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

chemical alteration in the micro weathering environment within a spheroidally weathered anorthosite boulder a chemical and mineralogical study of the innermost rindlets of a spheroidally weathered boulder from the wichita mountains oklahoma details incipient alteration of minerals along solution channels alteration products in plagioclase areas are highly aluminous whereas ferruginous residuum is commonly found in olivine alteration zones alteration of plagioclase along solution channels results in sharp weathering contacts as contrasted with wider zones of limonite bordering solution channels in olivine in certain instances cores of fresh olivine are surrounded by limonite indicating that diffusion may play a role in this type of alteration mobility of sesquioxides is evidenced by aluminum rich debris adjacent to mafic materials in solution channels and iron bearing material in channels cutting through plagioclase not resolved is whether the movement of sesquioxides in solution channels is by transport of colloidal particulates or by diffusion of dissolved species calcic plagioclase alters to calcite along the innermost solution channel it is postulated that solution exterior to the boulder and undersaturated with respect to calcite enters the rind system via solution channels cut off from the co2 reservoir of the atmosphere by the narrow confines of this solution channel the solution reacts with the bytownite to precipitate calcite apart from the occurrence of calcite no crystalline weathering products were found in the innermost rindlet samples a poorly crystallized illite was identified in the outermost rindlet these findings suggest that initial alteration products on weathering rinds are amorphous and that with time these products attain a degree of identifiable crystallinity

the relationship between quaternary volcanism in central mexico and the seismicity and structure of subducted ocean lithosphere late quaternary volcanism in central mexico is related to the subduction of young ocean lithosphere at the middle america trench along arc variations in seismicity volcano structure and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab morphological and petrographic characteristics of major volcanoes within the trans mexican volcanic belt tmvb serve to distinguish two calc alkaline subprovinces 1 a western arc averaging 60 km in width associated with aseismic subduction of the rivera plate the main cones of this region are dominated by two pyroxene andesites comprise volumes 70 km 3 and stand less than 3 000 m above sea level 2 a broad central and eastern arc related to subduction of a gently inclined segment of the cocos plate bounded by the rivera transform and the tehuantepec ridge major volcanic edifices possess summit elevations in the range 4 000 to 6 000 m have appropriately larger volumes typically 200 km 3 and are constructed with a high proportion of amphibole bearing lavas the boundary between these subprovinces is marked by a north south oriented structural depression the colima graben and it coincides with a 100 km offset in the volcanic front extensional tectonism in the colima graben accompanied by mixed calc alkaline and alkaline volcanism of potassic affinity is likely related to a hinge type transform fault which marks the cocos rivera plate juncture in the downgoing slab a third segment of ocean floor is presently interacting with continental lithosphere south of the gulf of tehuantepec where quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse cocos noam caribbean triple junction the northern limit of this triple junction is defined by the seismically active isthmus fault which may be related to alkaline volcanic activity at san andres tuxtla a tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate depth earthquakes along the arc is directly dependent upon the age of the subducted slab ocean lithosphere younger than approximately 20 m y is subducted aseismically at convergence rates approaching 9 cm yr the length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m y the tmvb overlies the aseismic extension of this young ocean lithosphere several aspects of this study have a bearing on the segmented nature of converging margins in general 1 the tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction 2 the complete record of volcanism in the tmvb over the past million years can be related to the present plate configuration 3 alkaline and calc alkaline volcanism have developed contemporaneously at a converging plate margin 4 lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab

petrography and provenance of the early permian fluvial warchha sandstone salt range pakistan abstract the warchha sandstone of the salt range of pakistan is a continental succession that accumulated as part of a meandering fluvial system during early permian times several fining upward depositional cycles are developed each of which is composed of conglomerate cross bedded sandstone and in their upper parts bioturbated siltstone and claystone units with distinctive desiccation cracks and carbonate concretions clast lithologies are mainly of plutonic and low grade metamorphic origin with an additional minor sedimentary component textural properties of the sandstone are fine to coarse grained poorly to moderately sorted sub angular to sub rounded and with generally loose packing based on modal analyses the sandstone is dominantly a feldspathoquartzose arkose to sub arkose detrital constituents are mainly composed of monocrystalline quartz feldspars more k feldspar than plagioclase and various types of lithic clasts xrd and sem studies indicate that kaolinite is the dominant clay mineral and that it occurs as both allogenic and authigenic forms however illite illite smectite mixed layer smectite and chlorite are also recognised in both pores and fractures much of the kaolinite was likely derived by the severe chemical weathering of previously deposited basement rocks under the influence of a hot and humid climate transported residual clays deposited as part of the matrix of the warchha sandstone show coherent links with the sandstone petrofacies thereby indicating the same likely origin illite smectite and chlorite mainly occur as detrital minerals and as alteration products of weathered acidic igneous and metamorphic rocks based primarily on fabric relationship the sequence of cement formation in the warchha sandstone is clay generally kaolinite iron oxide calcareous and siliceous material before iron rich illite and occasional mixed layer smectite illite and rare chlorite both petrographic analysis and field characteristics of the sandstone indicate that the source areas were characterised by uplift of a moderate to high relief continental block that was weathered under the influence of hot and humid climatic conditions the rocks weathered from the source areas included primary granites and gneisses together with metamorphic basement rocks and minor amounts of sedimentary rocks regional palaeogeographic reconstructions indicate that much of the warchha sandstone detritus was derived from the aravalli and malani ranges and surrounding areas of the indian craton to the south and southeast before being transported to and deposited within the salt range region under the influence of a semi arid to arid climatic regime

paleogene continental margin truncation in southwestern mexico geochronological evidence the reasons for and mechanisms of continental margin truncation in sw mexico where mesozoic cenozoic plutons are situated directly on the pacific coast are not yet well understood large scale dextral and or sinistral displacements of the continental margin terranes now forming parts of baja california or the chortis block have been proposed the well defined along coast nw se decreasing granitoid intrusion age trend 1 2 cm yr in the 100 ma 40 ma time interval between puerto vallarta and zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the chortis block from those latitudes toward the se changes in the dip and velocity of the nnw sse trending cretaceous tertiary subduction zone resulted in a landward migration of the magmatic arc taking into account certain stratigraphic affinities of chortis and the oaxaca and mixteca terranes together with the known displacement rates along the north america caribbean plate boundary the northwesternmost paleoposition of the chortis block with respect to sw mexico was near zihuatanejo in contrast between zihuatanejo and the isthmus of tehuantepec the cessation of the tertiary magmatism decreased more rapidly 7 7 cm yr although the trend is not so obvious starting in the late eocene chortis moved about 1100 km to the se along a transform boundary associated with the opening of the cayman trough based on our geochronological data and structural relationships between mylonite zones and plutons in the acapulco tehuantepec area we propose an approximately 650 km se movement of chortis from about 40 25 ma with a velocity of 6 5 4 3 cm yr since this is considerably slower than the decreasing age trend obtained by us using the geochronological data we consider batholith formation in this segment to predate and postdate the offshore passage of the north america farallon caribbean triple junction geological observations and paleomagnetic data do not give strong support for large scale right lateral displacements of crustal blocks like the baja california given the isotopic data presented the continental margin truncation in sw mexico seems to be the consequence of an interaction of mechanisms of these we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment on the other hand the lateral removal of material associated with the displacement of chortis is more important in the southeastern segment

chemical processes affecting the mobility of major minor and trace elements during weathering of granitic rocks abstract the behaviour of 38 major and trace elements as well as changes in the mineralogy have been examined in 10 weathering profiles developing on some portuguese granitic rocks element mobilities are calculated from geochemical data normalized with respect to ti in the fresh parent rock chemical elements are divided into two groups immobile and mobile on the basis of their geochemical distribution during weathering elements that are immobile during weathering are zr hf fe al th nb sc and the ree very mobile are ca na p k sr ba rb mg and si mobile elements are derived mainly from leachable minerals such as feldspars micas and apatites whereas immobile elements are either concentrated in resistate phases or strongly adsorbed by secondary minerals the geochemical behaviour of mn cr v fe and ce is very dependent on redox conditions redox transformations of these elements can be used to set limits on the oxidation state of a weathering suite the ree are mobilized or fractionated during late stages of weathering but not during moderate stages of weathering this fractionation is caused by selective leaching of rocks composed of both stable and unstable minerals containing ree

ongonite and topazite dikes in the flying w ranch area tonto basin arizona the petrogenesis of the topaz rich ongonite and topazite dikes in the flying w ranch area tonto basin arizona was investigated and the possible origin of the topaz in the flying w rhyolite is discussed the field textural geochemical and mineralogical characteristics of the topaz bearing dikes in the flying w ranch area all were found to support the genesis of the topaz bearing bodies from a magma the evidence obtained contradicts an origin for these rock types by greisenization or veining

mineralogical changes during intense chemical weathering of sedimentary rocks in bangladesh abstract bangladesh is situated in a subtropical to tropical climatic zone a recently weathered crust has developed on sedimentary bedrock sandstone siltstone shale and claystones of tertiary quaternary age weathered samples were collected from 16 sections totaling 68 samples and were analyzed mineralogically the main primary minerals identified in the weathered crust of sedimentary rocks are quartz plagioclase k feldspar biotite muscovite sparse carbonate and epidote the secondary minerals are kaolinite illite chlorite gibbsite and goethite weathering initiated along the grain boundaries and cleavage planes of the minerals forming small cloudy materials which were very difficult to identify in the advanced stage of weathering these cloudy materials have turned into secondary minerals in region 1 high rain fall 7100 mm yr and monsoonic climate resulted in a kaolinite gibbsite goethite suite through the weathering of feldspars and biotite the occurrence of gibbsite in the relatively elevated lands of sylhet and fe kaolinite throughout the study areas is indicative of a humid tropical climate during formation of the weathered crust

pleistocene to recent alkalic volcanism in the region of sanganguey volcano nayarit mexico forty five cinder cones and associated lava flows have erupted within the last 300 000 years along five parallel lines through the calc alkaline volcano sanganguey in the northwestern segment of the mexican volcanic belt lavas erupted from these cinder cones include ne and hynormative alkali basalts hawaiites mugearites and benmoreites it is unusual that this suite has erupted in a calc aikaline volcanic belt where volcanoes in the vicinity have been erupting calc alkaline andesites dacites and rhyodacites

chemical weathering of granite under acid rainfall environment korea chemical weathering was investigated by collecting samples from five selected weathering profiles in a high elevation granitic environment located in seoul korea the overall changes of chemistry and mineralogical textures were examined reflecting weathering degrees of the samples using polarization microscopy x ray diffraction xrd electron probe micro analysis epma x ray fluorescence spectroscopy xrf and inductively coupled plasma mass spectroscopy icp ms the chemical distribution in the weathering profiles shows that few trace elements are slightly immobile whereas most major particularly ca and na and trace elements are mobile from the beginning of the granite weathering on the other hand there were mineralogical changes initiated from a plagioclase breakdown which shows a characteristic circular dissolved pattern caused by a preferential leaching of ca cation along grain boundaries and zoning the biotite in that region is also supposed to be sensitive to exterior environmental condition and may be easily dissolved by acidic percolated water as a result it seems that some rock forming minerals in the granitic rock located in seoul are significantly unstable due to the environmental condition of acidic rainfall and steep slopes where they are susceptible to be dissolved incongruently leading some elements to be highly depleted

