated melt and mineral inclusions we demonstrate that the differences between melt fractions that can be seen most clearly in differing melt inclusion compositions are also visible in the composition of the resulting ore forming and accessory minerals and are visible on scales from a few micrometers to hundreds of meters

the mount st helens volcanic eruption of 18 may 1980 large short term surface temperature effects the surface temperature effects of the 18 may 1980 eruption of mount st helens volcano were examinedfor 1 day immediately after the eruption 24 hour temperature differences and model output statistics errors as well as the detailed temporal evolution of surface temperature at selected stations were used during the daytime hours immediately after the eruption the temperature was suppressed by the volcanic plume by as much as 8 c that night low level volcanic dust produced temperature enhancements of up to 8 c these effects quickly diminished the next day as the volcanic dust cloud dissipated and moved toward the east the net local effect of the eruption appears to be warming in contrast to cooling which might be expected over climatic time scales

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

the solubility of nb ta zr hf w in granitic melts with li and li f constraints for mineralization in rare metal granites and pegmatites tantalum mineralization is associated with granites and pegmatites that invariably are rich in lithium this in part is due to the extreme fractionation that is necessary for melts to reach saturation in either a ta or an li phase however there may also be chemical interactions in melts that explain this association experiments therefore were conducted to determine the effect of lithium on the solubility of tantalite mnta 2 o 6 in water saturated granitic melts other important elements in rare metal granites include nb zr hf and w consequently the effect of li on the solubilities of columbite mnnb 2 o 6 zircon zrsio 4 hafnon hfsio 4 and hbnerite mnwo 4 in granitic melts was also investigated the haplogranitic melt compositions used in these experiments have constant mole percent si and al and the al li na k ratio was 1 0 for all experiments i e the lithium content was the only compositional variable columbite and tantalite solubilities in the granitic liquids increase by a factor of approximately 2 to 3 with the addition of 2 wt percent li 2 o to haplogranite at 750 degrees to 1 035 degrees c 2 kbars and water saturated conditions the solubility data were extrapolated to a lower temperature more reasonable for columbite tantalite crystallization then compared to compositions representative of evolved liquids from which rare metal granites and pegmatites crystallize it is estimated that at 600 degrees c these melt compositions could be saturated in columbite but not tantalite however tantalite saturation is predicted if the melts contained less li and f therefore the genesis of tantalum mineralization may be explained by ta being retained in the melt because of high li f concentrations tantalite crystallization is delayed until an li or f or p mineral crystallizes which lowers tantalite solubility and results in a general association of ta with li in mineralized granitic rocks in contrast to columbite and tantalite the solubility of wolframite is not affected by the lithium content of the melt with up to 3 8 wt li 2 o in starting glasses and the solubilities of zircon and hafnon decrease with increasing li content of the melt one possible interpretation of the hbnerite solubility data is that the melt contains dominantly w super 6 which behaves as a network former the decrease in zircon hafnon solubility with li may reflect the lower field strength of zr super 4 and hf super 4 compared to nb super 5 and ta super 5 i e zr and hf are less able to compete with li for nonbridging oxygens whereas nb and ta are more able to compete it is also significant that the solubility of hafnon is greater than that of zircon similar to the higher solubility of tantalite compared to columbite this can explain why zr hf and nb ta ratios both decrease with fractionation

mineral paragenesis fluid inclusions h o isotopes and ore forming processes of the giant dahutang w cu mo deposit south china abstract the newly discovered giant dahutang w cu mo deposit in the central part of the jiangnan orogen hosts an estimated resource of 1 1 million tons mt of wo3 plus a proven resource of 0 65 mt cu and 0 08 mt mo the dahutang deposit is temporally and spatially associated with the late mesozoic s type granites emplaced into the neoproterozoic jiuling granodiorite batholith and the shuangqiaoshan group based on petrographic observations eight hydrothermal alteration mineralization stages have been recognized comprising pegmatitic stage i potassic alteration stage ii albite alteration stage iii greisenization and main mineralization stage iv polymetallic sulfide mineralization stage v late scheelite mineralization stage vi carbonate alteration stage vii and supergene alteration stage viii stage iv can be further divided into the early greisenization stage iv a the main wolframite mineralization stage iv b and the main scheelite mineralization stage iv c fluid inclusion and h o isotope analyses on nine stage iv to vii ore gangue minerals wolframite cassiterite scheelite sphalerite fluorapatite fluorite quartz calcite and chlorite suggest the presence of an early w and a late cu mo ore forming fluid system the early ore forming fluids belong to a medium to high temperature 200 420 c low salinity h2o nacl cacl2 system this primary magmatic hydrothermal fluid 18ofluid 5 4 to 8 8 and d 102 to 75 was likely exsolved from the late jurassic granites at dahutang wolframite and scheelite coexist closely with fluorapatite and fluorite respectively indicating that the volatile rich fluids at dahutang were also rich in tungsten fluorine and phosphorous tungsten was likely transported as tungstate species e g wo42 and hwo4 oxygen fluorine complexes e g wo2f4 2 wo3f2 2 and phosphorous heteropolytungstate e g p w12o40 3 in the fluids fluorapatite and wolframite may have precipitated first when the temperature dropped from 400 to 320 c along with ph increase at an estimated depth of 7 8 km this was likely followed by the extensive scheelite mineralization with fluorite precipitation that formed huge disseminated veinlet type w orebodies when the temperature further dropped to 200 c along with ph increase after the w mineralization extensive cu mo polymetallic mineralization and the associated sericite chlorite alterations may have formed by the granite porphyry and or muscovite granite emplacement the late ore forming fluids belong to a medium to high temperature 200 360 c low salinity h2o nacl cacl2 system molybdenum was likely transported mainly as mo complexes e g h2moo4 moo42 and copper as cu cl complexes molybdenite and chalcopyrite may have begun to precipitate when the fluids ascended and the temperature dropped to 330 c meteoric water incursion may have then occurred 18ofluid 1 9 6 9 and d 99 to 68 and further cooled the fluid system to 250 c forming large cu mo orebodies

palaeomagnetic properties of eocambrian sediments in northwestern scotland implications for world wide glaciation in the late precambrian abstract samples from two regions and from a range of rock types in northwest scotland have remanences that are stable to thermal and alternating magnetic field demagnetisation normal and reversed directions for different areas are also in closer agreement after correction for the different tectonic tilt in the different areas the age of the remanence is pre devonian and most considerations indicate but do not prove an eocambrian age these data are therefore thought to substantiate previous evidence proposed for the low latitude of deposition of the eocambrian tillites and associated thick limestones and dolomites on this basis it is not thought that the climatic changes indicated by the nature of the sediments can be attributed to rapid polar wandering during this period the data do however indicate that the glaciation not only took place at sea level but at latitudes which indicate that these glaciations at this time could represent a world wide glacial event

li and b isotopic fractionation at the magmatic hydrothermal transition of highly evolved granites abstract the isotopic compositions of li and b have great potential for the tracing of fluid sources and for the characterization of ore forming processes the fractionation of li and b isotopes during the magmatic hydrothermal transition in evolved granites however is not well understood as these rocks commonly show multiple late hydrothermal overprints tourmaline is sensitive to changes in melt and fluid compositions during its growth and resistant to alteration by later fluids which makes it a good recorder of li and b isotopic signatures and fractionation at late evolutionary stages of magmatic systems to characterize the evolution of a sn ore forming granitic intrusion we analyzed magmatic hydrothermal and detrital sedimentary tourmaline from the yuanbaoshan granite associated cassiterite tourmaline quartz veins and disseminated tourmaline bearing cassiterite sulfide ore in the sedimentary wall rocks the li and b isotopic compositions of tourmaline from these rocks do not covary largely due to mineral controlled decoupling during magmatic and hydrothermal processes because of the dominant role of tourmaline on the b budget of the rock magmatic and hydrothermal tourmalines show little variation in 11b 12 5 to 8 2 a signature reflecting the metasedimentary protoliths of the melt in contrast several rock forming minerals contribute to the budget of li which shows significant variation in 7li 0 3 to 6 9 the isotopic fractionation of li in tourmaline from rocks of the yuanbaoshan area is controlled by three major processes i fractional crystallization results in magmatic tourmaline having progressively higher 7li values and higher li contents ii the magmatic hydrothermal transition leads to higher 7li values at lower li contents in hydrothermal tourmaline and iii extensive fluid rock interaction adds isotopically light li to hydrothermal tourmaline in the wall rocks extensive fluid rock interaction also leached hree mg isotopically light b and probably sn from the granites and or wall rocks tourmaline seems to robustly record progressive changes in li isotopes in evolving magmatic hydrothermal systems and hydrothermal mineral deposits

crystal rich mass flow deposits related to the eruption of a sublacustrine silicic cryptodome early permian collio basin italian alps abstract sedimentological granulometric and petrographic data are presented from a detailed study on a crystal rich mass flow deposit which is presumably related to the eruption of a sublacustrine cryptodome the deposit forms a prominent intercalation in the lower permian collio formation in the italian alps north of brescia outcrops of the 10 20 m thick volcaniclastic deposit dasdana i beds db can be traced over 12 km from east to west the db consists of a thick crystal rich sandy gravelly lower subunit representing a sequence of amalgamated bouma a b divisions overlain by a thin well bedded sandy muddy subunit that is rich in outsize porphyritic silicic fragments modal and computer aided image analyses reveal that the crystal rich lower subunit contains up to 80 of volcanogenic crystals some samples contain up to 60 of porphyritic fragments which have a phenocryst content of about 20 the wide textural range from cryptocrystalline poikilomosaic to rarer medium grained granophyric groundmass the irregular to lensoid shapes of the porphyritic fragments and the presence of basement and sedimentary clasts suggest that the db originated from a sublacustrine eruption of a partially extrusive cryptodome ca 1 6 km3 two other porphyritic felsic cryptodomes dosso dei lupi dosso del bue described briefly here emplaced into the collio formation sometime after the db event and expose flat bases and tilted sediments at their sides textures observed in these domes are comparable to those found in the db porphyritic fragments

genesis of crystal rich volcaniclastic facies in the permian red beds of the central pyrenees ne spain abstract the permo carboniferous volcanism of the central pyrenees developed in subaerial conditions and is mainly represented by the products of explosive eruptions of calc alkaline silicic magmas occasionally associated with calderaforming events the sedimentological record of the permo carboniferous basins of the central pyrenees is characterized by the abundance of crystal rich volcaniclastic deposits reflecting a high crystal content in the erupting magmas 18 40 vol sedimentation during the early permian was dominated by massflows stream floods and braided channels organised in poorly developed alluvial fans at basin margins a wide spectrum of crystal rich deposits can be identified in the permian terrigenous red beds the grouping of these deposits into facies and facies associations allows the recognition of several volcaniclastic marker horizons extending through the central pyrenees these can be used to establish stratigraphical correlations between the different permo carboniferous basins the study of these crystal rich volcaniclastic deposits has revealed that the lower permian crystal rich voleaniclastic deposits of the central pyrenees originated from pyroclastic and reworking processes that sometimes acted simultaneously pyroclastic deposits derived from high intensity explosive magmatic eruptions of rhyodacitic and rhyolitic crystal rich magmas these eruptions generated high eruption columns that occasionally collapsed giving rise to the formation of widespread pyroclastic flows and associated pyroclastic surges the entrance of the eruption columns into the atmosphere had a significant effect on the local weather provoking sporadic storms that led to reworking of pyroclasts by rainfall run off during eruption different crystal concentration processes acted in the eruption columns and in the subsequent pyroclastic flows and overriding ash clouds producing the accumulation of a high volume of crystals in the resulting deposits reworking and redeposition of pyroclastic deposits by fluvial processes further enhanced crystal concentration