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

tectono stratigraphic terranes and mineral resource distributions in mexico about 80 of the southern part of the north american cordillera within the republic of mexico is made up of suspect terranes these terranes are suspect because their paleogeographic setting with respect to cratonic north america at various times through much of phanerozoic time is uncertain much of northeastern and southeastern mexico is underlain by basement accreted during late paleozoic time an extension of the appalachian ouachita orogeny this orogen has been considerably modified by jurassic strike slip translations related to the opening of the gulf of mexico western and southwestern mexico is largely made up of several distinct but coeval latest jurassic to late cretaceous submarine magmatic arc terranes with unknown basement that appear to have accreted against the disrupted north american margin by early tertiary time only northeastern sonora and the state of chihuahua appear to be floored by unmoved north american cratonic basement the combined effect of mesozoic accretions and translatio

model for bauxite formation paragominas brazil abstract samples from a profile of the paragominas bauxite brazil were analyzed extensively principal minerals were determined as well as the concentrations of 56 elements the major element chemistry can be described in terms of the oxides and hydroxides of al si ti and fe bound mineralogically as gibbsite kaolinite and hematite 30 of the 45 minor and trace elements analyzed often maintain their average crustal concentrations depletions are most obvious for the alkali metals the alkaline earths and the metals of the first row transition series enrichment is observed for b sc zr nb sn sb i hf and th differences in concentration patterns for some refractory metals al ti zr hf etc lead to the suggestion that some e g ti zr hf of these metals may be participating in biological reactions the economic bauxite zone is bracketed above and below by kaolinitic horizons and this succession of alteration could be explained by extraordinarily high flow rates of meteoric waters for millions of years through highly permeable terrains the formation and preservation of these highly leached profiles also require a quiescent tectonic setting for 10 100 ma such a model is consistent with conditions which may have prevailed in eastern amazonia for at least 100 ma

mobilities of alkalis alkaline earths and halogens during weathering abstract concentrations of alkali metals alkaline earths and halogens were monitored in an australian weathering profile and in brazilian soils and bauxite the materials analysed are considered to represent a range of chemical weathering histories which are distinguished with respect to their degree of leaching by the dominant aluminosilicate assemblages during initial weathering concentrations of alkalis and alkaline earths are similar or enriched relative to average crustal abundances as weathering proceeds depletion is attenuated by the capacity of a particular element to participate in adsorption and ion exchange processes strong depletion of alkalis and alkaline earths is common as weathering advances and clay minerals are replaced by al oxide phases e g gibbsite with much lower capacities for exchanging and incorporating ions halogen concentration trends are more variable f retention during weathering is likely due to substitution in mineral phases for hydroxyl groups cl and i are commonly observed to accumulate as weathering advances and some of these accumulations may be due to atmospheric additions

phase equilibria of a fluorine rich leucogranite from the st austell pluton cornwall highly evolved leucogranitic rocks in the st austell pluton cornwall of hercynian age contain accessory muscovite topaz and fluorite the authors have studied the h sub 2 o saturated melting behavior of one representative sample its solidus and liquidus pass through the points 663 and 725 sup 0 c respectively at 1 kbar 640 and 665 sup 0 c at 2 kbar 610 and 717 sup 0 c at 4 kbar and 608 and 700 sup 0 c at 8 kbar plagioclase is on the liquidus at low pressure and topaz is on the liquidus at 4 kbar the fluorite is consumed in the formation of the first formed liquid calcium can partition into an evolved granitic melt if complexed by fluorine the fluorite appears to be largely primary in fresh fluorite granite at st austell and not to reflect the albitization of oligoclase in the surrounding biotite granite such fluorine rich leucogranites can be expected to be of subsolvus character

genesis of the xianghualing sn pb zn deposit south china a multi method zircon study abstract the xianghualing sn pb zn deposit is situated near the center of the world class nanling w sn ore belt associated with the highly evolved laiziling granite of jurassic age previous studies focused mainly on geochemical and geochronological features of the mineralization related granite whereas research on the source evolution and geochronology of the ore forming fluids has been limited we carried out precise u pb dating trace element and hf isotopic analyses on magmatic and hydrothermally altered zircons from the skarn stage and sulfide stage ores in the xianghualing deposit zircon u pb dating results from the two ore forming stages show distinct geochronological populations the skarn stage ores contain mainly magmatic and hydrothermally altered zircons in three age clusters 150 ma 220 ma and 420 ma whereas the sulfide stage ores are characterized by abundant old detrital zircons ranging from 497 to 3409 ma with a peak at 800 1000 ma this pattern may indicate that magmatic hydrothermal events in the silurian and triassic were precursors to the main sn mineralization event in the jurassic when circulation of evolved fluids through older sedimentary rocks led to pb zn mineralization the jurassic zircons exhibit high trace element contents e g hf p y nb ta th u and low ti contents eu anomalies nb ta and th u ratios which imply that the laiziling magma experienced significant plagioclase biotite and muscovite fractional crystallization and that it interacted extensively with post emplacement f rich hydrothermal fluids the positive ce anomalies of these zircons ce ce 1 0 226 indicate that the magmatic hydrothermal fluids that led to the jurassic sn mineralization event were reducing the less negative ehf t values of triassic zircons 18 7 to 2 6 compared to the silurian zircons 14 6 to 2 3 suggest more limited interaction between the mantle and the magma source region the jurassic magmatic zircons in the skarns exhibit even more negative ehf t values 30 7 to 6 7 along with tdmc model ages ranging from 1627 to 3134 ma indicating that the jurassic magma was derived from partial melting of old continental crustal basement based on the combined study of zircons in the skarns and sulfide ores we propose a three stage ore genetic model for the xianghualing sn pb zn deposit

basaltic volcanism and extension near the intersection of the sierra madre volcanic province and the mexican volcanic belt three groups miocene pliocene and pleistocene off oceanic type basalts associated with normal faulting have been identified in an area surrounding the city of guadalajara mexico although most basalts within these groups have compositional characteristics of an asthenospheric source each group is also associated with lavas that have subduction related traits the first group the san cristobal plateau basalts has a minimum volume of 1 800 km 3 of predominantly alkali olivine basalt and lesser basaltic andesite and fills a pre existing extensional basin of unknown configuration 10 m y ago the relatively rapid eruption of these basalts along with their volume structural association and chemistry suggest an upwelling mantle plume beneath the guadalajara region this region may be the earliest indication of the separation of the jalisco block from north america the second basalt group the guadalajara basalts consists of small volumes of porphyritic basalts and basaltic andesites 3 3 5 0 ma in age that outcrop near the northern outskirts of guadalajara and near the town of hostotip aquillo to the northwest the youngest basalts in the guadalajara area are the 0 4 1 4 ma santa rosa suite of basalts hawaiites mugearites close to the contemporary 0 2 ma andesitic central volcano v tequila these either flowed from nearby cinder cones toward the rio santiago canyon or erupted in the canyon to dam the river the flat lying tertiary 23 27 ma ash flows of the sierra madre volcanic province that lie to the north of guadalajara are faulted north northeast into horsts and grabens that in one case can be dated as being older than 21 8 ma this fault trend has been reactivated closer to guadalajara as normal faults 200 to 300 m displacement cut the miocene plateau basalts but are hidden by a cover of younger volcanic rocks close to the city also hidden by the young volcanic succession there is the north west southeast fault zone that extends to the gulf of california as the tepic zacoalco graben system faulting of this zone is well displayed in the region of the santa rosa dam in the santiago canyon northwest of guadalajara two styles of faulting are found there an older 1 ma 3 has been approximately north northeast for at least 4 m y characteristic of the mexican basin and range province

geology of the northern colima and zacoalco grabens southwest mexico late cenozoic rifting in the mexican volcanic belt the south trending northern colima graben intersects two other grabens the northwest trending zacoalco graben and the east trending chapala graben to form a rift rift rift triple junction 50 km south southwest of guadalajara in southwest mexico the northern colima graben represents the northernmost extension of the 190 km long 20 to 65 km wide colima rift which overlies the projection of the subducting boundary of the rivera and cocos plates and represents a rift of probable pliocene pleistocene age of the east trending mexican volcanic belt mvb associated with the northernmost 90 km of the rift there are 4 6 m y old to holocene alkaline volcanic rocks rarely found in continental volcanic arc environments and which are more typically associated with either continental intraplate rifting or oceanic islands these alkaline lavas have erupted side by side with 10 m y old to holocene calc alkaline lavas that represent typical arc volcanism it is proposed that the grabens are associated with the initiation of a rifting of a portion of southwest mexico away from the mexican mainland similar to the rifting of baja california away from the mexican mainland the calc alkaline k ar dates are consistent with post mid miocene development of east trending mvb volcanism

present day kinematics of the rivera plate and implications for tectonics in southwestern mexico we model the present day motion of the rivera plate relative to the pacific north america and cocos plates and combine the results with the nuvel i model for cocos north america motion to examine present day deformation in southwestern mexico the pacific rivera data which include 25 three m y average spreading rates determined from original surface ship magnetic data and 22 azimuthal data along the rivera transform are systematically misfit by the nuvel i pacific north america and pacifie cocos euler vectors indicating that the rivera plate is kinematically distinct from north america and cocos an f test shows that the improvement in fit to the pacific rivera data from a model with a distinct rivera plate exceeds that expected solely from adding three model parameters the rivera north america euler vector derived from closure of the rivera pacific north america plate circuit predicts slower and more trenchnormal convergence along the acapulco trench than prior models in addition the model predicts convergence normal to the eastern tamayo fracture zone at a rate 60 slower than the right lateral strike slip predicted by prior models an observed systematic misfit of the pacific rivera euler vector to azimuths from the western rivera transform fault may have several causes there may be a time averaging problem between the 3 0 m y average rates and the shorter time average transform fault azimuths and earthquake slip vectors alternatively rivera transform trends may be biased by deformation within lithosphere adjacent to the transform valley or future detailed mapping of the transform may reveal the misfit to be an artifact of the presently available bathymetry along the entire pacific rivera rise spreading rates averaged over the past 0 7 m y are systematically faster than 3 0 m y average rates with the difference increasing southward along the rise the post 3 ma change in the rate gradient has caused a southward migration of the pacific rivera euler pole since 3 ma possibly in response to the 3 ma completion of a spreading reorganization that doubled the length of the rivera transform fault and halved the area of the rivera plate we hypothesize that deformation in southwestern mexico including opening along the colima rift is related to oblique subduction of the cocos plate along the northern middle america trench three unes of evidence support this hypothesis first the sense of oblique subduction predicted by nuvel i along the middle america trench is consistent with a model in which part of western mexico located southeast of the colima rift moves to the southeast relative to north america second field geologic data and landsat imagery suggest that in the past few million years the chapala oaxaco fault zone has accommodated several kilometers of sinistral motion and sinistral transtension has occurred along the trans mexican volcanic belt either or both of these fault zones could accommodate any southeastward coastal block motion that might result from oblique subduction third the observation that earthquake slip vectors from the northern middle america trench trend systematically counterclockwise from the predicted cocos north america convergence direction is also consistent with southeastward motion the discrepancy between the observed and predicted slip directions along the middle america trench suggests sinistral slip is 0 10 mm yr 1 if oblique subduction is driving southeastward transport of a coastal sliver the colima rift is a passive pull apart zone at the northwestern end of this sliver rather than an incipient spreading ridge that will replace the pacific rivera rise

provenance and depositional variability of the thin mud facies in the lower ganges brahmaputra delta west bengal sundarbans india the sundarbans is one of the largest coastal wetland sites in the world and covers an area of approximately one million hectares of the western delta of the ganges and brahmaputra g b rivers located across bangladesh and india since the late holocene the western delta has not been directly fluvially sourced due to the ganges shift to the east present day bangladesh the depositional facies thin mud facies of the late holocene abandoned western region the sundarbans is derived from dominant estuary tidal dynamics however the provenance of the associated tmf sedimentation in this far western zone indian sundarbans per se is as yet equivocal in this study sediment cores from the indian sundarbans saptamukhi thakuran estuary were closely examined for grain size distributions gsds mineralogy through x ray diffraction xrd and geochemistry with x ray fluorescence xrf the tmf in the west bengal sundarbans has been determined to show intensively weathered terrestrial sediment derived principally from the ganges alluvial plain gap there is a predominance of quartz mica and clay minerals with quartz interpreted as a product of low relief tropical weathering sourced via the g b rivers draining the himalayas lithofacies interpreted through gsd analysis of the tmf is indicative of a muddy tidal flat environment with aggradation and a general fining up trend between the adjacent estuaries the sediment provenance indicates a continuing g b sediment source which moves westward along the bay of bengal from the active delta front and is then reworked over the far western abandoned delta by tidal estuarine forcing

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

compositional polarity of triassic granitoids in the qinling orogen china implication for termination of the northernmost paleo tethys abstract the qinling orogen was formed from the closure of the northernmost paleo tethys sea and the tectonic suturing of the yangtze and north china cratons the timing of this collision and the tectonic framework are debated the widely developed triassic granitoids in the western qinling orogen offer a key to understand the tectonic evolution of this region here we compile the geological geochemical and geochronological data of the triassic granitoids from the qingling orogen and conclude that the granitoids north of the mian lue suture were emplaced in an active continental margin related to the northward subduction of the mian lue oceanic plate during 248 200 ma the granitoids can be classified into i and s types with the former constituting the major variety northward from the mian lue fault an s type granite belt appears followed by and locally overlapping with an i type granite belt the i type granite belt can be subdivided into four sub belts from south to north with increasing contents of k2o k2o na2o sio2 th and u and the ratios of k2o na2o rb sr and 87sr 86sr i but decreasing contents of na2o al2o3 mg and ehf t except for the northernmost belt which occurs in the huaxiong block of the north china craton this geochemical polarity of i type granitoids and the zoned distribution of s and i types granitoids cannot be explained through continental collision orogeny including syn to post collision but can be well interpreted by considering the progressive subduction of the northernmost branch of the paleo tethys as represented by the mian lue ocean thus the termination of the northernmost paleo tethys and the onset of the continental collision between the yangtze and the north china plates are considered to have occurred at about 200 ma during the transition from triassic to jurassic our model is also supported by the available data from other studies and provides a revised framework for the timing and tectonics of assembly of the yangtze and the north china cratons

zircon records multiple magmatic hydrothermal processes at the giant shizhuyuan w sn mo bi polymetallic deposit south china abstract the supergiant shizhuyuan w sn mo bi polymetallic deposit is located in the nanling range and is intimately related to the qianlishan granite complex intruded during the late jurassic period the complex consists of three intrusive phases porphyritic biotite granite pbg equigranular biotite granite ebg and porphyritic granite dyke in this work the distinct geochemical features of magmatic and hydrothermally altered zircons from the qianlishan complex are used to reveal the magmatic hydrothermal evolution of the deposit whole rock geochemistry and mineralogy show that ebg samples have experienced stronger fractional crystallization than pbg samples zircon u pb dating constrains the intrusive episodes at 155 153 ma and 152 ma for the pbg and ebg respectively and subsequent hydrothermal events revealed by hydrothermal zircon ages at 154 153 ma and 152 150 ma hydrothermally altered zircon and paragenetic mineral inclusions from ebg samples show textures and compositional properties suggesting they were intensely altered by a zirconium saturated aqueous fluid type 1 enriched in most trace and metallogenic elements e g zr rees hf p u th nb ta ti pb al fe mg ca mn w and sn and volatiles f cl li co2 and h2o this fluid was relatively lower in temperature pressure and oxygen fugacity average ce4 ce3 11 2 type 2 fluid represented by hydrothermally altered zircons found in pbg was depleted in trace elements and had a relatively higher temperature and pressure and low fo2 ce4 ce3 2 12 29 16 type 3 fluid as evidenced by fluid altered zircons from the greisenized granite was likely a further evolved product of the type 2 fluid given its moderate cargo of most trace elements but also with exceedingly low ce4 ce3 values 2 9 33 45 the type 3 fluids might be precursors of w sn mineralizing fluids whereas the type 1 fluids were likely to represent mature ore forming fluids which were activated during the late skarn stage but before the retrograde mineralization stage compared with the fo2 of pbg and ebg magmas calculated by non altered magmatic zircons average ce4 ce3 31 47 and 49 21 it seems that the hydrothermal fluids generated by them became more reduced during the further evolution a progressive process can thus be divided pbg type 2 type 3 ebg type 1 with the gradual accumulation of ore forming materials the enrichment of most elements in hydrothermal zircons further indicates a substitution of y rees p hf u th ti nb ta ca k fe and mn for zr4 and si4 in the zircon lattice most fluid altered zircons exhibit a similar 176hf 177hf distribution range 11 to 6 but evidently elevated 176lu 177hf and 176yb 177hf ratios with magmatic ones this indicates that three types of hydrothermal fluids were magmatic derived whereas lu and yb were highly mobile during the hydrothermal event based on the results a magmatic hydrothermal evolution model is presented for shizhuyuan zircons this research highlights the use of zircon as a powerful tool in revealing the complex magmatic hydrothermal processes and perhaps mineralization potential in highly evolved granite associated mineralization systems

vapor undersaturated experiments with macusani glass h 2 o at 200 mpa and the internal differentiation of granitic pegmatites vapor undersaturated fractional crystallization experiments with macusani glass macusanite a peraluminous rhyolite obsidian at 200 mpa yield mineralogical fabrics and zonation and melt fractionation trends that closely resemble those found in zoned granitic pegmatites and other granitoids of comparable composition typically peraluminous li be ta rich deposits the zonation from the edge of charges inward is characterized by 1 fine grained sodic feldspar quartz border zones 2 a fringe of very coarse grained graphic quartz feldspar intergrowths that flair radially toward melt and terminate with nearly monophase k feldspar 3 cores of very coarse grained nearly monominerallic quartz or virgilite lialsi5o12 mica and 4 late stage fine grained albite mica intergrowths that are deposited from alkaline na rich interstitial melt at vapor saturation similar experimental products have been observed in compositionally simpler less evolved systems liquid lines of descent from initially h2o undersaturated runs are marked by a decrease in sio2 and increases in na k b p f h2o and a variety of trace lithophile cations these trends are believed to be governed by three factors 1 disequilibrium growth of feldspars quartz via metastable supersaturation 2 fractionation of melt toward sio2 depleted na rich compositions due to increases in b p and f and 3 changes in nucleation and growth rates mostly as a function of the h2o content of melt x w m in contrast experiments that are cooled below the liquidus from the field of melt aqueous vapor london et al 1988 fail to replicate pegmatitic characteristics in most respects on the basis of these and other experiments we suggest that the formation of pegmatite fabrics stems primarily from fractional crystallization in volatile rich melts and that enrichments in normally trace lithophile elements result from melt differentiation trends toward increasingly alkaline silica depleted compositions although vapor saturation at near solidus and subsolidus conditions may promote extensive recrystallization an aqueous vapor phase does not appear to be necessary for the generation of most of the salient characteristics of pegmatites