quartz and feldspar zoning in the eastern erzgebirge volcano plutonic complex germany czech republic evidence of multiple magma mixing abstract zoned quartz and feldspar phenocrysts of the upper carboniferous eastern erzgebirge volcano plutonic complex were studied by cathodoluminescence and minor and trace element profiling the results verify the suitability of quartz and feldspar phenocrysts as recorders of differentiation trends magma mixing and recharge events and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale multiple resorption surfaces and zones element concentration steps in zoned quartz ti and feldspar phenocrysts anorthite content ba sr and plagioclase mantled k feldspars etc indicate mixing of silicic magma with a more mafic magma for several magmatic phases of the eastern erzgebirge volcano plutonic complex generally feldspar appears to be sensitive to the physicochemical changes of the melt whereas quartz phenocrysts are more stable and can survive a longer period of evolution and final effusion of silicic magmas the regional distribution of mixing compatible textures suggests that magma mingling and mixing was a major process in the evolution of these late variscan granites and associated volcanic rocks quartz phenocrysts from 14 magmatic phases of the eastern erzgebirge volcano plutonic complex provide information on the relative timing of different mixing processes storage and recharge allowing a model for the distribution of magma reservoirs in space and time at least two levels of magma storage are envisioned deep reservoirs between 24 and 17 km the crystallisation level of quartz phenocrysts and subvolcanic reservoirs between 13 and 6 km deflation of the shallow reservoirs during the extrusion of the teplice rhyolites triggered the formation of the altenberg teplice caldera above the eastern erzgebirge volcano plutonic complex the deep magma reservoir of the teplice rhyolite also has a genetic relationship to the younger mineralised a type granites as indicated by quartz phenocryst populations the pre caldera biotite granites and the rhyodacitic schonfeld volcanic rocks represent temporally and spatially separate magma sources however the deep magma reservoir of both is assumed to have been at a depth of 24 17 km the drastic chemical contrast between the pre caldera schonfeld westfalian b c and the syn caldera teplice westfalian c d volcanic rocks is related to the change from late orogenic geotectonic environment to post orogenic faulting and is considered an important chronostratigraphic marker

the formation of the yichun ta nb deposit south china through fractional crystallization of magma indicated by fluid and silicate melt inclusions abstract the yichun ta nb deposit which is located in jiangxi province south china can be divided into four lithological zones from bottom upward two mica granite muscovite granite albite granite and lepidolite albite granite zones it remains controversial whether these distinct vertical zones were formed through late magmatic hydrothermal metasomatic alteration or fractional crystallization of magma to investigate the evolution mechanism of rock and ore forming fluid in this deposit we studied fluid and melt inclusions in quartz and lepidolite in these four granite zones these fluid inclusions are mainly composed of h2o nacl and have homogenization temperatures ranging from 160 c to 240 c with densities between 0 86 and 0 94 g cm3 and salinities between 0 5 and 6 5 wt nacl equivalent raman spectroscopic analyses showed that the daughter minerals contained in silicate melt inclusions are mainly quartz lepidolite albite muscovite microcline topaz and sassolite from the lower to upper granite zones the albite contents in silicate melt inclusions increase while the muscovite contents decrease gradually until muscovite is substituted by lepidolite in the lepidolite albite granite zone additionally the calculated densities of the silicate melt inclusions exhibit decreasing trends from bottom upward the total homogenization temperatures of silicate melt inclusions which were observed under external pressures created in the sample chamber of a hydrothermal diamond anvil cell decreased from 860 c in the lower lithological zone to 776 c in the upper lithological zone and the initial melting temperatures of solid phases were 570 710 c the calculated initial h2o contents of granitic magma showed an increasing trend from the lower 2 wt in the two mica granite zone to the upper granitic zones 3 wt in the albite granite zone all of these features illustrate that the vertical granite zones in the yichun ta nb deposit formed through the continuous fractional crystallization of the granitic magma additionally the low h2o contents and gradual enrichment of incompatible elements f b li ta nb etc in the residual granitic magma favored the formation of a granite type ta nb deposit

regional kinematics inferred from magnetic subfabrics in archean rocks of northern ontario canada strain analysis observations of l s fabrics and studies of the anisotropy of magnetic susceptibility ams and of anisotropy of isothermal remanence airm reveal a sequence of progressive fabric development during regional deformation l s fabrics of quartz and feldspar grains are most transposed in regional transpression whereas the ams subfabrics controlled by the properties of late metamorphic silicates record a later orientation of the ambient stress field anisotropy of magnetic remanence is caused by the still younger minerals magnetite and pyrrhotite its anisotropy is less transposed than the ams subfabric consequently the relative orientations of the three types of fabric yield a kinematic sequence of subfabrics developed during transpression the l s fabrics are most transposed the ams less transposed and the airm fabrics least transposed toward the plane of regional flattening the results indicate that the deformation recorded by the three subfabrics was non coaxial in the most general sense with all three principal directions spinning with respect to the rocks during progressive straining

crystal structure refinement of a rubidian cesian phlogopite the crystal structure of a rubidian cesian phlogopite 1 m from pegmatite exocontacts at red cross lake manitoba monoclinic a 5 343 1 b 9 247 2 c 10 397 3 a 100 04 2 v 505 8 2 a 3 has been refined to an r index of 4 5 based on 519 observed reflections measured with graphite monochromated moka x radiation on an automated four circle diffractometer the crystal used in the collection of the x ray intensity data was also analyzed by electron microprobe giving the unit formula k0 46rb0 28cs0 23 mg1 20fe1 00al0 38li0 34mn0 04ti0 04 si2 91al1 09 o10 oh 1 55f0 45 the interlayer site x contains large amounts of rb and cs and cell dimensions and the distance are in accord with data from synthetic rb and cs phlogopites and plutonic phlogopites the interlayer coordination is much more regular in rubidian cesian phologopite than in other trioctahedral micas defining a trend of increasing regularity with increasing interlayer cation size

tracking magmatic and hydrothermal nb ta w sn fractionation using mineral textures and composition a case study from the late cretaceous jiepailing ore district in the nanling range in south china abstract the jiepailing mining district in the nanling range in south china is well known for its granite related sn be f mineralization recently drill holes have exposed an nb ta w sn mineralized granitic porphyry and topaz bearing granite greisen at depth which we have studied here using mineral columbite rutile wolframite cassiterite zircon and mica major and trace element compositional data mineral textures and zircon and columbite u pb geochronology our age data shows that the porphyry and the granite and their mineralization formed at 91 89 1 ma in the late cretaceous and thus subsequent to the main ore forming events of the region continuous mineral compositional trends indicate that the studied granitoids are related by progressive fractionation we propose that 1 subhedral euhedral low ta columbite crystallized from melt 2 euhedral subhedral rutile and wolframite and subhedral and subhedral cassiterite up to 30 m in size formed at the magmatic hydrothermal transition of the system and 3 high ta columbite and subhedral cassiterite up to 10 m in size formed from subsolidus hydrothermal fluids in combination with the nb ta w and sn compositions of zircon and mica their textures and compositional variation allow us to track the magmatic to hydrothermal rare metal fractionation concentration mobilization and deposition of the system in detail despite our limited access to it through only two exploration drill cores using the nb ta w and sn concentrations in zircon refractory early crystallized and in micas late equilibrated respectively was particularly useful for tracing the partial loss of sn and w ore components from the intrusion and to constrain the information which is crucial for any rigorous ore exploration

phyllosilicate fabric characterization by low temperature anisotropy of magnetic susceptibility lt ams 1 this paper examines the use and applicability of anisotropy of magnetic susceptibility at low temperature 77 k called lt ams paramagnetic susceptibility increases significantly with low temperature which permits high precision determination of the anisotropy of paramagnetic susceptibility of a rock from measurements at liquid nitrogen temperature we describe a new procedure that measures directional susceptibility in an oriented rock specimen while immersed in liquid nitrogen our procedure overcomes the problem of temperature heterogeneity within a rock sample while its susceptibility is being measured offering the opportunity to measure low temperature susceptibility in multiple orientations this non destructive method provides a way of characterizing the principal paramagnetic minerals present in a rock based on temperature induced changes of the anisotropy degree moreover the method offers a better resolved relationship between anisotropy degree and grain alignment which is used to quantify rock fabrics

anatomy of a strike slip fault controlled sedimentary basin permian of the southern pyrenees spain this study is mainly concerned with a 55 km long east west oriented strike section through a thick succession of dominantly clastic post variscan deposits in the spanish pyrenees south of andorra a subdivision into three formations can be made two formations of stephanian age composed of volcanic rocks and fluviatile sediments respectively and one of probable permian age which has been studied in detail and is entirely composed of terrestrial redbeds these three formations are characterised by large variations in thickness along strike thicknesses vary from 0 to 800 m in the volcanics from 0 to 300 m in the stephanian fluviatile sediments and from 0 to 1500 m in the permian redbeds the geometry of the sedimentary bodies in all three formations suggests deposition in an east west oriented graben like depression a conclusion which is confirmed by detailed sedimentological analysis of the permian rocks the vertical succession of permian sediments can be subdivided in six major basinfill sequences which unconformably overlie each other internally the basinfill sequences are built up of one or more coarsening upward fining upward cycles the genesis of these cycles is directly related to dip slip movement along the basin boundary faults whereas the unconformities between the basinfill sequences indicate that periods of basin subsidence and sedimentation were followed by periods of deformation and erosion these changes in tectonic regime as reflected in the sedimentary sequences are interpreted to indicate alternating phases of transtension and transpression in a strike slip zone other sedimentological observations substantiate this interpretation the basin depocentre shifted in space and time which resulted in the observed large variations in sediment thickness along strike together with eastward paleocurrent directions along the basin axis transtension was accompanied by outpouring of andesitic lavas and drape folding on a large scale during transpressional phases normal faults changed character to reverse faults while folding uplift and subsequent erosion resulted in the formation of intrabasinal unconformities alternation of transpressive and transtensive conditions should be the result of movement along non planar fault surfaces in a major strike slip zone a separate structural study of the variscan basement of the studied basin revealed the existence of a dextral simple shear deformation generation of probable stephano permian age simple shear in the basement on the one hand and brittle superficial strike slip faulting involving transtension and transpression on the other are considered to be expressions of the same regional stress field at different levels in the earth s crust

geochemical and petrogenetic features of mineralization in rare element granitic pegmatites in the light of current research abstract tremendous expansion of high technology applications of rare metals and specialty minerals requires periodic reassessment of their geological sources keeping in mind the long range technological potential of all hi tech materials available from a given type of deposit granitic pegmatites are a classic example of diversified sources li rb cs be ga sc y ree sn nb ta u th zr hf optical quartz and fluorite high purity feldpar petatite and refractory spodumene ceramic amblygonite e tc such rare element pegmatites can be subdivided into three principal families lct pegmatites with li rb cs be ga sn ta nb b p f nyf pegmatites with nb ta ti y ree zr th and u f pegmatites with a mixed geochemical signature the less evolved members of the lct family produce mainly only be nb ta and sn but exotic types with be ta nb f b p with li rb and cs and are also known so far no differences ini mineralization styles have been observed between the spodumene petalite and amblygonite subtypes of the complex type except the mineralogy of their substantial li contents however extensive divergence in k rb cs fractionation is observed violating the classic criteria for for example pollucite potential and economic ga concentration is recognized the lepidolite subtype is commonly enriched in y and h ree albitespodumene pegmatites have largely simple li sn nb ta be mineralization but a suprisingly broad range of alkali fractionation levels the not so common albite type seems to be typically enriched in sn nb ta ti the nyf family is much less abundant a general subdivision of these pegmatites is not yet available except for the recognition of sporadic lree enriched and much more widespread y hree enriched subtypes scandium is an element recognized recently as much more abundant than previously thought internal evolution of individual pegmatites is better understood today in terms of london s model of crystallization from highly hydrous but homogeneous melts experimental evidence does not support the jahns and burnham proposal of early separation of a fluid phase peraluminous fertile s granites generating lct pegmatites are derived by partial melting of upper middle crustal rocks undergoing their first anatexis and the pegmatites commonly show regional zoning metaluminous granites yeilding poorly zoned groups of nyf pegmatites are largely of the a type generated by second melting of short lived depleted lower crustal protoliths however a juvenile igneous or fluid component may be involved mixed nyf lct granite pegmatite suites may originate in several ways peraluminous lct sequences were traditionally regarded as orogenic in contrast to anorogenic nyf systems tectonic correlation with geochemistry is now considered secondary to the control by source lithologies it is the understanding of the association of pegmatite types with their geological setting and petrogenetic affiliation which assists in the search for selected hi tech materials