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

contemporaneous calc alkaline and alkaline volcanism at sanganguey volcano nayarit mexico sanganguey volcano located in the northwestern segment of the mexican volcanic belt is a calc alkaline stratovolcano that has erupted a chemically continuous suite of andesites and dacites during the past 200 000 yr alkalic volcanism however appears to have been predominant in the area since the pliocene alkalic basalts and their derivatives erupted along extensional fractures trending northwest through sanganguey and continued to erupt through the flanks of the stratocone following cessation of calc alkaline activity the calc alkaline andesite dacite volcanism was focused beneath sanganguey and appears to have been controlled by crystal fractionation in a shallow magma chamber no basalts that erupted in the area have chemical compositions that could be parental to the sanganguey calc alkaline suite the most basic calc alkaline rocks basaltic andesites have incompatible trace element concentrations nearly identical to the most basic alkaline rocks implying that the two suites were derived from distinct parental magmas alternatively it is suggested that stagnation of mantle derived alkalic basalts provided heat input to mobilize lower crustal rocks resulting in the generation of calc alkaline magmas

mesozoic large scale magmatism and mineralization in south china preface abstract south china is famous for its polymetallic deposits with more than 50 of the world s w and sb reserves and 20 of the sn reserves as well as abundant cu mo pb zn au ag ta nb hree and u this special issue reports recent progress on the tectonic evolution magmatism and ore deposits of south china which leads to a more comprehensive understanding of the mesozoic large scale magmatism and mineralization events 1 south china has been located in backarc settings during multiple subductions since the paleozoic which may have contributed to pre enrichments of the ore forming materials 2 the pacific plate subduction was a principle factor that controlled the late mesozoic large scale magmatism and mineralization in south china through flat subduction ridge subduction slab tearing and slab rollbacks

magma eruption rates constrained by 40ar 39ar chronology and gis for the ceboruco san pedro volcanic field western mexico 40ar 39ar geochronology is coupled with quantitative volume determinations utilizing field mapping digital elevation models orthophotos and geographic information system gis software to constrain magma eruption rates at the ceboruco san pedro volcanic field 1600 km2 in the western trans mexican arc ages are reported for 40 volcanic units including volcan ceboruco an active andesitic stratovolcano peripheral domes shields cinder cones and fissure fed flows after a hiatus of 3 m y volcanic activity recommenced to produce 80 5 3 5 km3 of magma at a rate of 63 m3 km2 per year over the past 0 8 m y however 75 of this volume erupted in the past 100 k y including the 51 2 5 km3 of volcan ceboruco equivalent to an eruption rate of 377 m3 km2 per year there have been at least two stages of cone building activity at volcan ceboruco the main edifice is composed of 38 km3 of precaldera andesites the youngest dated at 45 8 ka their eruption was followed by a hiatus interrupted by a plinian eruption at 1 ka the plinian eruption and subsequent lava flows are andesite to dacite in composition and constitute 13 km3 of the total volume of volcan ceboruco overall the relative proportions of lava types erupted in the past 0 8 m y are 18 19 basaltic andesite 56 60 andesite 18 22 dacite and 3 rhyolite the peripheral lavas are each of small volume geochemically diverse and show little evidence of prior storage in an upper crustal chamber the eruptive sequence proportions of lava types phenocryst assemblages textures and geochemistry imply that the lavas do not reflect the differentiation of a single parental liquid in a long lived magma chamber the distinct geochemical signatures were present prior to magma emplacement in the upper crust whereupon subsequent degassing and crystallization led to variable phenocryst abundances and assemblages

petrogenesis and geodynamic significance of silicic volcanism in the western trans mexican volcanic belt role of gabbroic cumulates in the western trans mexican volcanic belt voluminous silicic volcanism has been associated with the rifting of the jalisco block from mainland mexico rhyolitic volcanism started at 7 5 ma after a major pulse of basaltic volcanism aged 11 8 5 ma associated with slab detachment this was followed by a second period between 4 9 and 2 9 ma associated with rhyolitic domes and ignimbrite coexisting with basaltic volcanism the similarity in rare earth element contents between basalts and rhyolites excludes a simple liquid line of descent the low ba and sr contents and the ferroan character of the rhyolites suggest extensive fractional crystallization late miocene early pliocene rhyolite sr isotope values are only slightly more radiogenic than the basalts whereas nd isotope ratios are indistinguishable we successfully modelled the 7 5 3 ma silicic magmatism as a result of partial melting of crustal gabbroic complexes that we infer to have formed in the mid lower crust due to the high density fe enriched composition of the late miocene basaltic volcanism slab rollback since 7 5 ma favoured decompression melting and arrival of additional mafic magmas that intruded in the lower crust these basalts heated and melted the gabbroic complexes forming the silicic magmas which subsequently underwent assimilation and fractional crystallization processes the first silicic pulse was emplaced during a period of low tectonic activity extensional faulting since the pliocene favours the eruption of both silicic magma and lesser amount of mafic lavas

weathering and global denudation a negative correlation between sediment yield and weathering history as measured by the chemical alteration cia of the suspended sediment is observed for many of the world s major rivers and other regions of denudation the weathering history is a first order control on the sediment yield of such areas termed equilibrium denudation regions for other areas data scatter with either apparent increases or decreases of sediment yield for given cia values these areas are termed nonequilibrium denudation regions low sediment yields can be attributed to moderated erosion either natural or human induced and or the incorporation of unweathered glacial debris accelerated erosion resulting in high sediment yield is primarily human induced and results from cultivation and other land use each of these effects has a profound influence on global sediment discharge from the continents pre human suspended sediment discharge from the continents is estimated to be 12 6 times 10 15 g yr or about 0 6 the

petrogenesis of tourmaline granites and topaz granites the contribution of experimental data abstract fluorine and boron are generally only minor components of granitic rocks but may be found highly concentrated in certain tourmaline granites and topaz granites up to 1 wt b2o3 and 3 2 wt f respectively studies of various examples of these granite types which occur as small bodies emplaced at shallow levels p lithostatic textural and petrographic data indicate that the tourmaline and topaz granite magmas crystallized at low to moderate temperatures below 700 c and that a fluid phase may have been present during the crystallization interval both granites varieties are peraluminous co up to 5 characterized by the presence of white micas lithium micas or muscovite aluminium silicates and other al rich minerals garnet cordierite boron and fluorine are hosted by tourmaline schorl rich member of the schorl dravite series topaz and mica respectively these granites have low ca fe mg and ti contents each the crystallization of tourmaline granite and topaz granite magmas is discussed using experimental data derived from studies of granitic systems with added boron and fluorine both elements act as a flux enabling boron and fluorine enriched melts to persist to temperatures as low as 650 c at 1 kbar in addition the effect of increasing fluorine content displaces the minimum in q ab or at ph2o 1 kbar toward the albite apex plots of fluorine rich topaz granites coincide with this trend suggesting that the role of fluorine in magmatic processes may have affected the rock composition as well as crystallization temperatures boron rich granites plot near the low ph2o minima in qz ab or suggesting that boron has less of an influence on minimum melt compositions the low temperatures of crystallization permits phases such as muscovite or andalusite topaz and two alkali feldspars to crystallize directly from f and b enriched melts at low pressures in addition the limited available experimental data are used to compare the effects of fluorine and boron on major and trace element distribution during melt vapour interaction

a new style of rare metal granite with nb rich mica the early cretaceous huangshan rare metal granite suite northeast jiangxi province southeast china

active rifting in southwestern mexico manifestations of an incipient eastward spreading ridge jump a major ongoing episode of continental rifting has formed a system of three intersecting grabens in the western mexican volcanic belt these structures may mark the initial stage of an eastward spreading ridge jump beneath continental crust eastward ridge segment jumps have been propagating northward along this part of the east pacific rise for the past 12 m y but this is the first plate boundary reorganization event to involve continental crust since the separation of baja california in the miocene since pliocene time small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant subduction related calc alkaline magmas erupted from andesitic stratovolcanoes the alkaline magmas are varieties found in zones of active rifting elsewhere in the world

significance of the sedimentary al ti ratio as an indicator for variations in the circulation patterns of the equatorial north atlantic this paper presents results of geochemical investigations conducted on three gravity cores recovered along an e w transect 3 5 n in the equatorial atlantic considering the high terrigenous content of the sediments approximately 12 74 wt al and ti concentrations are predominantly associated with the input of lithogenous phases the terrigenous fraction of western equatorial atlantic sediments is more influenced by material derived from the amazon river than by deposition of african dust particles as a possible route of transport the north equatorial countercurrent is considered fluctuations in al ti records generally reflect changes in the low latitude insolation cycle 19 23 kyr and thus indicate a close coupling to variations of the trade wind systems and continental climate changes however characteristics of the terrigenous signals in the eastern and western equatorial atlantic differ significantly while the precessional signal in the al ti record and low latitude portion in the 18o record are in phase in the eastern equatorial atlantic variations in the terrigenous fraction westward of the mid atlantic ridge precede the marine signal by 2 3 kyr which we ascribe to a shift between trade wind systems of the northern and southern hemispheres

properties and genesis of a soil and the underlying gibbsite bearing saprolite cape breton island canada brownish yellow saprolite derived from granite gneiss is exposed beneath 1 2 m of reddish brown till along a roadcut in the cape breton highlands the strongly acid soil is similar to other podzolic soils of the region except for the presence of gibbsite in the silt and clay fractions in the saprolite feldspar and biotite have weathered to poorly crystalline kaolinite gibbsite and a minor component of iron oxide the poorly crystalline kaolinite does not occur in the overlying till the evidence presented is consistent with the view that the area was covered either by cold based or by stagnant warm based glaciers that eroded the pre quaternary surface only slightly and deposited a thin discontinuous veneer of till derived mainly from local bedrock and saprolite weathering products from the saprolite are probably incorporated commonly in tills of the area during the formation of soils in these tills the poorly crystalline kaolinite and most of the gibbsite are either transformed to other minerals

monazite chemical weathering rare earth element behavior and paleoglaciohydrology since the last glacial maximum for the loch vale watershed colorado usa rare earth element ree release from weathering of accessory monazite ree po4 since the last glacial maximum at 18 1 ka was investigated in sediment recovered from an outlet lake within the glaciated loch vale watershed colorado usa labile rees in the sediments reveal monazite weathering increased during the younger dryas chronozone ydc 13 2 to 11 1 ka when alpine glaciers advanced as climate cooled and bedrock comminution increased monazite dissolution peaked at approximately the pleistocene holocene transition 10 5 ka during the holocene ree concentrations decline reflecting a reduction in monazite weathering the ree cerium ce may occur as trivalent or tetravalent cations and the ce anomaly ce n ce n in sediment permits interpretations of paleoredox conditions the ce n ce n decreases from 18 1 ka to the pleistocene holocene boundary initially oxidizing conditions within the watershed increased because of enlargement of the proglacial area as the ice retreated at the onset of the ydc oxidation further escalated in response to enhanced chemical weathering associated with glacial advance a more stable holocene climate and landscape resulted in relatively small changes in ce n ce n values slightly decreasing ce n ce n values over the last several hundred years are consistent with present day stream water values and may reflect the onset of present day hydrobiogeochemical conditions

halloysite versus gibbsite silicon cycling as a pedogenetic process in two lowland neotropical rain forest soils of la selva costa rica abstract halloysite and gibbsite although known to require quite different conditions for their formation commonly occur together in the same horizon in oxisols derived from andesitic parent materials in tropical costa rica we selected two soils of similar parent material but of different ages and soil moisture regimes to identify possible clues to the coexistence of these two minerals we employed selective dissolution procedures x ray fluorescence analysis and x ray diffraction on field moist and air dry bulk soil samples to investigate how mineralogy changes with depth we further separated the size fraction we found both soils to be depleted of primary minerals and pedogenesis to have progressed to advanced weathering stages particularly in the subsoils gibbsite xrd signal intensities were linearly and significantly related to weathering indices corroborating the residual nature of gibbsite as an endproduct of weathering processes the si bearing quartz and kaolinite group minerals were enriched in the topsoils indicating i their independence from a primary mineral si source and ii the existence of a mechanism capable of protecting them against the continuous tropical weathering pressure as we found no indications for retrospective additions of soil material through mass movement or aeolian additions we believe a vegetation dependent biological pumping mechanism to be the most plausible explanation for the presence of silica bearing minerals in the la selva topsoils the vertical distribution of 1 0 nm halloysite and its accumulation in the lower reaches of the wetter alluvial soil suggest that this metastable mineral forms as a result of si enrichment where the residence time of the pore water is long enough to allow for si concentrations to exceed the halloysite precipitation threshold taken together our evidence indicates gibbsite in the la selva soils to be the endproduct of intense tropical weathering while the presence of hydrated halloysite seems to have mainly kinetic reasons and is most probably coupled to the contemporary soil moisture regime

geochemical and isotopic composition of volcanic rocks of the heterogeneous miocene 23 19 ma tepoztlan formation early transmexican volcanic belt mexico abstract we present the first geochemical data major and trace elements as well as sr nd and pb isotopes on volcanic rocks from the tepoztlan formation in the central transmexican volcanic belt the tepoztlan formation is up to 800 m thick and comprises a wide range of primary volcanic rocks lavas pyroclastic density current deposits pyroclastic fall deposits and their secondary reworked products due to mass flow lahars and fluvial processes magnetostratigraphy combined with k ar and ar ar geochronology suggests an age of early miocene 23 19 ma for this formation lava flows pyroclastic rocks dykes and volcanic clasts range from basaltic andesite to rhyolite with a clear dominance of andesites and dacites all samples are subalkaline and hy normative these rocks show homogeneous ree patterns with lree enrichment and higher lile concentrations with respect to hfse notably the typical negative anomaly of nb ta and ti suggesting a subduction related magma genesis major and trace element concentrations show that either assimilation of heterogeneous continental crust or crustal recycling by subduction erosion and fractional crystallization are important processes in the evolution of the tepoztlan formation magmas isotopic compositions of the tepoztlan formation samples range from 87sr 86sr t 0 703693 to 0 704355 and 143nd 144nd t 0 512751 to 0 512882 falling within the mantle array all geochemical characteristics indicate that these rocks originated from a heterogeneous mantle modified and evolved through assimilation of country rock and fractional crystallization in the upper crust

searching for an oxygenation event in the fossiliferous ediacaran of northwestern canada abstract late neoproterozoic ediacaran strata from northwestern canada provide a thick and rich sedimentological record preserving intercalated carbonates and shale extending from the 635 million year old marinoan glacial deposits up through the 541 million year old precambrian cambrian boundary this region also holds one of the classic localities for the study of early animal life with the ensuing suggestion that this temporal interval captures a gross change in the o 2 content of the earth s atmosphere to test this hypothesis and bring records of northwestern canada into line with other ediacaran fossil bearing basins we provide a detailed geochemical reconstruction from the wernecke mountains of the yukon where possible we also extend these records to the ogilvie mountains to the west and previously published data from the mackenzie mountains to the east our work in the wernecke mountains is set against a composite 13 c record for carbonate that preserves three distinct ediacaran isotope excursions the lowermost of which preserved in the gametrail formation is a putative shuram excursion equivalent what emerges from a multi proxy fe speciation sulfur isotopes major and trace element analyses reconstruction is a picture of a persistently anoxic and ferruginous ediacaran ocean notably absent is geochemical evidence for a prominent oxygenation event an expectation given the appearance of animals and large swings in 13 c the new insight gained through these data challenges the idea of an ediacaran jump in atmospheric oxygen which in turn muddles the link between animal evolution and local geochemical environments

clay transformation and pedogenic calcite formation on a lithosequence of igneous rocks in northwestern iran abstract knowledge about the pathways of pedogenic calcite formations and clay transformations on igneous rocks in semiarid regions plays a vital role in the commentary of soil development on these rocks the present study was conducted to explore the weathering stages and pathways for the formation of pedogenic calcite in soils developed on selected plutonic and volcanic rocks in a semiarid climate in north western iran the soils developed on plutonic rocks were found to be in the primary stage of weathering with very few pedogenic calcites except for soil developed on alkali granite which contained few calcitic pedofeatures calcitic crystallitic b fabrics and calcitic pedofeatures were found in all soils developed on volcanic rocks in situ weathering of ca bearing minerals such as plagioclases amphiboles and pyroxenes could be the possible explanation for the accumulation of pedogenic calcite in the absence of calcareous parent rocks in the studied area micritic calcite was observed to be the dominant form in most of the soils developed on volcanic rocks indicating a rapid precipitation of calcium carbonate smectite kaolinite and mica illite were present in the soils developed on plutonic rocks and the clay content in these soils was less than that of the volcanic rocks feldspars plus mica were supposed to be the most likely source minerals for the formation of pedogenic illite which could be consequently transformed into smectite in these semi arid environments smectite mica illite and kaolinite were the most abundant clay minerals in the soils developed on dacite and andesite rocks while a minor quantity of vermiculite occurred only in andesite derived soils smectite as the dominant mineral and some kaolinite with fairly constant variation in content with depth were observed in the soils formed on hornblende andesite and pyroxene andesite this could indicate that the smectite had the authigenic origin and was formed most likely from the weathering of plagioclase hornblende and pyroxene in general in situ crystallization of pedogenic carbonates and high quantities of smectite in the clay fraction confirmed that the soils formed on volcanic rocks were more developed than those on plutonic rocks in the studied semiarid region moreover our results showed that releasing calcium from ca bearing minerals in igneous rocks in conjunction with biological activities resulted in pedogenic calcite formation

volcanism and tectonism in western mexico a contrast of style and substance the jalisco block of southwestern mexico records the initial stages of continental rifting superimposed on a convergent continental margin pliocene quaternary eruptions in north northwest trending graben systems within the block produced extraordinarily diverse lava types including minette leucitite absarokite and andesite interspersed with these are several basaltic shield volcanoes that have compositional similarities to lavas from oceanic islands evidence of miocene basin and range extension which is seen around much of the gulf of california is found only along the northern margin of the jalisco block large volumes of alkali basalt were also erupted during the late miocene at several locations along this northern margin we propose that pliocene quaternary extension and associated volcanism within the confines of the jalisco block are consequences of complex plate boundary reorganizations that are causing the block to rift away from the mexican mainland

weathering dynamics and geosphere mixing with reference to the potassium cycle abstract continental weathering and erosion are discussed from a planetary perspective as important links in the chain of chemical and physical exchanges between surface geospheres atmosphere hydrosphere crust and the mantle the influence of weathering on geosphere chemistry is explored by considering potassium exchanges between geospheres k is transferred from the continents by weathering and erosion to the oceans from which dissolved k is removed during spilitization and by incorporation into sediments k carried to the mantle by subduction of spilites and sediments 8 10 13 g a 1 could balance the dissolved k added to the oceans from continental river runoff and from seafloor hydrothermal springs the largest k flux is that 2 7 10 14 g a 1 physically carried to the oceans in suspended sediments the weakest link in quantifying geosphere exchanges is the lack of understanding of the construction and restoration of continents and hence in defining the parameters involved in their chemical and physical renewal

topaz albite granites and rare metal mineralization in the limu district guangxi province southeast china the topaz albite granites of the limu district are ultra acidic peraluminous li f na rich and sn ta nb mineralized a distinct vertical zonation is developed in the granite stocks there is an upward systematic transition from leucocratic microcline albite granite through albite microcline granite topaz albite granite pegmatite stockscheider and layered pegmatite aplite dikes to k feldspar quartz veins and lepidolite fluorite stringers in the country rocks snow ball textures homogeneous distribution of rock forming and accessory minerals disseminated mineralization and melt inclusions in quartz topaz and albite are typical features indicative of their crystallization from the late stage li f na rich and sn ta nb bearing residual granitic melts at a higher intrusion level a comparison with rare metal bearing pegmatite ongonite topaz rhyolite and obsidian glass from other regions shows the worldwide existence of these specialized residual melts their emplacement and crystallization in a variety of geological environments result in the formation of a series of chemically similar rocks with different petrographic textures and mineral associations the topaz albite granites and associated mineralization in the limu district provide a good example of highly evolved magmatic fractionation in the f rich granite system and fluid melt partitioning behavior of rare metal elements during magmatic hydrothermal evolution

an f rich sn bearing volcanic intrusive complex in yanbei south china the igneous complex that hosts the yanbei porphyry tin deposit is situated along the eastern margin of the southeast china caledonian fold belt it intrudes a late jurassic granitic batholith and was unconformably covered by late cretaceous red beds the complex consists mainly of volcanic ignimbrites and minor rhyolitic lavas tuffs and subvolcanic rhyolite porphyry and intrusive rocks topaz bearing granite porphyry and granite these rocks are generally peraluminous with f and vi al rich biotite as the main mafic mineral and f rich and oh poor topaz as an accessory mineral in the granite porphyry and granite and as an essential mineral in the topazite topaz in the granites is usually interstitial and crystallized during the late magmatic stage the volcanic rocks and granites are rich in si k f sio 2 75 wt na 2 o k 2 o 0 03 0 72 f 0 06 6 48 wt and incompatible trace elements rb zr nb w sn and poor in mg ti ca p and compatible trace elements sr eu the chondrite normalized rare earth element ree patterns for the granites generally have a distinct v shape with strong eu depletion eu eu super whitesunwithrays 0 04 0 20 the main rocks of the complex have low epsilon nd t values 9 3 to 8 9 for volcanic rocks 4 0 to 4 8 for granites and high delta 18 o values of quartz from dacite 9 5ppm and granites 9 6 10 1ppm the difference in epsilon nd t between the volcanic rocks and granites indicates that they are not comagmatic the source of the volcanic rocks probably contains a greater metasedimentary component than that of the granites the yanbei porphyry tin deposit is located along the contact zone between topaz bearing granite porphyry and rhyolite porphyry and consists principally of one orebody which makes up 99 percent of the total reserves of the metallic minerals 5 by volume cassiterite pyrite and chalcopyrite are dominant sphalerite wolframite and galena are less important and siderite molybdenite and stannite are minor cassiterite accounts for 96 percent of the total sn reserves and stannite is the subordinate sn bearing mineral that occurs as rims around chalcopyrite cassiterite occurs mostly in disseminated form and as veinlets the wall rock alteration is intense and can be roughly grouped into three zones spatially arranged around the granite porphyry namely topaz quartz chloritic and sericitic zones most of the orebody occurs in the topaz quartz and chloritic zones hydrothermal alteration caused a decrease in k and na and an increase in al fe ca sn w and cu the delta 18 o values of quartz and cassiterite from the mineralized zones vary from 6 0 to 11 per mil and the calculated delta 18 o sub h 2 o values of the ore forming fluid range from 4 2 to 7 8 per mil the altered rocks have elevated delta 18 o sub h 2 o values 14 5 9 7ppm relative to fresh rocks the pyrite from the ore has a limited range of delta 34 s values 0 3 1 5ppm these data suggest a magmatic source for the ore forming hydrothermal fluid