sulfide saturation and magma emplacement in the formation of the permian huangshandong ni cu sulfide deposit xinjiang northwestern china the eastern tianshan orogenic belt northwestern china hosts 120 mafic ultramafic intrusions and at least six of them host significant ni cu sulfide mineralization of which the permian huangshandong intrusion is the largest the mafic ultramafic rocks of the huangshandong intrusion comprise lherzolite gabbronorite gabbro and diorite individual orebodies are composed of pyrrhotite pentlandite and chalcopyrite as disseminated to net textured or massive to semimassive sulfide mineralization oxide bearing sulfide mineralization contains 1 to 5 magmatic magnetite that has ilmenite and spinel exsolution lamellae silicate rocks and sulfide mineralization have variable whole rock platinum group elements pge 0 38 362 ppb cu 42 97 000 ppm and ni 26 33 000 ppm concentrations the mineralization is broken out into two types termed oxide rich and oxide poor the oxide rich sulfide mineralization has high pge concentrations with low 187re 188os ratios 38 9 151 whereas oxide poor sulfide mineralization has low pge concentrations with high 187re 188os 159 781 both oxide rich and poor sulfide mineralization have highly variable os values 49 189 and 30 278 respectively oxide rich sulfide mineralization contains magnetite with variable amounts of mgo tio2 al2o3 feototal cr v zn and sn indicating that they crystallized from different stages during magma differentiation we propose that the host intrusion and associated sulfide mineralization were derived from high mg basaltic magmas from a mantle source that was previously modified by subducted oceanic slab oxide poor sulfide mineralization of the huangshandong intrusion formed from a magma that underwent 0 03 sulfide removal before emplacement and was thus pge depleted oxide rich sulfide mineralization formed from magma that had much lower amounts of sulfide 0 003 removed in the early stage before entering the huangshandong magma chamber an early sulfide saturation event was likely associated with fractionation of silicate minerals in a deep staging magma chamber the second sulfide saturation event that formed the sulfide mineralization was probably triggered by selective crustal contamination and fractionation in the high level chamber where mixing of magmas and accumulation of immiscible sulfide occurred

magnetic mineralogy of some granites from the french massif central origin of their low field susceptibility abstract with a view to a better understanding of the origin of the low field susceptibility in the variscan granitoids of the french massif central for which extensive structural mapping using the anisotropy of magnetic susceptibility ams technique is currently applied a detailed study of magnetic carriers has been undertaken origin of the susceptibility has been studied using high field and low temperature measurements in a cryogenic magnetometer together with standard rock magnetic techniques additional determinations of the opaque phases have been performed with the metallographic microscope x ray microdiffraction and electron microprobe techniques in almost all the granite types the magnetic susceptibility is essentially carried by the paramagnetic biotite crystals minor amounts of ubiquitous oxides ilmenite rutile titano haematite do not contribute significantly to the susceptibility this is also the case for rare occurrences of magnetite in some granitoids mainly the cordierite bearing anatexites sulphides are found to give a large contribution magnetic mineralogy identifies a mixture of ferromagnetic pyrrhotite and antiferromagnetic fe9s10 hexagonal pyrrhotite hpo the latter species is identified as forming the larger unaltered pyrrhotite grains the ferromagnetic pyrrhotite is identified to be mainly smythite fe9s11 a species formed by supergenic alteration of monoclinic pyrrhotite located along the basal cleavage planes the microfractures and the grain boundaries of the hpo this study which constitutes the first report of the magnetic properties of smythite in rocks indicates that magnetic carriers derived from low temperature alteration have to be taken into account for purposes of structural studies using ams

textural and chemical evolution of a fractionated granitic system the podlesi stock czech republic the podlesigranite stock czech republic is a fractionated peraluminous f li and p rich and sn w nb ta bearing rare metal granite system its magmatic evolution involved processes typical of intrusions related to porphyry type deposits explosive breccia comb layers rare metal granites stockscheider and rare metal pegmatites extreme f p li enrichment nb ta sn minerals layering geological textural and mineralogical data suggest that the podlesigranites evolved from fractionated granitic melt progressively enriched in h2o f p li etc quartz k feldspar fe li mica and topaz bear evidence of multistage crystallization that alternated with episodes of resorption changes in chemical composition between individual crystal zones and or populations provide evidence of chemical evolution of the melt variations in rock textures mirror changes in the pressure and temperature conditions of crystallization equilibrium crystallization was interrupted several times by opening of the system and the consequent adiabatic decrease of pressure and temperature resulted in episodes of nonequilibrium crystallization the podlesigranites demonstrate that adiabatic fluctuation of pressure bswinging eutecticq and boundary layer crystallization of undercooled melt can explain magmatic layering and unidirectional solidification textures usts in highly fractionated granites d 2004 elsevier b v all rights reserved

diversity of lithium mica compositions in mineralized granite greisen system cinovec li sn w deposit erzgebirge the li sn w deposit cinovec zinnwald in the eastern erzgebirge czech republic and germany represents one of the worldwide best known examples of complex rare metal plutons with pervasive greisenization mica from altogether 38 samples representing all granite and greisen varieties from the present surface to the depth of 1596m was analysed using electron microprobe empa and laser ablation inductively coupled plasma mass spectrometry la icp ms for major including f rb and trace li sc ga ge nb in sn cs ta tl w elements respectively

caldera events in a rift depocentre an example from the jurassic neuquen basin argentina we analyse the volcanic stratigraphy of the lower jurassic synrift sequence of the sanico depocentre which is a sub basin of one of the most important hydrocarbon basins in south america the neuquen rift basin the sanico depocentre comprises a complex multicyclic volcano sedimentary succession the oldest part of the succession preserves a volcano sedimentary sequence interpreted as the record of a stratovolcano the middle part of the succession is characterized by the emplacement of two thick ignimbrites that completely filled the depocentre finally the uppermost part of the succession records subordinate volcanism characterized by andesitic lavas associated with epiclastic sediments the sanico depocentre shows a complex subsidence history that involves an interplay between tectonics and volcanism during the early and late synrift phases subsidence was relatively slow and permitted the accumulation of thick sequences of volcanic and sedimentary deposits throughout a long period of time during the middle synrift phase subsidence was very rapid and allowed the accumulation of a thick ignimbrite sequence in a very short time interval in this study this middle episode is interpreted as corresponding to a caldera like event in which rapid subsidence occurred along the same normal faults that controlled tectonic subsidence during the previous and subsequent episodes

melt inclusions in quartz from an evolved peraluminous pegmatite geochemical evidence for strong tin enrichment in fluorine rich and phosphorus rich residual liquids abstract we have investigated the magmatic evolution of a late stage f and p rich pegmatite forming aluminosilicate liquid and the geochemical controls on magmatic mineralizing processes by remelting totally crystallized melt inclusions in quartz and analyzing the quenched glass by epma and sims the quartz phenocrysts were sampled from a pegmatite that occurs in a variscan granite genetically associated with cassiterite and wolframite mineralized greisen veins at the ehrenfriedersdorf sn w deposit central erzgebirge se germany the melt inclusion compositions imply that the pegmatite forming liquid achieved extreme levels of chemical differentiation it contained high abundances of sn f p li rb cs nb ta and be and abnormally low concentrations of ca y sr and ree for a granite and it was strongly peraluminous the molar al 2 o 3 cao na 2 o k 2 o ranged from 1 3 to 2 0 fractions of the pegmatite forming liquid were extremely enriched in p 2 o 5 f al 2 o 3 and the molar abundances of f p in the glasses correlate strongly with moles of network modifying al ions implying that the bulk liquid included f p and al bearing complexes formation of these complexes reduced the activities of f p and al in bulk liquid suppressed the crystallization of magmatic topaz and p rich minerals and allowed the liquid to become enriched in these constituents some fractions of the ehrenfriedersdorf aluminosilicate liquid contained 1000 2000 ppm sn these levels of sn enrichment were up to 2 orders of magnitude greater than that ever reported for nonmineralized metaluminous and peraluminous igneous materials and are consistent with some experimentally derived sn solubilities in cassiterite saturated granitic liquids at geologically relevant pressures and temperatures this concordance implies that cassiterite could have crystallized directly from this highly evolved p and f rich peraluminous granitic liquid without the involvement of hydrothermal fluids

magmatic hydrothermal rare element mineralization in the songshugang granite northeastern jiangxi china insights from an electron microprobe study of nb ta zr minerals abstract the songshugang granite hidden in the sinian metasedimentary stratum is a highly evolved rare element granite in northeastern jiangxi province south china the samples were systematically taken from the ck 102 drill hole at the depth of 171 423 m four types of rocks were divided from the bottom upwards topaz albite granite as the main body greisen nodules topaz k feldspar granite and pegmatite layer electron microprobe study reveals that the rare element minerals of the songshugang granite are very different from those of other rare element granites mn mn fe mn and ta ta nb ta of columbite group minerals and hf hf zr hf of zircon are nearly constant within each type of rocks however back scattered electron imaging revealed that nb ta oxides and zircon of the songshugang granite especially those of topaz albite granite topaz k feldspar granite and greisen are commonly characterized by a specific two stage texture on the crystal scale the early stage nb ta oxide is simply subhedral shaped columbite fe cgm i with low mn 0 16 0 37 and ta 0 05 0 29 columbite fe is penetrated by the later stage tantalite veinlets cgm ii or surrounded by complex nb ta sn w mineral assemblages including tantalite fe wodginite sl cassiterite and ferberite tantalite has wide range of mn values 0 15 0 88 from fe dominance to mn dominance wodginite with ta nb has large variable concentrations of w sn and ti cassiterite and ferberite are all enriched in nb and ta nb 2 o 5 ta 2 o 5 up to 20 12 wt and 31 42 wt respectively with high ta 0 5 similar to nb ta oxides and nb ta sn w mineral assemblages the early stage zircon is commonly included by the later stage zircon with sharply boundary they have contrasting hf contents and hfo 2 of the later stage zircon is up to 28 13 wt petrographic features indicate that the early stage of columbite and zircon were formed in magmatic environment however the later stage of rare element minerals were influenced by fluxes enriched fluids tantalite together with wodginite cassiterite and ferberite implies a ta dominant media an interstitial fluid rich melt enriched in ta and flux at the magmatic hydrothermal transitional stage is currently a favored model for explaining the later stage of rare element mineralization

anisotropy of magnetic remanence a brief review of mineralogical sources physical origins and geological applications and comparison with susceptibility anisotropy the magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low field magnetic susceptibility ams however alternative methods based on remanent magnetization measured in the absence of a magnetic field rather than induced magnetization measured in the applied field have distinct advantages for certain geological applications this is particularly true for 1 adjunct studies in paleomagnetism in order to assess the fidelity with which a natural remanence records the paleofield orientation 2 studies of weakly magnetic or weakly deformed rocks for which susceptibility anisotropy is very difficult to measure precisely and 3 quantitative applications such as strain estimation the fundamental differences between susceptibility and remanence and their respective anisotropies are due to several factors 1 susceptibility arises from all of the minerals present in a sample whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals 2 ferromagnetic minerals are generally more anisotropic than para and diamagnetic minerals 3 for ferromagnetic minerals remanence is inevitably more anisotropic than susceptibility and 4 a number of common minerals including single domain magnetites possess an inverse anisotropy of susceptibility i e they tend to have minimum susceptibility parallel to the long axis of an individual particle remanence is immune to this phenomenon as a consequence of all these factors remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample

subaqueous rhyolitic dome top tuff cones a model based on the devonian bunga beds southeastern australia and a modern analogue the relics of two late devonian subaqueous rhyolitic dome top tuff and pumice cone successions are preserved in the bunga beds outlier of the boyd volcanic complex southeastern australia these cone successions and other rhyolitic volcanics of the bunga beds are associated with turbidite and other deep water massflow sedimentary rocks the two cone successions have a generally similar stratigraphy at the base flow banded variably autobrecciated and quench fragmented rhyolite representing an intrusive to extrusive dome is overlain by rhyolitesediment breccia representing extrusion of the dome through the deep water sediment pile and resedimentation down its flanks in the northern cone succession an overlying succession of bedded pumiceous crystal rich to crystal poor tuffs represents the onset of pyroclastic activity and growth of a tuff cone an overyling debris flow deposit represents degradation of part of the cone the topmost unit a stratified pumice succession is thought to represent another cone building eruptive phase and is separated from the underlying strata by a major slide surface the southern cone succession contains less tuff and abundant pumice and is also terminated by a debris flow deposit indicating cone degradation a modern analogue for the inferred eruptive style and sequence is the 1953 1957 rhyolite eruption that formed the tuluman island lava tuff cone complex in the bismarck sea the eruptions were often cyclical consisting of an initial inferred submarine lava forming stage passing into a pumicecone forming stage in some cases a subaeriallava forming stage and a final stage following the cessation of volcanism during which the cones collapsed gravitationally or were destroyed by wave erosion using observations from both the tuluman island eruptions and the preserved stratigraphies of the devonian tuff cones a dynamic model is proposed for the formation of subaqueous rhyolitic dome top tuff and pumice cones