zircon geochronology and geochemistry of the xianghualing a type granitic rocks insights into multi stage sn polymetallic mineralization in south china abstract highly evolved sn polymetallic mineralization related granites are intensively developed in the nanling range of south china in this study two types of granitic rocks albite granite and greisenized granite from the laiziling pluton in the xianghualing area were studied to elucidate magma evolution and the genesis of mineralization the a type xianghualing granitic rocks are characterized by high sio2 69 16 85 04 rb 759 2110 ppm ta 17 2 50 8 ppm nd 2 8 82 4 ppm and 87sr 86sr 0 907275 4 181262 and low mgo 0 01 0 98 tio2 0 01 0 03 sr 1 4 70 2 ppm ba 1 4 132 5 ppm eu 0 03 0 21 ppm and 143nd 144nd 0 512283 0 512326 the whole rock end t values range from 6 3 to 6 0 with t2dm ages of 1426 1454 ma the zircon ehf t values mostly range from 10 to 5 with dominant tdmc values of 1400 1800 ma this suggests that the xianghualing granitic rocks were derived from partial melting of mesoproterozoic basement rocks in the cathaysian block in a lithospheric extension setting zircon u pb dating yielded five age clusters proterozoic 900 ma n 9 silurian 430 ma n 4 triassic 220 ma n 16 jurassic 150 ma n 62 and cretaceous 80 ma n 28 zircon internal textures trace element contents and hf isotopic compositions indicate that they formed from a range of sources including residual zircons from the magma source region detrital zircons from local sediments zircons captured from pre existing magmatic rocks with obvious mantle signatures magmatic and fluid altered zircons and late stage hydrothermal zircons this suggests significant fractional crystallization weak wall rock assimilation contamination and intense fluid differentiation metasomatism during the genesis of the xianghualing granitic rocks m type ree tetrad effects are seen not only in bulk rock compositions but also in individual zircon grains indicating that highly fractionated and reduced f rich fluids played an important role in the formation of sn polymetallic mineralization guided by the major mesozoic w sn ore forming events occurring in the cathaysian block of south china a multi stage genetic model is proposed for the xianghualing granitic minerogenic system initial enrichment of w sn in the triassic 220 ma dominant sn polymetallic metal precipitation in the jurassic 150 ma and superimposed mineralization in the cretaceous 80 ma

quaternary volcanism near the valley of mexico implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions the valley of mexico and surrounding regions of mexico and morelos states in central mexico contain more than 250 quaternary eruptive vents in addition to the large composite volcanoes of popocatepetl iztaccihuatl and nevado de toluca the eruptive vents include cinder and lava cones shield volcanoes and isolated andesitic and dacitic lava flows and are most numerous in the sierra chichinautzin that forms the southern terminus of the valley of mexico the chichinautzin volcanic field cvf is part of the e w trending mexican volcanic belt mvb a subduction related volcanic arc that extends across mexico the crustal thickness beneath the cvf 50 km is the greatest of any region in the mvb and one of the greatest found in any arc worldwide lavas and scoriae erupted from vents in the cvf include alkaline basalts and calc alkaline basaltic andesites andesites and dacites both alkaline and calc alkaline groups contain primitive varieties that have whole rock mg mgo and ni contents and liquidus olivine compositions fo90 that are close to those expected of partial melts from mantle peridotite primitive varieties also show a wide range of incompatible trace element abundances e g ba 210 1080 ppm ce 25 100 ppm zr 130 280 ppm data for primitive calc alkaline rocks from both the cvf and other regions of the mvb to the west are consistent with magma generation in an underlying mantle wedge that is depleted in ti zr and nb and enriched in large ion lithophile k ba rb and light rare earth la ce elements extents of partial melting estimated from ti and zr data are lower for primitive calc alkaline magmas in the cvf than for those from the regions of the mvb to the west where the crust is thinner the distinctive major element compositions low cao and al2o3 high sio2 of the primitive calc alkaline magmas in the cvf indicate a more refractory mantle source beneath this region of thick crust in contrast primitive alkaline magmas from the cvf and other regions of the mvb show compositional similarities to intraplate type alkali basalts erupted behind the arc in the mexican basin and range province these similarities are consistent with the hypothesis that slab induced convection in the mantle wedge beneath the mvb causes advection of asthenospheric mantle from behind the arc to the region of magma generation trace element systematics of primitive magmas in the mvb reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub alkaline basaltic melts

geochemical evolution of a palaeolaterite the interbasaltic formation northern ireland abstract the interbasaltic laterites of northern ireland were formed during a period of relative volcanic inactivity by extensive chemical weathering of tertiary basalts they reach a maximum thickness of 30 m and once provided a major source of iron and aluminium ore an extensive database comprising major minor and trace elements has been compiled for 240 samples in order to study the effects of weathering in terms of the changes in whole rock chemistry and mineralogy from basalt through to iron rich crust percolating waters caused degradation of the parent basalt mineralogy and precipitation of neoformed phases principally through incongruent dissolution processes primary olivine plagioclase feldspar and augite were successively broken down and replaced by a mineral assemblage consisting of hematite gibbsite goethite anatase meta halloysite and kaolinite changes in mineralogy facilitated concomitant changes in element concentrations mass balance calculations indicate that all elements were depleted in the iron crust enrichment of al loi cr cu and v occurred in the laterite horizon while enrichment of al loi ba ce cr cu ni and rb occurred in the lithomarge notably yttrium was found to be mobile indicating that weathered basalts should not be used in discrimination of original tectonic environments the severe leaching conditions evidenced by yttrium depletion local aluminium redistribution and iron crust formation are indicative of weathering under a humid sub tropical monsoon climate

geochemical and isotope data from the acatlan volcanic field western trans mexican volcanic belt origin and evolution abstract the quaternary acatlan volcanic field avf is located at the western edge of the trans mexican volcanic belt tmvb this region is related to the subduction of the pacific cocos and rivera plates beneath the north american plate since the late miocene avf rocks are products of pleistocene volcanic activity and include lava flows domes erupted basaltic andesite trachyandesite trachydacite and rhyolite of calc alkaline affinity most rocks show depletion in high field strength elements and enrichment in large ion lithophile elements and light rare earth elements as is typical for magmas in subduction related volcanic arcs 87sr 86sr values range from 0 70361 to 0 70412 while nd values vary from 2 3 to 5 2 sr nd isotopic data plot along the mantle array on the other hand lead isotope compositions 206pb 204pb 18 62 18 75 207pb 204pb 15 57 15 64 and 208pb 204pb 38 37 38 67 give evidence for combined influences of the upper mantle fluxes derived from subducted sediments and the upper continental crust involved in magma genesis at avf additionally 18o whole rock analyses range from 6 35 in black pumice to 10 9 in white pumice of the acatlan ignimbrite a fairly good correlation is displayed between sr as well as o isotopes and sio2 emphasizing the effects of crustal contamination compositional and isotopic data suggest that the different avf series derived from distinct parental magmas which were generated by partial melting of a heterogeneous mantle source

genesis of the giant shizhuyuan w sn mo bi pb zn polymetallic deposit south china constraints from zircon geochronology and geochemistry in skarns abstract the giant shizhuyuan w sn mo bi pb zn polymetallic skarn type deposit which lies in the world famous nanling w sn metallogenic belt has a close affinity with the qianlishan granite complex previous studies focused on the qianlishan granitoids rather than relevant skarns where the evolutionary processes of skarn forming fluids mineralized fluids and sources of metals are debated this study carried out systematic epma and la icpms zircon u pb dating major and trace element and lu hf isotopic analyses for three types of skarns ore free greisen stockwork and sulfide bearing skarns at shizhuyuan the whole skarn zircons were classified into six age clusters 125 187 ma 235 ma 345 509 ma 677 772 ma 811 921 ma and 1000 ma and three genetic types detrital magmatic and hydrothermally altered among them the majority of the skarn zircons are of detrital origin 345 to 2225 ma peaking at 410 ma 710 ma and 905 ma and showing a wide range of th u ratios 0 05 7 09 they share similar characteristics of zircons hosted by devonian strata in south china suggesting the inheritance from wall rock the magmatic zircons in skarns can be further subdivided into two clusters triassic 235 ma and jurassic 150 ma which are derived from the corresponding granite ore free skarn contains only detrital zircons that have experienced the weakest alteration indicating the minor participation of granitic fluid the greisen stockwork skarn consists of both detrital zircons and jurassic magmatic hydrothermal zircons implying that granitic and stratigraphic materials may both contribute greatly to the w sn polymetallic mineralization sulfide bearing skarn includes abundant detrital zircon a few of triassic zircons but no jurassic zircons indicating that the latest sulfide mineralization was dominated by sediments rather than granitic rocks the two mineralized skarn zircons display elevated concentrations of most trace elements e g u ree ti y p nb and ta coupled with decreased si and zr which reflects strong fluid rock reaction the lowest ratios of ce ce 1 22 89 78 mean 24 13 in greisen stockwork skarn zircons reveal the most reducing environment for the main w sn mineralizing event jurassic zircons are homogeneous in 176hf 177hf 0 282352 0 282454 and ehf t 11 58 to 7 27 in well accord with local granitic rocks triassic zircons have variable ehf t 9 5 to 1 6 values suggesting more mantle input of precursor pluton before the jurassic main mineralization event based on the zircon geochronology in skarns we speculate that concealed triassic magmatism exists in the deep shizhuyuan area consequently a three stage ore forming model was established for the giant shizhuyuan deposit this study further illustrates that zircon can be used as an important indicator for ore genesis in complex metallogenic systems