use of grain size and magnetic fabric analyses to distinguish among depositional environments the combined use of grain size and magnetic fabric analyses provides the ability to discriminate among depositional environments in deep sea terrigenous sediments we analyzed samples from three different depositional settings turbidites pelagic or hemipelagic interlayers and sediment drifts results indicate that sediment samples from these different environments can be distinguished from each other on the basis of their median grain size sorting as well as the intensity and shape of magnetic fabric as determined from an examination of the anisotropy of magnetic susceptibility we use these discriminators to interpret downcore samples from the bermuda rise sediment drift we find that the finer grains of the bermuda rise relative to the blake outer ridge do not result from lower depositional energy current speed and so may reflect a difference in the nature of sediment being delivered to the site i e distance from source between the two locations

sedimentary fabric on deep sea sediments from kodos area in the eastern pacific abstract an anisotropy of magnetic susceptibility ams study on three sediment cores taken from the korea deep ocean study kodos area in eastern pacific was carried out to understand the relationship between magnetic fabric patterns and paleo depositional conditions of deep sea sediments most ams measurements reveal normal sedimentary fabric patterns indicating a stable sedimentation mode however some intervals show deformed ams patterns magnetic lineation and this anomalous fabric is mainly observed at the boundary where sediment color changes the abrupt change of ams parameters at this boundary provides evidence that the linear fabric was induced by input of intensified bottom currents based on the trend of microfossil occurrences it is interpreted that the discontinuous color boundary was formed by hiatus between tertiary and quaternary age the change of magnetic fabric at the topmost color boundary coincides with the time of prevailed antarctic bottom water aabw activity in this study the ams properties are shown to be useful tools for making inferences concerning the paleoenvironment of abyssal basins

mineral chemistry of columbite tantalite from spodumene pegmatites of kolmozero kola peninsula russia abstract compositional variation results of electron microprobe analyses and mass spectrometry analyses of columbite group minerals cgm from fully differentiated albite spodumene pegmatites at kolmozero in the kola peninsula is evaluated concentric zoning typical of rare metal pegmatites was not observed in the kolmozero pegmatites columbite group minerals occur in all main parageneses of the pegmatites and form four generations reflecting the sequence of pegmatite formation these minerals demonstrate wide variations in the content of major and trace elements the composition of cgm ranges from columbite fe to tantalite mn fractionation trends were observed in mn mn fe versus ta ta nb diagrams and trace element abundances plotted versus xta and xmn the early cgm paragenesis is characterized by homogeneous oscillatory and progressive oscillatory zoning and corresponds to a primary magmatic type late generation cgm show patchy irregular internal textures replacing earlier regular patterns of zoning the irregular zoning points to metasomatic replacement processes for the first time it is shown that distributions of rare earth elements ree in cgm reflect the evolution of a pegmatite forming system at kolmozero the main trend of ree variation from early to late generations of cgm involves decreasing total ree contents due to a decrease in heavy ree and y decreasing negative eu anomaly and decreasing magnitude of m shape tetrad effect between gd and ho these changes are accompanied by gradual flattening of the bird like patterns of chondrite normalized ree distribution all these features are typical for late differentiates of granitic volatile rich magma late metasomatic tantalite mn is characterized by sharp changes in its ree distribution pattern decreasing total ree contents changing shape of the ree distribution pattern the absence of eu anomaly and tetrad effects and the appearance of a negative ce anomaly the textural characteristics and mineral chemistry of cgm indicate that the pegmatite forming system underwent several stages of evolution the earliest magmatic stage can be divided into two sub stages involving direct crystallization and collective recrystallization respectively and was succeeded by a late hydrothermal metasomatic post magmatic stage variations in chemical composition among the different generations of cgm are explained by the interplay of several processes fractional crystallization competitive crystallization of main rock forming feldspar muscovite spodumene and accessory triphylyte lithiophilite spessartine fluorapatite zircon microlite minerals and evolution of the mineral forming environment from a melt to a hydrothermal metasomatic fluid

mineralogy and radiation effects of microlite from the harding pegmatite taos county new mexico microlite rangrng from crystalline to metamict is a principal accessory in several lithologic units of the harding pegmatite taos county new mexico from the sequence of lithologic units within the pegmatite crystallization of microlite from the pegmatite magma is inferred to have begun relatively late after the formation of the beryl and quartz zones and continued throughout the formation of the core zones and subsolidus replacement units over 200 chemical analyses of microlite determined by electron microprobe are reported and they are consistent with the accepted structural formula a2 b2x6y phro where principally a ca na ij mn b ta nb ti x o and y f oh o general features of microlite crystal chemistry identified include l a positive correlation between a site vacancies and the maximum y site vacancies 2 a positive correlation between na and f and 3 a negative correlation between u and f the latter is consistent with the interpretation that u at the a site in the pyrochlore structure is analogous to the uranyl group uo3 requiring o in place of f at the y site microlite compositions from four of five lithologic units examined are chemically distinct in terms of linear combinations of the variables ij fe ti bi ca ce pb f mn ba sb th ta and na the cleavelanditeunit microlites the exception not surprisingly have chemistries like those of the quartzlath spodumene zone which the cleavelandite unit in part has replaced the earliest formed microlites from the quartz lath spodumene zone have the highest ta na and f and are low in u and mn in contrast microlites from the later microcline spodumene zone and the replacement units are generally higher in ti mn and u and lower in f and na chemical changes ascribed to primary hydrothermal alteration from residual pegmatitic fluids include increases in ca mn and ti and an overall loss of a site cations alteration effects and chemical zoning within crystals analyzed in terms of simple end members identifies the following principal substitution schemes bnbbta bta bnb acayo anayf and aeye anayf for core to rim zoning in crystals and acayo anayf and acayo aey for primary alteration secondary weathering alteration results in decreases in na ca and f and increases in hro the effects of alpha recoil damage due to the decay of constituent u have been examined because of the wide variation in u content 0 1 10 wto o uor the microlites exhibit the full range of structural periodicity from completely crystalline 20 dpa based on x ray and electron ditrraction analysis the progressive structural modification of microlite udth increasing alpha dose involves 1 formation of isolated defect aggregates i e individual alpha recoil tracks up through doses of l0 4 alphas mg with no detectable effect on the materials s ability to diffract x rays or electrons 2 continued damage and overlap of these defect aggregates yielding coexisting regions of amorphous and crystalline domains at l0rs to 1016 alphas mg and 3 complete amorphization at doses greater than 10 7 alphas mg

the role of diffusion controlled oscillatory nucleation in the formation of line rock in pegmatite plite dikes the george ashley block gab located in the pala pegmatite introduction district san diego county california is a composite pegmatite modal or phase layering is a phenomenon that is seen aplite dike of 8 m thickness displaying striking mineralogical layering in plutonic igneous rocks of all compositions from highin the aplite portion of the dike referred to as line rock rhythmic temperature low viscosity mafic complexes to low temlayering is characterized by garnet rich bands alternating with perature high viscosity felsic complexes the sizes of albite quartz muscovite rich bands cumulus textures are notably such plutonic bodies can range from the large layered absent from the layered portion of the dike elongated quartz intrusive complexes such as the 3000 m thick skaergaard megacrysts are oriented perpendicular to the garnet rich layers and complex down to dikes a few meters wide there is poikilitically include garnet albite and muscovite calculated however no consensus as to the origin of the layering crystal free magma viscosity with 3 h2o is 10 pa s and the modal layering that results from the gravitational settling calculated settling velocity for garnet is 0 51 cm year conductive of crystals from a magma is a model often invoked to cooling calculations based on emplacement of a 650 c dike into explain the layering in mafic magmatic systems this is 150 c fractured gabbroic country rock at 1 5 kbar and accounting more problematic in granitic systems because of the for latent heat of crystallization demonstrate that the line rock higher viscosities of the melts and the smaller density portion of the dike cools to 550 c in about 1 year crystal difference between crystals and melt size distribution studies also suggest very rapid nucleation and this paper presents a temperature crystallization crystallization diffusion controlled gel crystallization experiments model for the george ashley composite pegmatite aplite yield textures virtually identical to those observed in the layered dike which displays well developed rhythmic layering aplite including rhythmic banding colloform layering and band line rock in the aplite portion of the dike kleck 1996 discontinuities thus observed textures and calculated magmatic proposed that the layering in the george ashley block parameters suggest that mineralogical layering in the gab results gab a float slide block from this dike could be exfrom an in situ diffusion controlled process of oscillatory nucleation plained by the gravitational settling of garnet and albite and crystallization we propose that any event that promotes from the melt and that many of the textural features strong undercooling has the potential to initiate rapid heterogeneous which characterize the line rock such as offsets in the nucleation and oscillatory crystal growth leading to the development layers formed as a result of soft sediment deformation of a layer of excluded components in front of the crystallization of a crystal liquid mush the purpose of this investigation front and the formation of line rock was to evaluate crystal settling as a possible mechanism

li nb ta mineralization in the jurassic yifeng granite aplite intrusion within the neoproterozoic jiuling batholith south china a fluid rich and quenching ore forming process abstract the neoproterozoic jiuling batholith in northwestern jiangxi province south china is one of the largest batholiths in the nanling range and it is intruded by mesozoic granitic intrusions associated with mineralization the jurassic yifeng granitic pluton intrudes to the southern part of the jiuling batholith and comprises li rich muscovite li mus granite trilithionite tri granite and aplite dikes the granitic rocks are enriched in rare metals li rb cs nb ta we conducted whole rock geochemical analyses undertook columbite u pb dating and examined the textures and chemistry of feldspars micas and the accessory minerals montebrasite columbite group minerals cgms microlite cassiterite and zircon all three granitic rock types have high li contents up to 6200 ppm and the 7li values of the li mus granite and aplite are 3 55 0 14 and 5 34 0 04 respectively the isolated grains of cgms and cassiterite in the aplite are magmatic products but intense fluid activity in the later stages and quenching resulted in unique mineral aggregates of cassiterite cgms zircon the nb ta mineralization of the li mus granite took place at 144 5 ma according to columbite u pb dating and this age is similar to that of the neighbouring yichun granitic pluton to the south moreover both the yifeng and yichun plutons contain p rich rare metal bearing li f granitic rocks that contain p rich feldspars montebrasites amblygonites cs rich lepidolites cgms close to the mn endmember and hf rich zircons it is inferred that a metallogenic district characterized by li nb ta mineralization exists in northwestern jiangxi province thus providing a reliable guide for further exploration

magnetic anisotropy of some phyllosilicates magnetic susceptibility anisotropy of susceptibility and hysteresis of single microcrystals of chlorite biotite phlogopite muscovite zinnwaldite and fuchsite were measured in low and high magnetic fields with an alternating gradient force magnetometer micromag their properties are sufficient to account for the low field susceptibility ams of most micaceous rocks nearly all samples show some ferromagnetic contribution at low fields due to inclusions of pseudosingle domain and multidomain magnetite the paramagnetic contribution isolated at high fields usually exceeds the ferromagnetic contribution the paramagnetic susceptibility is intrinsic to the silicate lattice and agrees with values predicted from chemical composition within the limits of error the minimum susceptibility is nearly parallel to c another axis is parallel to b and the third susceptibility usually the maximum is close to a the paramagnetic susceptibility has a disk shaped magnitude ellipsoid with strong anisotropy p 2 the ferromagnetic contributions at low fields have more variably shaped ellipsoids with greater eccentricity p 5 the silicate lattice does not constrain their orientation our technique cannot determine the principal axes of the ferromagnetic component however its principal values usually correspond with the paramagnetic principal susceptibilities in order of magnitude thus the combined paramagnetic ferromagnetic anisotropy recognised in routine studies of ams should faithfully represent the petrofabric of most micaceous rocks nevertheless nearly 10 of our samples have incompatible anisotropy ellipsoids for the silicate host and magnetite inclusions these yield a net inverse ams that does not correctly represent the orientation of the silicate lattice therefore some caution is necessary in petrofabric ams studies of micaceous rocks

the beauvoir topaz lepidolite albite granite massif central france the disseminated magmatic sn li ta nb be mineralization the beauvoir topaz lepidolite albite granite french massif central is the latest intrusion in a variscan peraluminous granitic complex composed of three units successively emplaced the concealed la bosse granite the colettes two mica granite and the beauvoir granite the 900 m deep drill hole of the geologie profonde de la france deep drilling program of the continental crust has provided a continuously cored section of the beauvoir granite the exposed section of the beauvoir granite is presently mined for kaolin in the upper 200 m it also represents a huge subeconomic disseminated li sn ta be deposit sharing many characteristics with rare metal bearing pegmatites compared to similar li f rich igneous bodies the beauvoir granite is strongly enriched in sn 200 1 400 ppm ta 20 400 ppm and be 20 300 ppm li is located in lepidolite and amblygonite ta and nb mainly in columbo tantalite and uranium rich microlite and sn in cassiterite be is located in lepidolite crystals and in unidentified mineral s the major element composition of the beauvoir granite is similar to that of macusani volcanic glasses of peru and ongonite subvolcanic rocks of mongolia the main structural textural mineralogical and geochemical characteristics of the beauvoir granite can be explained by extreme fractionation of f and li rich magmas and associated fluid phases most trace elements show a strong upward enrichment the high f sub o 2 low solidus temperature and high fluorine content of the beauvoir magma have considerably reduced sn and w fractionation in the expelled magmatic fluid despite its high chlorine content 25 30 wt nacl equiv however f and li enriched geochemical halos have developed in the enclosing mica schist in response to the boiling of the magmatic fluid the relatively deep emplacement of the granite 3 km has limited the development of hydraulic fracturing and subsequent hydrothermal circulation and mineralization thus despite the very high specialization of the beauvoir granite no significant vein type mineralization is directly related to it the la bosse quartz ferberite stockwork was developed before the emplacement of the colettes and beauvoir granites and is related to the earlier concealed la bosse granite

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

the magnetic zone axis a new element of magnetic fabric for the interpretation of the magnetic lineation abstract the magnetic zone axis is the common direction of the intersection of the magnetic foliations of different samples it is easily determined using tensors from the minimum susceptibility axes it provides useful information for the interpretation of the magnetic fabric particularly of the maximum axis a bootstrap method reveals small local disturbance of the magnetic foliation it yields also the confidence zone of the magnetic zone axis

textural features and chemical evolution in tantalum oxides magmatic versus hydrothermal origins for ta mineralization in the tanco lower pegmatite manitoba canada tantalum a key element in the electronics industry is produced mainly from rare element granitic pegmatites although their internal structure mineralogy and petrogenesis have been extensively investigated the processes that control tantalum mineralization remain poorly understood in particular the role of fluids in the crystallization of tantalum ore one of the major problems arises from the difficulty in distinguishing primary magmatic from secondary hydrothermal textures in such complex rocks in the tanco pegmatite manitoba canada tantalum mineralization shows a complexity that reflects the complex petrogenesis of its host pegmatite eight different families of tantalum oxides occur in various associations and compositions the tanco lower pegmatite is an isolated body beneath the main pegmatite body that contains abundant tantalum associated with mica alteration tantalum mineralization in the tanco lower pegmatite occurs as three different styles facies 1 is hosted by the wall zone and hosts primary magmatic ta oxides with simple textures progressive and oscillatory zoning facies 2 is hosted by the lower and upper intermediate zones where most mineralization occurs with dendritic amblygonite facies 3 is hosted by the central zone which is affected by mica alteration in this latter facies the oxides show particularly complex textures evident from x ray mapping by combining information obtained from textural observations and chemical analyses we are able to determine the paragenesis for each ta oxide bearing mineral assemblage and hence to evaluate the relative contributions of magmatic versus hydrothermal processes in complex associations we observe relics of primary ta phases that are replaced or overgrown by secondary ta phases we propose the paragenetic sequence columbite group minerals microlite early primary magmatic columbite wodginite group minerals microlite late primary magmatic wodginite group minerals microlite secondary magmatic ferrotapiolite secondary magmatic in addition chemical variations were identified in the columbite and wodginite group minerals both at the crystal scale and at the pegmatite scale columbite and wodginite group minerals show the typical ta ta ta nb and mn mn mn fe atomic ratios increase from earlier to later zones at the crystal scale the increase in ta and mn with fractionation in the columbite group is explained by the higher solubility of the ta end member relative to the nb end member and the crystallization of other minerals that shift the melt composition toward mn enrichment the fractionation trend in the wodginite group shows fe enrichment which is consistent with experimental results that show a higher solubility of the fe end member relative to the mn end member in columbite group minerals considering the available experimental data as well as the intimate association of ta oxides with zircon observed in this study we conclude that tantalum mineralization is a product of direct crystallization from the melt rather than hydrothermal in origin fluids are attributed an indirect role only as they could have brought minor elements fe mn or ca into the melt which resulted in the crystallization of secondary ta phases textural evidence shows that the late secondary phases were formed at the same time as mica alteration we suggest that mica alteration was due to a late melt rather than a hydrothermal fluid that reacted with the blocky k feldspar of the central zone during the alteration event tantalum was remobilized from primary phases and incorporated in new secondary ta oxide species

permian rhyolitic volcanism changing from subaqueous to subaerial in post variscan intra continental sirinia basin sw romania eastern europe abstract the sirinia basin is a well exposed upper carboniferous lower permian intra continental extensional basin containing 1 2 km of permian interbedded terrigeneous sediments and primary volcanic and volcaniclastic rocks of rhyolitic composition these are mostly the products of subaqueous volcanism passing into a subaerial volcanism the intra continental basin lies on the danubian metamorphic units in the south westernmost part of the carpathians sw romania and is n s oriented 18 8 km as result of alpine tectonic evolution and deformation the sirinia basin setting formed a dynamic closed intra continental sedimentary system that experienced a successive subsidence it started with lacustrine sediments and then with large volumes of rhyolitic volcanic and volcaniclastic material dominantly sited in the central part of the basin volcanism included 1 subaqueous extrusion of domes surrounded by hyaloclastite deposits closely associated with secondary sedimentation 2 extrusive and explosive surtseyan type and subaerial eruptions from shallow water to emergent volcanoes on dome cap and associated reworked deposits towards the southern side of the basin and 3 subaerial extrusive domes fluvial erosion and deposition completed the evolution of the emerged marginal part of the basin most of the erupted material of the first and second phase filling the central part of the basin along extensional normal faults was rapidly transported on sides of the basin by turbidite sedimentation debris flows slumping and sliding while turbidites prevail toward the central part of the basin the debris flow sedimentation predominated at the margin of the basin infrequently interrupted by deposition of fallout ash from hydromagmatic explosive eruptions related to the dome cap tuff and pumice cones

mineralogical constraints on the genesis of w nb ta mineralization in the laiziling granite xianghualing district south china abstract the laiziling granitic pluton located in the xianghualing district hunan province china comprises typical rare metal granites with a pronounced vertical lithological zonation this pluton includes three granitic phases comprising from base to top medium grained mg alkali feldspar granite fine grained fg alkali feldspar granite and albite granite here we report the results of textural and chemical analyses of w nb ta oxide minerals from the laiziling granitic pluton the chemical compositions of whole rock samples rock forming minerals and zircon have allowed us to identify the magmatic hydrothermal processes that operated during the formation and rare metal mineralization of the laiziling pluton the granites are slightly peraluminous to peraluminous highly evolved and fractionated as inferred from whole rock geochemical and mineralogical micas and zircon signatures all the micas in different granitic phases are classified into the zinnwaldite group li concentration in the primary micas 2 6 4 wt li 2 o and hf concentration 1 8 wt hfo 2 in the zircon increase for the base to the top granitic phases both alkali feldspar granites contain oxide mineral aggregates including columbite group minerals wolframoixiolite qitianlingite wolframite and cassiterite the columbite group minerals wolframite and cassiterite also occur as isolated grains within the albite granite the presence of the oxide minerals suggests that the w nb ta mineralization developed during the latter stages of the magmatic evolution related to fluids that were compositionally evolved from the granites the composition of the fluids controls the aggregate of the oxide minerals the wolframite and cassiterite that precipitated within the laiziling skarn type deposit are chemically different from those within the granite indicating a hydrothermal origin for the oxide minerals that formed as a result of pervasive alteration

magnetic fabric methods and applications fabric is a ubiquitous and significant feature of geological materials the processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains petrofabrics thus provide essential keys to understanding the history of geological materials magnetic anisotropy is directly related to petrofabric and has become one of the most rapid sensitive and widely used tools for its characterization the relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition concentration and grain size of mineral grains ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved the papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations mineral fabric development technical advances and rock magnetic properties

aridification across the carboniferous permian transition in central equatorial pangea the catalan pyrenean succession ne iberian peninsula abstract the carboniferous permian terrestrial successions record a global climatic shift from icehouse to hothouse conditions our multidisciplinary study documents an aridification trend throughout the 1000 m thick composite terrestrial succession of the western catalan pyrenees ne iberian peninsula representing this time period the detailed stratigraphic framework integrates sedimentology paleopedology biochronology plant fossils and tetrapod footprints and geochronology paleomagnetism additional absolute age correlation is also carried out the new and reviewed data show that the late carboniferous wet environments with short drought periods progressively changed to a strong seasonal semi arid and arid climate with short humid periods through the early permian this paleoclimatic trend supports the previously suggested aridification of the pangean pan tropical belt and supports the hypothesis of the influence of the recurrent climatic fluctuations in central pangea being tentatively correlated to the southern gondwanan glaciation deglaciation periods therefore the carboniferous permian terrestrial succession from the catalan pyrenees emerges as a continuous record that can help to constrain late paleozoic paleoenvironmental events

caldera like structures related to permo carboniferous volcanism of the catalan pyrenees ne spain abstract the permo carboniferous volcanism of the catalan pyrences produced calc alkaline orogenic rocks with a predominance of rhyolitic and rhyodacitic pyroclastic deposits on the basis of the regional stratigraphic tectonic relationships and facies analysis of sedimentary and pyroclastic deposits the existence of four permo carboniferous basins which acted as volcanotectonic depressions has been deduced the origin of these basins seems to be related to the movement of regional strike slip faults which formed during the permo carboniferous compressive deformation caldera like structures which in some cases affected the entire basin occurred in each basin these caldera like structures seem to follow a similar pattern in three of these basins each taking place when the basin structure was already developed and continental sedimentation had started volcanism caused a high subsidence rate during the most important explosive periods by a mechanism similar to that of caldera collapse but after the subsidence structure has been established moreover a caldera structure formed by the collapse of a central volcanic edifice is also suggested in the western margin of the fourth basin

columbite solubility in granitic melts consequences for the enrichment and fractionation of nb and ta in the earth s crust the behaviour of niobium and tantalum in magmatic processes has been investigated by conducting mnnb2o6 and mnta2o6 solubility experiments in nominally dry to water saturated peralkaline aluminium saturation index a s i 0 64 to peraluminous a s i 1 22 granitic melts at 800 to 1035 c and 800 to 5000 bars the attainment of equilibrium is demonstrated by the concurrence of the solubility products from dissolution crystallization mn doped and nb or ta doped experiments at the same pressure and temperature the solubility products of mnnb2o6 ksp nb and mnta2o6 ksp ta at 800 c and 2 kbar both increase dramatically with alkali contents in water saturated peralkaline melts they range from 1 2 10 4 and 2 6 10 4 mol2 kg2 respectively in subaluminous melt a s i 1 02 to 202 10 4 and 255 10 4 mol2 kg2 respectively in peralkaline melt a s i 0 64 this increase from the subaluminous composition can be explained by five non bridging oxygens being required for each excess atom of nb5 or ta5 that is dissolved into the melt the ksp nb and ksp ta also increase weakly with al content in peraluminous melts ranging up to 1 7 10 4 and 4 6 10 4 mol2 kg2 respectively in the a s i 1 22 composition columbite tantalite solubilities in subaluminous and peraluminous melts a s i 1 02 and 1 22 are strongly temperature dependent increasing by a factor of 10 to 20 from 800 to 1035 c by contrast columbite tantalite solubility in the peralkaline composition a s i 0 64 is only weakly temperature dependent increasing by a factor of less than 3 over the same temperature range similarly ksp nb and ksp ta increase by more than two orders of magnitude with the first 3 wt h2o added to the a s i 1 02 and 1 22 compositions whereas there is no detectable change in solubility for the a s i 0 64 composition over the same range of water contents solubilities are only slightly dependent on pressure over the range 800 to 5000 bars the data for water saturated sub and peraluminous granites have been extrapolated to 600 c conditions at which pegmatites and highly evolved granites may crystallize using a melt concentration of 0 05 wt mno 70 to 100 ppm nb or 500 to 1400 ppm ta are required for manganocolumbite and manganotantalite saturation respectively the solubility data are also used to model the fractionation of nb and ta between rutile and silicate melts predicted rutile melt partition coefficients increase by about two orders of magnitude from peralkaline to peraluminous granitic compositions it is demonstrated that the nb2o5 ta2o5 activity coefficient ratio in the melt phase depends on melt composition this ratio is estimated to decrease by a factor of 4 to 5 from andesitic to peraluminous granitic melt compositions accordingly all the relevant accessory phases in subaluminous to peraluminous granites are predicted to incorporate nb preferentially over ta this explains the enrichment of ta over nb observed in highly fractionated granitic rocks and in the continental crust in general