precise molybdenite re os and mica ar ar dating of the mesozoic yaogangxian tungsten deposit central nanling district south china the yaogangxian deposit in the central nanling region south china consists of vein type ore bodies hosted in cambrian to jurassic strata and mesozoic granitic intrusions wolframite and molybdenite are the dominant ore minerals intergrown with gangue minerals of quartz feldspar phlogopite and muscovite we have carried out molybdenite re os and phlogopite and muscovite 40ar 39ar dating to better understand the timing and genesis of mineralization re os dating of eight molybdenite samples yielded model ages ranging from 152 0 3 5 to 161 1 4 5 ma with an average of 156 0 ma the re os analyses give a well defined 187re 187os isochron with an age of 154 9 2 6 ma mswd 2 4 hydrothermal phlogopite and muscovite display extremely flat 40ar 39ar age spectra phlogopite yields a 40ar 39ar plateau age of 153 0 1 1 ma whereas muscovite yields a plateau age of 155 1 1 1 ma both 40ar 39ar ages are in good agreement with the re os ages placing the timing of tungsten mineralization at about 154 ma this age is consistent with the field relationships our new data when combined with published geochronological results from other major deposits in this region suggest that large scale w sn mineralization occurred throughout the central nanling region in the late jurassic

the geology and petrology of volcan san juan nayarit mexico and the compositionally zoned tepic pumice abstract volcan san juan is the westernmost quaternary composite volcano in the mexican volcanic belt its activity is divided into three stages of decreasing eruptive volume during stage 1 activity the main cone of san juan and the adjacent cerro alto grew to a combined volume of 60 km3 through eruption of hornblende bearing andesites dacites and rhyodacites stage 1 included explosive eruptions of hornblende dacite and an andesite dated at 33 750 a years ago and 23 010 a stage 2 activity involved a major plinian eruption 14 770 a that produced the hypersthene hornblende bearing compositionally zoned rhyodacitic andesitic tepic pumice and led to formation of an oval shaped caldera 4 1 km in diameter tepic the capital city of nayarit state with a population of 250 000 lies at the ne foot of volcan san juan this location was buried under 2 9 m of rhyodacitic pumice fall deposits from the tepic pumice eruption erosion has since removed virtually all exposures of tepic pumice 15 km from the caldera the estimated volume of erupted tephra is 5 6 km3 equivalent to 1 2 km3 of magma modeling of the eruption dynamics indicates that as the eruption evolved the column height increased from 15 km to 24 km and maximum mass discharge rate increased from 3 106 kg s to 6 107 kg s during stage 3 activity a hornblende andesitic lava dome partially filled the caldera and sent block lava flows down the north slope of the cone total erupted volume was 0 55 km3 compared to other composite volcanoes of the western mexican volcanic belt the products from san juan have the highest sr contents and the lowest rb y yb and lu contents resulting in high la yb and sr y with low rb sr these observations indicate a strong slab melt component at san juan consistent with its proximity to the middle america trench where young hot lithosphere is subducting the san juan andesites dacites and rhyodacites also have relatively high 87sr 86sr 206pb 204pb 207pb 204pb and 208pb 204pb and low end values compared to these other volcanoes which appear to reflect significant crustal contamination of san juan s magmas

phase equilibria of a be u and f enriched vitrophyre from spor mountain utah abstract the equilibrium melting relations of a vitrophyre representative of topaz rhyolites have been experimentally determined in order to constrain the pre eruption temperature and h2o content of topaz rhyolite magmas the vitrophyre sample is strongly enriched in f and incompatible elements and consists of approximately 70 unaltered volcanic glass and 30 crystals of biotite quartz sanidine plagioclase magmatic fluorite and a variety of accessory phases biotite stability is strongly dependent on temperature pressure and ah2o in the melt and is apparently independent of the variations in o2 over the range investigated in this study the thermal stability of biotite increases relative to that of quartz sanidine and plagioclase in water saturated melts as pressure increases from 1 2 to 2 kbar fluorite is stable to temperatures below but within 50 to 75 c of the biotite stability curve and topaz is present in the products of runs at and near the solidus for all fluid conditions investigated the 1 2 wt f in the vitrophyre results in very low solidus temperatures of 525 c for all fluid conditions studied comparison of the experimentally determined crystallization sequence of the vitrophyre with the observed crystallization sequence of the vitrophyre and of the overlying topaz rhyolites indicates that the phenocrysts in the vitrophyre and in the topaz rhyolites crystallized at a pressure 1 kb that the magma contained at least 4 wt water during initial crystallization of biotite and that the magma contained a pre eruption water content of 5 wt

alkalic ocean island basalt type and calc alkalic volcanism in the mexican volcanic belt a case for plume related magmatism and propagating rifting at an active margin the mexican volcanic belt has been traditionally regarded as a classic case of subduction related calc alkalic volcanism however a series of geologic geophysical and petrological arguments makes this simple relationship doubtful a seismic gap beneath the belt a large scale mantle anomaly a graben triple junction domain and the presence of volumetrically important oceanic island basalt oib volcanism throughout the belt suggest a more complex tectonic scenario involving plume and subduction related processes we here propose a model involving the development of a propagating rift opening from west to east in response to plume activity the process started in miocene time within the western sector of the belt guadalajara and gave rise to a graben triple junction and oib type and calc alkalic volcanism extension and volcanism proceeded to the east giving rise to progressively younger ages for the initiation of oib type volcanism 1 miocene in the west e g guadalajara 2 pliocene in the central zone e g michoacan guanajuato and 3 quaternary farther east e g chichinautzin geochemical evidence suggests that part of the modern calc alkalic volcanism e g chichinautzin may be derived from magma mixing between the oib mafic magmas and silicic crust derived magmas however we do not preclude some influence of the subducting slab in the generation of other e g jorullo calc alkalic volcanic rocks our model suggests a currently unrooted upper plume attached to the subcontinental lithosphere which defines a hot zone beneath the mexican volcanic belt

geochemistry of the amazon 1 precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the amazon river system draining past obidos 80 of the basin at the time of peak discharge in june amazon precipitation chemistry can be devided into two principal components marine and terrestrial the marine component determined from analyses of marine rain consists of na k mg ca and cl in approximately sea salt proportions with s doubly enriched the excess sulfur is probably derived from gas phase inputs the terrestrial component makes an important contribution of k ca s and n much of which can be related to biological emissions the emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural acid rain in the amazon region with a ph from 4 7 to 5 7 this is about one tenth the acidity of polluted urban rain the chloride content of lowland rivers which drain regions lacking significant geologic sources of chloride shows a systematic decrease in chloride with increasing distance from the ocean this trend is used to define the cyclic salt background for amazonian surface waters cyclic salts in general make only a minor contribution relative to terrestrial inputs to the chemistry of amazon basin rivers even those draining intensely weathered terrains an estimated 17 6 cl 6 9 na 1 3 mg 3 6 s 0 4 k and 0 1 ca of the dissolved load at obidos during peak discharge is cyclic

partitioning of f between h2o and co2 fluids and topaz rhyolite melt implications for mineralizing magmatic hydrothermal fluids in f rich granitic systems fluid melt distribution coefficients for f have been determined in experiments conducted with peraluminous topaz rhyolite melts and fluids consisting of h2o and h2o co2 at pressures of 0 5 to 5 kbar temperatures of 775 1000 c and concentrations of f in the melt ranging from 0 5 to 6 9 wt the major element f and cl concentrations of the starting material and run product glasses were determined by electron microprobe and the concentration of f in the fluid was calculated by mass balance the h2o concentrations of some run product glasses were determined by ion microprobe sims the solubility of melt in the fluid phase increases with increasing f in the system the solubility of h2o in the melt is independent of the f concentration of the system with up to 6 3 wt f in the melt no evidence of immiscible silica and fluoriderich liquids was detected in the hydrous but water undersaturated starting material glasses 8 5 wt f in melt or in the water saturated run product glasses f concentrates in topaz rhyolite melts relative to coexisting fluids at most conditions studied however df wt f in fluid wt f in melt increases strongly with increasing f in the system maximum values of df in this study are significantly larger than those previously reported in the literature linear extrapolation of the data suggests that df is greater than one for water saturated peraluminous granitic melts containing 8 wt f at 800 c and 2 kbar df increases as temperature and as h2o h2o co2 of the fluid increase for topaz rhyolite melts containing 1 wt f and with h2o rich fluids df is independent of changes in pressure from 2 to 5 kbar at 800 c for melts containing 1 wt f and in equilibrium with co2 bearing fluids the concentrations of f in fluid increases with increasing pressure f and lithophile element enriched granites may evolve to compositions containing extreme concentrations of f during the final stages of crystallization if f in the melt exceeds 8 wt df is greater than one and the associated magmatic hydrothermal fluid contains 4 molal f such f enriched fluids may be important in the mass transport of ore constituents i e f mo w sn li be rb cs u th nb ta and b from the magma

high fluorine rhyolite an eruptive pegmatite magma at the honeycomb hills utah the honeycomb hills rhyolite dome in western utah displays chemical and mineralogical features characteristic of a rare element pegmatite magma the lavas show extreme enrichments in such trace elements as rb 1960 ppm cs 78 li 344 sn 33 be 270 and y 156 phenocrysts 10 50 by volume include sanidine or 66 70 plagioclase ab 83 92 quartz biotite approaching fluorsiderophyllite and fluortopaz as well as accessory phases common to highly differentiated granites and pegmatites including zircon thorite fluocerite columbite fergusonite and samarskite low temperatures 600 to 640 c coupled with high phenocryst and silica content might normally preclude eruption due to the extremely high viscosity of the melt however high concentrations of fluorine 2 3 could domal lavas significantly reduce viscosity and allow eruption of domal lavas even after dewatering of the mama during the initial pyroclastic phase of the eruptive cycle fractionation of phenocrysts and accessory phases for which partition coefficients have been measured is sufficient to account for most compositional gradients inferred in the preeruptive magma body although transport by a fluid phase formed a may have caused upward enrichments in li be and cs if the honeycomb hills magma had crystallized at depth it would have formed a rare element pegmatite

the weathering of basalt and relative mobilities of the major elements at belbex france abstract a basalt and its weathered profile have been analysed for major elements graphical examination shows al fe and ti to have been essentially immobile determining mobility sequences for this deposit is complicated by the long pedogenetic history of the area three different sequences are presented two for different stages of weathering integrated over time and one that reflects the weathering regime of the present day

neogene and quaternary development of the neotropical rain forest the forest refugia hypothesis and a literature overview abstract the upheaval of the northern andes in miocene and pliocene time changed the drainage system in northern south america significantly and caused the present day rain forest areas of choco and the lower magdalena valley became separated from amazonas plant diversity may have reached the highest level in the miocene or pliocene and excessive present day phytodiversity may be regarded as a legacy of the tertiary rather than an evolutionary product of the quaternary in the quaternary strong temperature oscillations related to the series of ice ages were superposed on the late tertiary forest dynamics which included river displacement and latitudinal migrations of the equatorial rain belt caloric equator with the rhythm of the precession cycle of orbital climate forcing the hypothesis that claims a permanent rain forest cover all over the amazon basin during the last glacial is in contrast with the forest refugia hypothesis which accepts replacement of rain forest by savanna or savanna forest during dry climatic intervals both scenarios have been evidenced by pollen records in this paper it is suggested that both hypotheses are not necessarily conflicting and apparently did occur in different parts of the amazon basin and in different periods depending on the climatological constraints a compilation of the most important literature concerning the vegetational climatic and environmental history of the rain forest areas of amazonas and choco and surrounding dry ecosystems has been included

enrichment of ion exchangeable rare earth elements by felsic volcanic rock weathering in south china genetic mechanism and formation preference abstract felsic volcanic rocks are an important source for generating ion adsorption type rare earth element ree resources to better understand the supergene enrichment of rees related to felsic volcanic rock weathering this study conducted an investigation of indosinian felsic volcanic rocks from guangxi southwest china which have been subjected to deep weathering the weathering has formed large areas of thick regolith with the potential to host ion adsorption type ree mineralization the felsic volcanic rocks belong to the dacite rhyolite series and are peraluminous high k calc alkaline and ree rich 325 376 ppm abundant ree bearing accessory minerals are present in the felsic volcanic rocks including titanite average ree 15 1 wt allanite average ree 14 9 wt and apatite average ree 1510 ppm and these three minerals contain an estimated 88 7 of the whole rock rees content significant ree enrichment is present in the felsic volcanic rock derived regolith a typical rhyolite derived regolith profile has ree contents that increase from 376 ppm in the rhyolite to 1737 ppm in the regolith representing an almost five fold enrichment due to weathering the rees in the regolith are present in an ion exchangeable form irees which accounts for 52 87 of the total rees present trees the occurrence of irees is closely linked to clay minerals showing an affinity in the order halloysite kaolinite illite continuous operations of rees by an eluviation illuviation process from the source minerals titanite allanite apatite to sink minerals kaolinite halloysite illite results in an iree enriched zone in the middle and lower parts of the rhyolitic regolith notably besides the studied indosinian felsic volcanic rocks there are multi epochs of such lithology outcropped in south china their related irees mineralizations however are preferentially developed in the weathered terrains of early yanshanian and indosinian felsic volcanic rocks and not in those of the yanshanian and late yanshanian felsic volcanic rocks a comparison of the mineralized and barren units indicates that the irees mineralization hosted in felsic volcanic rocks regolith is controlled by some key endogenic and exogenic ore forming factors high initial ree concentrations in the unaltered felsic volcanic rocks a suitable climate and a relatively quiescent tectonic setting are favorable for the formation and preservation of irees mineralization

continental boundaries of the jalisco block and their influence in the pliocene quaternary kinematics of western mexico extensional faulting observed in southwestern mexico has been related to the incipient rifting of the jalisco block from the mexican mainland since the pliocene on the basis of new structural and geophysical data we propose that 1 the continental boundaries of the jalisco block are ancient structures reactivated since the pliocene at a low 1 mm yr rate of deformation and 2 pliocene quaternary extensional faulting at the edges of jalisco block is a basement controlled intraplate deformation related to plate boundary forces rather than to active continental rifting the jalisco block boundaries first developed in response to the uplift of the puerto vallarta batholith in pre neogene time and underwent a complex contractile deformation before the pliocene during pliocene quaternary times north northeast extension reactivated the northern boundary forming the tepic zacoalco rift whereas east southeast extension formed the northern colima rift south of the colima volcano active extension is found only west of the so called southern colima rift and partly reactivates old northeast trending basement faults the parallelism between the subducted rivera cocos plate boundary zone and the eastern neotectonic boundary of the jalisco block supports east southeastward motion of the southern mexican blocks induced by the differential motion and oblique subduction of the cocos and rivera plates on the other hand we envisage pliocene quaternary extension along the northern boundary as an upper plate response to the low convergence rate and the steep subduction angle of the rivera plate

etch pit size and shape distribution on orthoclase and pyriboles in a loess catena abstract etch pits on 50 100 m orthoclase and pyribole grains from the e b and c horizons of a catena or drainage sequence of well drained somewhat poorly drained and poorly drained loessal soils were analyzed using sem and digital image analysis the objective was to determine the influence of soil drainage conditions and the associated geochemical environments on dissolution of primary silicates as determined from the distribution of etch pits on the grain surfaces etch pit size frequency distributions indicated that the poorly drained soil contained the smallest and most numerous etch pits for both minerals this indicated low growth rates for etch pits in the poorly drained soil analysis of etch pit shapes indicated that etch pits on pyriboles were more elongated and irregular in the poorly drained soil possibly resulting from a greater growth rate whereas etch pits on orthoclase grains were more elongated and irregular in the well drained soil the larger more elongated and irregularly shaped etch pits on orthoclase from the well drained soil indicative of a greater pit growth rate is consistent with a flushing model of mineral weathering in soils the etch pit size distribution on pyriboles is consistent with the flushing concept but the shape data indicates that other mechanisms of weathering may be involved

crystallization and melting experiments of a fluorine rich leucogranite from the xianghualing pluton south china at 150 mpa and h2o saturated conditions abstract crystallization and melting experiments were conducted with a fluorine rich granite from the xianghualing pluton south china in order to determine its crystallization sequence and melt fractionation trends at 150 mpa and h 2 o saturated conditions which are close to the crystallization conditions of this pluton the experimental results showed that quartz is the liquidus phase and is sequentially followed by k rich alkali feldspar zinnwaldite albite and topaz there are obvious evolutionary trends in the compositions of alkali feldspar and residual melt with decreasing temperature from liquidus to 600 c the or kalsi 3 o 8 content in alkali feldspar increases and the contents of ab naalsi 3 o 8 and an caal 2 si 2 o 8 decrease the most apparent fractionation trends in residual melt compositions are decreases in the sio 2 and k 2 o contents and si al ratio and increases in the contents of na 2 o f al 2 o 3 cao and the ratios of a nkc and na k these experimental results indicate that a low silica and sodic granitic melt could be derived from a relatively high silica and potassic f rich leucogranitic magma via fractional crystallization they also shed light on the fractional crystallization and vertical zoning of the xianghualing pluton and other similar high level f rich leucogranite plutons

upper pleistocene amazon deep sea fan muds reflect intense chemical weathering of their mountainous source lands abstract upper pleistocene muds of the amazon deep sea fan contain an abundance of residual weathering products as indicated by their bulk chemical and mineralogical compositions the muds are chemically more mature than average shale average pelagic mud or the surrounding demerara abyssal sediments they are however very similar to the bulk chemical compositions of contemporary suspended sediments of both the amazon estuary and those carried by the major andean tributaries the modern sediment compositions reflect the rapid weathering now occurring in the andean source regions of the basin the major element geochemical signature of the upper pleistocene deep sea fan sediments suggests that intense chemical weathering in the mountainous source lands has controlled the chemistry of amazon river suspended sediments since at least the upper pleistocene minor and trace element signatures appear uniform throughout the fan and element concentrations are in general higher than in the intensely weathered amazon lowland soils but lower than in deep sea sediments

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

eruption transport and deposition of ignimbrite a case study from mexico abstract the acatlan ignimbrite covers 150 km 2 around a vent situated 40 km southwest of guadalajara mexico the upper and main ignimbrite flow unit is compositionally zoned and has white or pink rhyolitic pumice in its lower part and black andesite in its upper these parts are believed to have been erupted continuously from the same magma chamber this zonation has been important in correlating an extremely coarse lithic rich deposit a co ignimbrite lag fall deposit with this ignimbrite the co ignimbrite lag fall deposit indicates that the flow unit formed by continuous collapse of an explosive eruption column and enables the source vent to be located the vertical compositional grading of the flow unit also provides a means of testing the various proposed mechanisms of ignimbrite emplacement turbulent vs high concentration flow the preservation of compositional zonation and other features in the deposit indicate that flow was not turbulent details of the grain size distribution indicate that laminar flow was important early in the flow history when collapsing material from the eruption column was superimposed in stratigraphical sequence initial high gas flow rates associated with flow segregation and formation at the site of column collapse rapidly decayed away from vent and later in its history the flow degassed to a semi rigid plug overriding a sheared basal layer

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle

the effect of fluorine on viscosities in the system na2o al2o3 sio2 implications for phonolites trachytes and rhyolites the effect of fluorine on melt viscosities of five compositions in the system na2o al2o3 sio2h as been investigateda t one atmospherea nd 1000 1600 cb y concentric cylinder viscometry the compositions chosen were albite jadeite and nepheline on the join naalolsio2 and two others of the join at 75 mole percent sio2 one peralkaline and one peraluminous all melt viscosities were independent of shear rate over two orders of magnitude indicating newtonian behavior all viscosity temperature relationships were arrhenian within error fluorine reduces the viscosities and activation energies of all melts investigated the viscosity reducing power of fluorine increases with the sio2 content of melts on the join naalo2 sio2 and is a maximum at na al molar i for melts containing 75 mole percent sio2 fluorine and water have similar effects on aluminosilicate melt viscosities probably due to depolymerization of these melts by replacement of si o si al bridges with si oh and si f bonds respectively evidence from slag systems shows that fluorine also reduces the viscosity of depolymerized silicate melts the viscous flow of phonolites trachytes and rhyolites will be strongly afected by fluorine it appears that fluorine contents of igneous rocks may be combined with water in calculation schemes for determining the viscosity of natural melts