low temperature magnetic anisotropy in micas and chlorite abstract phyllosilicates such as micas and chlorite are common rock forming minerals and often show preferred orientation in deformed rocks in combination with single crystal anisotropy this leads to anisotropy of physical properties in the rock such as magnetic susceptibility in order to effectively use the magnetic anisotropy to understand a rock fabric it is necessary to identify the minerals responsible for the magnetic anisotropy techniques have been developed to separate contributions of the ferrimagnetic antiferromagnetic paramagnetic and diamagnetic susceptibilities to the anisotropy of magnetic susceptibility because diamagnetic and paramagnetic susceptibility are both linearly dependent on field separation of the anisotropic contributions requires understanding how the degree of anisotropy of the paramagnetic susceptibility changes as a function of temperature note that diamagnetic susceptibility is not dependent on temperature the increase in paramagnetic anisotropy at low temperature is used to separate the paramagnetic and diamagnetic subfabrics and can be expressed by the p77 factor in this study we determined p77 which is the change in the degree of anisotropy k between room temperature 298 k and liquid nitrogen temperature 77 k for a series of micas and chlorite the paramagnetic susceptibility ellipsoid is highly oblate with the minimum principal susceptibility normal to the silicate layers at both 77 k and rt the degree of anisotropy k increases by a factor of approximately 6 3 8 7 for individual samples of muscovite phlogopite and chlorite on cooling from rt to 77 k and between 11 2 and 12 4 for biotite a decrease in temperature enhances the paramagnetic anisotropy in a mineral biotite exhibits a relatively stronger enhancement due to the onset of magnetic ordering below 100 k this can have important implications for interpreting low temperature anisotropy in mudstones mica schists and gneisses

palaeoenvironmental reconstruction and early permian ichnoassemblage from the ne iberian peninsula pyrenean basin this work was supported by the synthesys project e m de taf 2560 at mfn and fr taf 3621 and fr taf 4808 at mnhn http www synthesys info and secretaria d universitats i de recerca del departament d economia i coneixement de la generalitat de catalunya e m expedient number 2013 ctp 00013 at ise m for visits to collections e m received financial support from the pif grant of the geology department at uab field works have been developed on the project vertebrats del permia i el triasic de catalunya i el seu context geologic and evolucio dels ecosistemes amb faunes de vertebrats del permia i el triasic de catalunya ref 2014 100606 based at institut catala de paleontologia and we acknowledge the financial support of the departament de cultura generalitat de catalunya

a middle miocene 13 5 12 ma deformational event constrained by volcanism along the puna eastern cordillera border nw argentina abstract the features of middle miocene deposits in the puna eastern cordillera transition valles calchaquies indicate that cenozoic deformation sedimentation and volcanism follow a complex spatiotemporal relationship the intense volcanic activity recorded in the eastern puna border between 14 and 11 5 ma coincides with the occurrence of one of the most important deformation events of the neogene tectonic evolution in the region studies performed across the puna eastern cordillera transition show different relationships between volcanic deposits of ca 13 5 12 1 ma and the oligocene miocene angastaco formation in this paper we describe the ash flow tuff deposits which are the first of this type found concordant in the sedimentary fill of valles calchaquies several analyses performed on these pyroclastic deposits allow a correlation to be made with the alto de las lagunas ignimbrite ca 13 5 ma of the pucarilla cerro tipillas volcanic complex located in the puna outcrops of the ca 13 5 ma pyroclastic deposits are recognised within the puna and the valle calchaqui however in the southern prolongation of the valle de hualfin tiopampa pucarilla depression that separates the puna from the valle calchaqui at these latitudes these deposits are partially eroded and buried and thus their occurrence is recorded only by abundant volcanic clasts included in conglomerates of the angastaco formation the sedimentation of the angastaco formation was aborted at ca 12 ma in the tiopampa pucarilla depression by the pucarilla ignimbrite which unconformably covers the synorogenic units on the contrary in the valle calchaqui the sedimentation of the angastaco formation continued until the late miocene the different relationships between the miocene angastaco formation and the ignimbrites with ages of ca 13 5 and ca 12 ma reveal that in this short period 1 5 m y a significant deformation event took place and resulted in marked palaeogeographic changes as evidenced by stratigraphic sedimentological and chronological records in the angastaco formation

magnetic anisotropy in the trenton limestone results of a new technique anisotropy of anhysteretic susceptibility a new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described the magnetic fabric of a collection of trenton limestone specimens has been determined using this method as well as by conventional anisotropy of magnetic susceptibility the results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the trenton in which the bulk magnetic susceptibilities are low a model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses the original fabric is inferred to have been isotropic the anisotropy resides in secondary magnetite of late paleozoic age it is argued that the observed magnetic fabric must therefore be alleghenian or younger in age our method has the potential to determine paleostress directions in carbonates elsewhere provided our assumptions are correct

in situ analyses of micas in the yashan granite south china constraints on magmatic and hydrothermal evolutions of w and ta nb bearing granites article i nfo article history most rare metal granites in south china host major w deposits with few or without ta nb mineralization however the yashan graniticpluton located inthe yichun areaof western jiangxiprovince south china hosts a major nb ta deposit with minor w mineralization it is thus important for understanding the diversity of w and nb ta mineralization associated with rare metal granites the yashan pluton consists of multi stage intrusive units including the protolithionite muscovite granite li mica granite and topaz lepidolite granite from the early to late stages bulk rock ree contents and la yb ratios decrease from protolithionite granite to li mica granite to topaz lepidolite granite suggesting the dominant plagioclase fractionation this variation together with increasing li rb cs and ta but decreasing nb ta and zr hf ratios is consistent with the magmatic evolution in the yashan pluton micas are protolithionite muscovite li mica and lepidolite and zircons show wide concentration ranges of zro2 h fo2 u o2 t ho2 y 2o3 and p2o5 compositional variations of minerals such as increasing f rb and li in mica and increasing hf u and p in zircon are also in concert with the magmatic evolution from protolithionite granite to li mica granite to topaz lepidolite granite the most evolved topaz lepidolite granite has the highest bulk rock li rb cs f and p contents consistent with the highest contents of these elements and the lowest nb ta ratio in mica and the lowest zr hf ratio in zircon ta nb enrichment was closely related to the enrichment of volatile elements i e li f and p in the melt during magmatic evolution which raised the proportion of non bridging oxygens nbos in the melt the rims of zoned micas in the li mica and topaz lepidolite granites contain lower rb cs nb and ta and much lower f and w than the cores and or mantles indicating an exotic aqueous fluid during hydrothermal evolution some columbite group minerals may have formed from exotic aqueous fluids which were originally depleted in f rb cs nb ta and w but such fluids were not responsible for ta nb enrichment in the yashan granite the interaction of hydrothermal fluids with previously existing micas may have played an important role in leaching concentrating and transporting w fe and ti ta nb enrichment was associated with highly evolved magmas but w mineralization is closely related to hydrothermal fluid thus these magmatic and hydrothermal processes explain the diversity of w and ta nb mineralizations in the rare metal granites

subaqueous pyroclastic flows and ignimbrites an assessment an assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary hot gas rich pyroclastic flows mass flows which resulted from the transformation of gassupported flows into water supported ones and secondary mass flows carrying redeposited pyroclastic debris based on subaerial pyroclastic flows the term pyroclastic flow should be restricted to demonstrably hot gas rich mass flows of pyroclastic debris using this definition very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described in the majority of these cases the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate the only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the ordovician nearshore shallow marine ignimbrites of ireland and wales and miocene ignimbrites of southwest japan resulting from the passage of subaerially erupted pyroclastic flows into shallow water other possible examples are near vent dense clast deposits in the donzurobo formation of japan possible submarine intra caldera ponded ignimbrite successions in california and wales and near vent pumiceous deposits of ramsay island wales all other purported cases are either clearly the result of water supported mass flow transportation and deposition debris avalanches debris flows turbidity currents or lack adequate supporting evidence regarding the heat state or the palaeoenvironment only the shallow marine ignimbrites of ireland and wales show adequate evidence of welding but even these could have been nearly wholly exposed above sea level when welding occurred we conclude that when pyroclastic flows enter water they are generally disrupted explosively and or ingest water and transform into water supported mass flows and we suggest the various scenarios in which this occurs there is no evidence to suggest that welding in wholly subaqueous environments is common

the effect of rock fabric on p wave velocity distribution in amphibolites abstract this study presents contribution to the laboratory investigation of elastic properties and rock fabric of amphibolites p wave velocity was determined on four spherical samples prepared from a shallow borehole core the measurement was conducted in 132 directions under various conditions of hydrostatic pressure up to 400 mpa the rock fabric was investigated by image analysis of thin sections that enabled precise determination of grain size modal composition and shape parameters of rock forming minerals laboratory measurement of p waves revealed pseudoorthorhombic symmetry of rock fabric in amphibolites studied this symmetry reflects rocks macro and microfabric maximum p wave velocity corresponds to the macroscopically visible stretching lineation minimum p wave velocity is oriented perpendicular to the foliation plane the average grain size is the main microstructural factor controlling mean p wave velocity

glacial or non glacial origin for the bigganjargga tillite finnmark northern norway bigganjargga is the classic locality for the varangian ice age in northern scandinavia the presence of two sets of striations on the underlying quartzite basement has been taken as evidence for a glacial origin of the bigganjargga diamictite however soft sediment surface structures on the underlying sandstone indicate that it was not consolidated when the diamictite was emplaced together with structures in the diamictite this shows that the deposit formed as a debris flow moving northwestwards over an unconsolidated substrate there is no evidence for glacial origin or contribution for the diamictite material

fiamme formed by diagenesis and burial compaction in soils and subaqeuous sediments pumice clasts can be diagenetically altered at low temperature and flattened by compaction to form fiamme examples are described from trachytic pumice fall layers alternating with palaeosoils on sao miguel in the azores and from ordovician lacustrine volcaniclastic sedimentary rocks from the english lake district

the late palaeozoic early mesozoic from the catalan pyrenees spain 60 myr of environmental evolution in the frame of the western peri tethyan palaeogeography the eastern catalan pyrenees form the southernmost segment of the pyrenean axial zone they experienced complex multistage process as the result of both the variscan and the pyrenean orogenic cycles after the late palaeozoic several small extensional sub basins were filled by continental successions as a response to the changing tectonic setting within the frame of the southwestern europe this stratigraphic record perfectly exposed in the studied eastern pyrenean areas preserves the signatures of the late palaeozoic pangea break up during the progressive dismantling of the variscan chain up to the triassic diffused extension despite structural network being severely affected by cenozoic reworking the stratigraphic framework can be unravelled by the detailed sedimentological and stratigraphical analysis of the well exposed sequences in short this paper addresses facies patterns and architectural elements of late carboniferous permian triassic continental sections studied in the catalan pyrenees and aims to use these signatures for correlation not only at a regional scale but also within the broader picture of the southwestern europe our stratigraphical reconstruction clearly demonstrates that the thick studied succession consists of fluvial lacustrine basal units affected by intense volcanic activity and overlaying predominantly fluvial fining upwards units for each unit several lithofacies architectural elements and bounding surfaces of different order have been identified and their lateral and vertical staking characterized deposition and erosion processes are controlled by alternating periods of tectonic activity producing different stages of high and low subsidence the uplift of the sub basin margins and subsequent extended erosive events marked by angular unconformities on the strength of the major rank bounding surfaces analysis the studied sedimentary packages have been grouped into different tectono stratigraphic units and then correlated they have been contextualized within the latest palaeogeographic reconstructions of the studied sector contributing to detail the role of the study area within the frame of the s european palaeogeography

geochemical criteria for distinguishing magmatic and metasomatic albite enrichment in granitoids examples from the ta li granite yichun china and the sn w deposit tikus indonesia two examples of albite rich granitoids of different genesis are discussed the ta li granite at yichun china exhibits dominantly igneous textures and the sodium enrichment zone in biotite granite of the sn w deposit at tikus is characterized by metasomatic textures the yichun stock shows a magmatic evolution from biotite muscovite granite to lepidolite granite with an increase of na2o and f concentrations and a decrease of cao and sr concentrations the composition of the different granite phases is close to pseudoternary minima in the haplogranite h2o li f system the composition of the f rich albitization zone at tikus deviates strongly from the pseudoternary minima and shows a tendency to become monomineralic nearly pure albitite cao and sr concentrations are increased or remain unchanged with respect to the unaltered biotite granite at tikus albitization produced by fluid rock interaction in f rich systems is characterized by that cao or sr released by the breakdown of plagioclase in the granite protolith are fixed in newly formed fluorite or albite respectively

stratigraphy petrography and dispersion of the lower permian syn eruptive deposits in the viar basin spain abstract this paper presents a stratigraphic petrographic and sedimentologic analysis of felsic syn eruptive deposits of the southernmost permian basin in europe viar basin an inner orogenic transtensional half graben located between two major geotectonic zones of the iberian variscan belt sw spain the syn eruptive deposits of the viar basin represent a mappable lithostratigraphic unit that is herein named the los canchales formation this unit forms a wedge shaped body bounded by alluvial fluvial deposits red layers that are in gradational contact the stratigraphic architecture of the los canchales formation is complex with frequent facies changes and interfingering among pyroclastic rocks resedimented volcaniclastic rocks and sedimentary rocks abundant fossil flora is also recorded a wax wane cycle of volcanic activity is recognized during the syn eruptive period two short lived volcanic episodes were recorded including ignimbrites and a possible phreatomagmatic event explosive volcanism generated pyroclastic flow surge and ash fall deposits in cases collapsing the drainage network they also modified the vegetation and created a new depositional setting with four successive sedimentary environments vegetated floodplain ephemeral sandy braidplain palustrine environment and again vegetated floodplain input of pyroclasts followed a nw to se longitudinal axial dispersal pattern

magma tectonic interaction and the eruption of silicic batholiths abstract due to its unfavorable rheology magma with crystallinity exceeding about 50 vol and effective viscosity 106 pa s is generally perceived to stall in the earth s crust rather than to erupt there is however irrefutable evidence for colossal eruption of batholithic magma bodies and here we analyze four examples from spain mexico usa and the central andes these silicic caldera forming eruptions generated deposits characterized by i ignimbrites containing crystal rich pumice ii co ignimbritic lag breccias and iii the absence of initial fall out the field evidence is inconsistent with most caldera forming deposits which are underlain by initial fall out indicating deposition from a sustained eruption column before the actual collapse sequence in contrast the documented examples suggest early deep level fragmentation at the onset of eruption and repeated column collapse generating eruption volumes on the order of hundreds of cubic kilometers almost exclusively in the form of ignimbrites these examples challenge our understanding of magma eruptability and eruption initiation processes in this paper we present an analysis of eruption promoters from geologic theoretical and experimental considerations assessing relevant dynamics and timescales for failure of crystal melt mush we propose a framework to explain eruption of batholithic magma bodies that primarily involves an external trigger by near field seismicity and crustal failure strain rate analysis for dynamic and static stressing chamber roof collapse and rapid decompression indicates that large solid like silicic reservoirs may undergo catastrophic failure leading to deep level fragmentation of batholithic magma at approximately 2 orders of magnitude lower strain rates than those characteristic for failure of crystal poor magmas or pure melt eruption triggers can thus include either amplified pressure transients in the liquid phase during seismic shaking of a crystal melt mush decompression by block subsidence or a combination of both we find that the window of opportunity for the eruption of large silicic bodies may thus extent to crystallinities beyond 50 vol for strain rates on the order of 10 3 to 10 4 s 1

a 20 m y long lived successive mineralization in the giant dahutang w cu mo deposit south china abstract the connection between prolonged granitic magmatism and the formation of giant tungsten w polymetallic deposits has long been disputed in this study we present 6 mica ar ar plateau ages and 22 molybdenite re os model ages data on the newly discovered giant dahutang w cu mo deposit in south china which is one of the largest w deposits in the world new and published zircon u pb mica ar ar and molybdenite re os age data reveal that the mesozoic dahutang magmatism and mineralization occurred in two major periods 1 the late jurassic ca 153 147 ma forming the hydrothermal breccia large wolframite bearing quartz vein and scheelite dominated disseminated veinlet type orebodies which is mainly associated with the emplacement of porphyritic biotite granite 2 the late jurassic to early cretaceous ca 146 130 ma forming the cu mo w sn mineralization overprinting the late jurassic w mo cu orebodies which is mainly related to the successively emplacement of the early cretaceous granites we suggest that continuous accumulation of mineralization for a long period of time 151 130 ma have contributed to the formation of the giant dahutang deposit

moist convection and the injection of volcanic ash into the atmosphere if unsaturated water vapor is carried upward by a volcanic eruption column it may eventually become saturated owing to the decrease in temperature of the column as it expands through decompression and transfers heat to entrained air heat released as a result of the subsequent condensation of water vapor causes the air within the column to expand we show that this increases the buoyancy and therefore the total height of rise of the column the increase in height is significant in relatively small sub plinian and strombolian eruptions in which mass eruption rates lie in the range 103 to 106 kg s in such eruptions the latent heat released as the entrained water vapor condenses may provide the main source of heat which drives the ash and clasts upward the height of rise then becomes relatively insensitive to the mass flux erupted at the vent and depends primarily upon the vapor loading of the atmosphere in a moist atmosphere ash may rise several kilomtres higher than in an eruption of comparable strength in a dry environment moist convection leads to much wider ash dispersal particularly from very small eruptions subsequently rain flushing of ash from umbrella clouds may result from the water which forms through the condensation of entrained vapor the ash provides natural condensation nuclei for some of this entrained vapor whose mass may be much greater than that of the ash in small columns 107 kg s the latent heat released by condensation of vapor is relatively small in comparison with the thermal energy provided by the hot clasts and therefore moisture has no significant effect upon the eruption column dynamics furthermore the mass of water vapor originating from the erupted volatiles is usually comparable to or greater than that entrained from the ambient air our model also shows that if the erupting mixture becomes buoyant then the eruption columns associated with phreatomagmatic eruptions ascend nearly as high as plinian columns with the same mass eruption rate this is because the water which is vaporized by the hot ash at the source condenses higher in the column and thereby restores its latent heat to the ascending ash

magnetic anisotropy rock fabrics and finite strain in deformed sediments of sw sardinia italy abstract although some empirical relationships have been established between the anisotropy of magnetic susceptibility ams and the state of finite strain in studies from specific geologic areas the mechanisms governing correlations are not well understood a comparative study has been made in order to elucidate the ams strain relationship by investigating mineral preferred orientations and the responsible microstructural deformation processes detailed analyses are presented from low grade metamorphic slates of a palaeozoic sequence exposed in the variscan of sw sardinia rock magnetic experiments suggest that paramagnetic minerals dominate the ams in these rocks excellent agreement was found between magnetic fabrics and lattice preferred orientations 001 of mica and chlorite determined from x ray texture goniometry measurements depending on the oblate or prolate ellipsoidal shape of anisotropy the minimum principal axes of ams and x ray textures can coincide with the poles to slaty cleavage or the maximum principal axes can coincide with the bedding cleavage intersection respectively the measured ams and x ray textures only partially reflect the finite strain determined from deformed micro pebbles and reduction spots axial orientations of magnetic fabrics mica and chlorite preferred orientations and finite strain ellipsoids are generally in good agreement depending on development of oblate or prolate fabric ellipsoidal shapes linear correlations can be established for magnitudes of minimum and intermediate axes of the ams and finite strain ellipsoids but shape comparisons are complex microstructural studies by scanning electron microscopy sem indicate that the preferred orientation of the phyllosilicates is caused by different grain scale deformation mechanisms such as kinking micro folding and preferential growth thus magnetic fabrics and mineral preferred orientations evidently result from different deformation mechanisms than the finite strain they reflect the heterogeneous deformation due to micro mechanical reorientation processes of mineral grains while finite strain reflects the homogeneous deformation accommodated cumulatively by the strain markers

inverse magnetic fabric in deformed limestones of the morcles nappe switzerland anisotropy of magnetic susceptibility ams was measured at 15 sites in deformed helvetic limestones of the morcles nappe in western switzerland inverse magnetic fabrics characterized by axes of maximum susceptibility subperpendicular to bedding or cleavage were observed at four sites in the root zone of the nappe acquisition and thermal demagnetization of isothermal remanent magnetization show that the ferromagnetic mineralogy is dominated by pyrrhotite with various proportions of magnetite hematite and goethite the matrix minerals were studied in thin section with a staining technique to distinguish different carbonates by x ray texture goniometry and by microprobe analysis observations of ams at room and liquid nitrogen temperatures and of the temperature dependence of bulk susceptibility lead to the conclusion that the inverse fabrics result from a paramagnetic component due to fe2 ions in the calcite matrix

volcanic and tectonic controls of lacustrine and alluvial sedimentation in the stephanian coal bearing sequence of the malpas sort basin catalonian pyrenees the infill of a small variscan basin shows a complex succession of lacustrine and alluvial sediments whose pattern of distribution through time appears to have changed in response to contemporaneous volcanism following initial deposition of pyroclastic breccias and extrusion of lavas small lakes were established on the floor of the basin their infills were probably locally controlled by relief on top of the lavas but the pattern was disrupted by phases of mass movement from the basin margin which emplaced debris flows and a sedimentary melange containing large blocks of limestone and led to extensive soft sediment deformation this complex sequence of events culminated in the emplacement of a widespread ash flow bed an alluvial plain established during a period of rather uniform subsidence was rapidly drowned to give an extensive lake whose progradational infill led to re establishment of an alluvial plain the deposition of a widespread airfall tuff coincided with the onset of differential subsidence during which major channel sandstones were stacked in association with poorly drained palaeosols in the east of the area whilst a mudstone dominated sequence characterized by well drained palaeosols developed in the west a return to more uniform subsidence appears to have coincided with the cessation of volcanic activity

magmatism in the intracratonic central iberian basins during the permian palaeoenvironmental consequences abstract recent studies have confirmed the important role played by magmatism during the late carboniferous permian transition in this report we describe the characteristics of this magmatism in the central iberian realm iberian ranges and eastern spanish central system and discuss its palaeoenvironmental impact in the studied area the late variscan transtensional to extensional tectonic regime favoured the emplacement of a calc alkaline magmatism during the late stephanian permian this magmatism is expressed both as pyroclastic rocks filling small half graben basins and hypabyssal intrusions sills and dykes pyroxenic and amphibolic andesites are the main rock compositions while basalt dacite and rhyolite are less represented the available k ar isotopic data suggests a lower permian age for this magmatism also supported by its complete macro and microflora content and by features related to its emplacement hypabyssal intrusions crosscut stephanian rocks and are in turn overlain by late permian triassic sediments frequent xenoliths of middle and lower crustal levels indicate considerable magma crust interaction this interaction may have led to substantial crustal assimilation during the first emplacement stages in conditions of a low extension rate crustal contamination barely affected the final magmatic products emplaced under conditions of greater extension the palaeoecological effects of explosive manifestations of this volcanism are expressed in the varied macroflora record in volcaniclastic basins the number and volume of hypabyssal intrusions suggest that the original geological record of explosive activity in these basins was much greater and was reduced by subsequent erosion processes

estimates of flow direction for calc alkaline welded tuffs and paleomagnetic data reliability from anisotropy of magnetic susceptibility measurements central san juan mountains southwest colorado abstract flow directions are estimated from the measurement of the magnetic fabric of 106 samples collected at 18 sites in four welded tuff units in the central san juan mountains of southern colorado the estimates assume that the tuffs generally flowed directly away from the extrusive vents and that the lineations of magnetic grains within the tuffs represent the flow direction at individual sites errors in the estimation may arise from topographic variation rheomorphism post emplacement mass flow within the tuff and other factors magnetic lineation is defined as the site mean anisotropy of magnetic susceptibility maximum azimuth a test on the flow directions for individual units is based on the projection of lineation azimuths and their intersection within or near the known source caldera for the tuff this test is positive for the four units examined paleomagnetic results for these tuffs are probably reliable indicators of the geomagnetic field direction in southwest colorado during the time 28 2 26 5 ma of emplacement

electric potential gradient changes during explosive activity at sakurajima volcano japan we report electric potential gradient measurements carried out at sakurajima volcano in japan during 1 explosions which generated ash plumes 2 steam explosions which produced plumes of condensing gases and 3 periods of ashfall and plume induced acid rainfall sequential positive and negative deviations occurred during explosions which generated ash plumes however no deflections from background were found during steam explosions during periods of ashfall negative electric potential gradients were observed while positive potential gradients occurred during fallout of plume induced acid rain from the same eruption these results suggest that a dipole arrangement of charge develops within plumes such that positive charges dominate in the volcanic gas rich top and negative charges in the following ash rich part of the plume the charge polarity may be reversed for other volcanoes hatakeyama and uchikawa 1952 we suggest that charge is generated by fracto emission donaldson et al 1988 processes probably during magma fragmentation within the vent rather than by frictional effects within the plume

pencil structure and strain in weakly deformed mudstone and siltstone abstract weakly deformed mudstone and siltstone middle ordovician knobs formation of the appalachian valley and ridge province south western virginia u s a show strain dependent transitions between bedding fissility pencil structure and cleavage pencil structures are associated with a bulk inhomogeneous shortening deformation where minimum principal strain z ranges between 9 and 26 shortening assuming a plane strain and constant volume where strains are less bedding fissility dominates pencil fragments are defined by intersecting fracture sets subparallel to the pre existing bedding fissility and cleavage their long axes are both parallel to the bedding cleavage intersection and to the inferred y axis of the tectonic strain ellipsoid pencil development is considered to result from fracturing along both fabric anisotropies during weathering and post tectonic stress relaxation pencils show variations in size and shape depending on lithology grain size and composition degree of initial clay preferred orientation degree of cleavage development type of cleavage total bulk strain and degree of strain homogeneity their shape l w however is a direct measure of total y z strain since strain determinations from chlorite pressure fringes on framboidal pyrite within the pencil fragments give y z 0 913 0 019 l w pencil structure is therefore a potential strain marker in weakly deformed rocks

facies and preservation of glaciogenic sequences from the varanger ice age in scandinavia and other parts of the north atlantic region abstract thickness and facies variations and state of preservation of varangerian glacial sequences are related to the geotectonic settings of the depositional environments from stable continental platform sites only thin remnants of a single glacial advance retreat sequence are preserved in fault basins thickness and lithofacies vary in response to fault movements in the continental platform to shelf region the sequence ranges from thin beds of one single glacial event on the craton to up to some hundred meterswith several glacial advance retreat cycles from shelf sites the stable and slowly subsiding shelf environment has given rise mainly to a thin sequence of diamictite overlain by mudstone in more rapidly subsiding shelves several glacial advance retreat units are preserved together with interstadial and interglacial sediments with total thickness up to several hundred meters the shelf slope environment is probably represented by up to 3000 m thick diamictites the preservation potential during the glacial epoch is primarily controlled by the property of glacier ice sheets to be either erosive or depositional the long term preservation is determined by various factors controlling relative sea level changes the state of preservation during orogenic processes generally decreases from sequences on platform sites to those on distal shelves and shelf slopes

differentiated rare element mineralization in an ongonite topazite composite dike at the xianghualing tin district southern china an electron microprobe study on the evolution from niobium tantalum oxides to cassiterite our study characterizes in detail the mineralogical textural and compositional features of a highly evolved composite ongonite topazite dike and its magmatic differentiation history we present compositional data collected by established techniques i e by electron microprobe and wet chemical analysis which provide a detailed framework for future studies that employ state of the art analytical techniques the studied dike referred to as the no 431 dike crops out within the xianghualing area in the nanling range of southern china in close spatial association with jurassic sn nb ta granite plutons the rock samples in the no 431 dike were collected from a structurally lower drill hole and a trench at higher level the ongonite is encountered throughout the dike but the topazite is only revealed along the margin of the upper near surface dike the results of whole rock major and trace element analyses show that the rocks of the no 431 dike are strongly peraluminous with an average acnk value of 1 5 for ongonite and 3 9 for topazite they are enriched in f 1 7 wt and 5 4 wt on average for ongonite and topazite respectively the rocks have low zr hf and nb ta ratios and high levels of ore forming elements including nb ta sn and w silicate and oxide mineral assemblages textures and compositions are also distinct for the two rock types studied in the lower ongonite of the dike there are abundant phenocrysts of k feldspar quartz and albite and microphenocrysts of topaz and zinnwaldite in a matrix dominated by quartz k feldspar and albite characteristic oxides are columbite tantalite tapiolite and microlite but cassiterite is absent the upper ongonite of the dike has a silicate assemblage similar to the lower ongonite columbite mn uranomicrolite and limited amounts of cassiterite are the dominant accessory minerals the topazite is characterized by large amounts of topaz and zinnwaldite intergrown with quartz while k feldspar albite and quartz phenocrysts have rounded shapes and are relatively rare cassiterite is the most abundant ore mineral while nb ta oxide minerals are less abundant we interpret the whole rock compositional trends mineral textures assemblages and compositions to reflect the differentiation of an evolved initially homogeneous magma that separated into aluminosilicate and hydrosaline melts corresponding to crystallization of ongonite and topazite respectively the crystallization of nb ta and sn bearing ore minerals was strongly controlled by the separation of the two melt phases we hypothesize that dike propagation widening subsequent to the initial dike emplacement may have driven the separation of the aluminosilicate and hydrosaline melt phases that crystallized to ongonite in the core and topazite along the margins of the structurally higher part of the dike

magnetic fabric interpretation complicated by inclusions in mafic silicates abstract the expected relationships between anisotropy of low field magnetic susceptibility ams and crystal symmetry of mafic silicates are disturbed by the presence of magnetite inclusions measurements of ams anisotropy of anhysteretic remanence aarm and theoretically predicted bulk susceptibilities from chemical composition all favour the exercise of great caution in the interpretation of preferred orientation distributions of silicates from a rock s ams these results pertain mainly to the mafic silicates of lower crustal rocks pyroxene orthopyroxene amphibole and some of their low grade metamorphic alterations serpentine epidote

identification of multicomponent anisotropies in rocks using various field and temperature values in a cryogenic magnetometer abstract a new technique involving magnetization measurements on a rotating sample in a cryogenic magnetometer with variable field from 0 to 4 t and variable temperature is presented it can be used to separate and identify multi component anisotropy in rocks the results obtained for anisotropy due to magnetite haematite pyrrhotite or phyllosilicates are interpreted according to different magnetization models

submarine crystal tuffs their origin using a lower devonian example from southeastern australia summary the submarine crystal rich volcaniclastics of the lower devonian merrions tuff contain concentrations of angular to euhedral volcanic quartz plagioclase and orthoclase lithic fragments largely altered vitriclasts are minor components and the average matrix content is 37 6 associated dacite andesite and rhyodacitic lavas have average groundmass contents of 64 2 rare shards in the graded pelitic tops of the thick volcaniclastic sedimentation units suggest that the mode of fragmentation originally was by explosive magmatic eruptions the sedimentology of the volcaniclastics suggests subsequent redeposition by cold state mass flow processes the volcaniclastics form an isotopically coherent suite and so redeposition must have occurred essentially contemporaneously with eruption the high crystal fragment concentration in these volcaniclastics is higher than lavas and ignimbrites and suggests some process whereby the groundmass fraction of the erupting magma is selectively removed so concentrating the crystal fraction the crystal rich character of the crystal tuffs is not simply due to explosive eruption several primary and secondary factors processes could have interacted to ultimately produce the crystal rich character these being i eruption of highly crystallized magmas 65 phenocrysts ii concentration of crystals in primary eruption columns iii concentration of crystals in any resulting pyroclastic flows fine vitric ashes being elutriated out and being carried away in accompanying trailing ash clouds iv concentration of crystals in secondary eruption columns generated by the flow of hot pyroclastic flows into the ocean and v concentration of crystals by the elutriation of fines into the trailing fine sediment cloud accompanying submarine mass flows resulting from the slumping of volcaniclastic aggregates from shallow marine subaerial settings

thermal enhancement of magnetic fabric in cretaceous sandstones heating of samples of the san juan cretaceous sandstones ne venezuela results in large increases in their low field susceptibility mostly above 650 c which allows measurement of their low field susceptibility ellipsoids where measurements are available before and after heating the same fabric is almost always enhanced samples previously too weak for measurement showed thermally induced fabrics which are consistent within each site but vary between different localities the susceptibility changes are all largely attributed to magnetite formation probably from vermiculite at 450 c in the proximal sediments and from illite at 850 c in all areas the induced magnetic fabrics can be interpreted in terms of either sedimentary structures or low grade metamorphism related to cleavage parallel to the fold axes it is considered that this thermal enhancement technique offers a method for measuring low field magnetic fabrics in a wider range of geological conditions and lithologies than reviously although the processes involved need considerable study

genesis of crystal rich epiclastic rocks from subaqueous silicic lava domes role of thermal shock on quartz phenocrysts abstract deposits in the palaeozoic marine basin of the sarrabus region se sardinia western mediterranean italy show the presence of rhyolitic micropillow lithofacies in the uppermost sector of extrusive subaqueous acid domes these particular volcanic rocks contain quartz phenocrysts affected by curved fractures attributed to thermal shock produced in the magma water interface the flanks of subaqueous domes contain proximal epiclastic sandstones and greywackes characterized by the presence of non mature angular quartz clasts very similar to the occurrences detected in the rhyolitic micropillows therefore a syngenetic referred to acid domes and proximal unreworked character is inferred for these epiclastic deposits a genetic model is proposed to explain the association of lithofacies volcanic and epiclastic in direct relationship with the subaqueous growth and erosion of the domes this process of genesis of texturally unmature epiclastic rocks not related to explosive acid volcanism seems rather new and the conclusions of the paper may be in sharp contrast with some of the criteria previously described for distinguishing between pyroclastic and epiclastic crystal rich rocks

solubility of manganotantalite zircon and hafnon in highly fluxed peralkaline to peraluminous pegmatitic melts the behavior of tantalum and zirconium in pegmatitic systems has been investigated through the determination of ta and zr solubilities at manganotantalite and zircon saturation from dissolution and crystallization experiments in hydrous li f p and b bearing pegmatitic melts the pegmatitic melts are synthetic and enriched in flux elements 0 7 1 3 wt li2o 2 5 5 wt f 2 8 4 wt p2o5 and 0 2 8 wt b2o3 and their aluminum saturation index ranges from peralkaline to peraluminous asili al na k li 0 8 to 1 3 with various k na ratios dissolution and crystallization experiments were conducted at temperatures varying between 700 and 1 150 c at 200 mpa and nearly water saturated conditions for dissolution experiments pure synthetic end member manganotantalite and zircon were used in order to avoid problems with slow solid state kinetics but additional experiments using natural manganotantalite and zircon of relatively pure composition i e close to end member composition displayed similar solubility results zircon and manganotantalite solubilities considerably increase from peraluminous to peralkaline compositions and are more sensitive to changes in temperature or asi of the melt than to flux content a model relating the enthalpy of dissolution of manganotantalite to the asili of the melt is proposed hdiss kj mol 304 asili 176 in the peralkaline field and hdiss kj mol 111 asili 245 in the peraluminous field the solubility data reveal a small but detectable competitivity between zr and ta in the melt i e lower amounts of zr are incorporated in a ta bearing melt compared to a ta free melt under the same conditions a similar behavior is observed for hf and ta the competitivity between zr or hf and ta increases from peraluminous to peralkaline compositions and suggests that ta is preferentially bonded to non bridging oxygens nbos with al as first neighbors whereas zr is preferentially bonded to nbos formed by excess alkalies as a consequence zr ta ratios when buffered by zircon and manganotantalite simultaneously are higher in peralkaline melts than in peraluminous melts

low temperature ams and the quantification of subfabrics in deformed rocks abstract we evaluate the application and significance of low temperature anisotropy of magnetic susceptibility lt ams measurements in deformed mudrocks originally conceived as a way to enhance paramagnetic relative to ferromagnetic susceptibility lt ams studies offer significant potential in constraining the coexistence of subfabrics that are due to phyllosilicate grains with different preferred orientations in this study we report a detailed procedure to obtain such directional susceptibilities measuring samples in multiple orientations at liquid nitrogen temperatures in order to determine the lt ams due to unequal changes of magnetic susceptibility in micas at low temperature the enhancement of standard ams at low temperature better separates interacting fabrics in natural rocks particularly depositional fabrics versus deformational fabrics lt ams is a non destructive technique that readily offers an ability to separate ferromagnetic and paramagnetic fabrics and allows the characterization and quantification of multiple fabrics in natural rocks