complex concentric granitoid intrusions in the coastal mobile belt espirito santo brazil the santa angelica pluton an example the brazilian coastal mobile belt in southern espirito santo consists of archean to upper proterozoic units which have been metamorphosed and deformed to a gneiss migmatite and granulite facies rock assemblage during archean and proterozoic tectono thermal events

metamorphic history and geodynamic significance of high grade metabasites from the ophiolitic melange beneath the yarlung zangbo ophiolites xigaze area tibet abstract blocks of highly foliated amphibolite are locally embedded within a serpentinite melange underlying the yarlung zangbo ophiolites in the xigaze area of southern tibet the ophiolites are remnants of an early cretaceous back arc basin within the permo cretaceous tethys ocean which are exposed along in the yarlung zangbo suture zone yzsz these amphibolites are interpreted as fragments of a dismembered dynamothermal sole three types of amphibolite are present 1 common amphibolite with assemblages of hbl pl ep ap ttn 2 clinopyroxene bearing amphibolite with hbl pl cpx ep ttn qtz ap and 3 garnet clinopyroxene bearing amphibolite characterized by the assemblages hbl cpx grt pl rt and grt hbl pl corona assemblage in all three types plagioclase is pseudomorphed by late albite prehnite retrograde cataclastic veins containing assemblages of prh ab ep chl are also present p t estimates indicate that the amphibolites reached peak metamorphic conditions of 13 15 kbar and 750 875 c partial replacement of pyrope rich up to 35 mole garnet by al tschermakite al2o3 up to 21 wt reflects a high pressure 18 kbar 600 c metamorphic event followed by rapid exhumation soon after exhumation the amphibolites were intruded by very fine grained diabase dykes that were then hydrothermally altered the field relationships and metamorphic history of the amphibolites indicate formation during inception of subduction within a back arc basin prior to obduction of the ophiolites onto the indian passive margin

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

significance of ferrodioritic liquids in magma mixing processes a general model to explain the various igneous associations found in mixed magma intrusives and extrusives belonging to high level volcanic centres is presented this model is based on detailed mapping in the british tertiary volcanic centre on skye which suggests that basic and acid magmas were continually available throughout its development new major and trace element data from these rocks indicate that they were co genetic and that the basic magmas which combined with acid magmas to produce hybrid magmas were ferrodioritic in composition and not basaltic as had previously been thought

winamphcal a windows program for the ima 04 amphibole classification a microsoft visual basic 6 0 program called winamphcal has been developed to calculate structural formulae of both wet chemical and microprobe derived amphibole analyses on the basis of the standard international mineralogical association ima 04 classification procedure winamphcal classifies amphibole analyses into five groups and then determines a specific amphibole name with prefixes and modifiers this software is developed to predict cation site allocations at the different structural positions as well as to estimate stoichiometric fe3 and h2o contents from microprobe analyses if fe2o3 content is unknown the program calculates ferric iron content on the basis of the minimum fe3 15enk and maximum 13ecnk criteria considering the site assignments and stoichiometric constraints other user defined calculation and normalization factors can also be carried out by current software for the process of amphibole chemical analyses using the charge balance method winamphcal permits the user to determine the mn2 and mn3 states from mn3 rich microprobe derived sodic amphiboles winamphcal stores all the calculated results in an excel file hence output of the program can also be displayed and processed by any other software for general data manipulation and graphing purposes

the metamorphic sole of new caledonia ophiolite 40ar 39ar u pb and geochemical evidence for subduction inception at a spreading ridge amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of new caledonia termed peridotite nappe recrystallized in the high temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the eocene metamorphic complex amphibolites mostly display the geochemical features of morb with a slight nb depletion and thus are similar to the youngest late paleocene eocene babb components of the allochthonous poya terrane thermochronological data from hornblende 40ar 39ar zircon and sphene u pb suggest that these mafic rocks recrystallized at 56ma using various geothermobarometers provides a rough estimate of peak recrystallization conditions of 0 5gpa at 800 950c the thermal gradient inferred from the metamorphic assemblage 60 ckm 1 geometrical relationships and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction obduction system and recrystallized when they subducted below young and hot oceanic lithosphere they were detached from the down going plate and finally thrust onto unmetamorphosed poya terrane basalts this and the occurrence of slab melts at 53ma suggest that subduction inception occurred at or near to the spreading ridge of the south loyalty basin at 56ma 2012 american geophysical union all rights reserved

behavior of alkalies during diffusive interaction of granitic xenoliths with basaltic magma a series of three experiments was made at 125 c and 10 kbar in which a 1 gram reservoir of molten oceanic tholeiite was maintained in contact with one end of a 6 5 mm diameter cylinder of partially molten fine grained granite detailed microprobe analysis 450 spots total of the quenched samples revealed progressive basalt granite interaction as run duration increased from 0 75 to 8 to 24 hours interdiffusion of melt components is limited for structure controlling species such as si 4 and al 3 and for divalent cations which all mix over a zone less than 0 5 mm wide in 24 hours at run conditions in contrast k and na are extremely mobile resulting in considerable uptake of k by the basalt melt and eventual loss of na from the basalt to the granite by uphill diffusion the chemical diffusivity of k in the basalt can be accurately estimated at 3 times 10 18 cm 2 sec for the specific experimental conditions used although this value is somewhat lower than previous estimates k o

stratigraphy of upper paleozoic volcanic rocks and regional unconformities in part of the northern sierra terrane california upper paleozoic volcanic rocks in the southern part of the taylorsville strike belt are unconformably truncated at the north fork of the american river by a lens of upper triassic conodont bearing limestone and lower and middle jurassic graywacke slate flysch deposits of the sailor canyon formation the volcanic sequence consists of submarine felsic pyroclastic debris flows and tuffaceous slate of the late devonian sierra buttes formation andesitic turbidites of the taylor formation a lower tuff siltstone member and an upper chert member of the peale formation and fine grained tuff of the reeve formation newly discovered radiolarians date the chert member of the peale as latest late mississippian and or early pennsylvanian shelly fossil fragments found 50 m above the chert member date the lower part of the reeve in this area as late early permian these rocks formed in the distal part of the submarine volcanic apron presumably in an island arc setting and were deposited unconformably on quartz rich clastic rocks and tectonically interlayered chert of the pre late devonian shoo fly complex the peale marks a period of quiescence in the late paleozoic volcanism that culminated in deep water chert deposition during latest mississippian to the middle pennsylvanian the peale reeve contact is a regional hiatus probably nondepositional in the south and becoming erosional to the north that spans most or all of the late pennsylvanian and early permian post late early permian to pre late triassic deformation produced northeast trending folds and a southeast trending fault in the late paleozoic volcanic rocks that are truncated nearly orthogonally by the late triassic unconformity and northwest trending folds in the jurassic sailor canyon formation

magma mingling and melt modification between granitic pipes and host diorite guernsey channel islands within the northern igneous complex of guernsey cylindrical pipe intrusions 20 40 cm in diameter developed by buoyant emplacement of granite magma into overlying denser diorite magma the high viscosity of the partially crystallized magmas and lack of mechanical stirring inhibited physical mixing and mesoscopically sharp well defined interfaces developed the granitic pipes are surrounded by aureoles up to a metre in diameter which exhibit changes in bulk chemical composition and partial mineralogical re equilibration these formed in response to the modification of interstitial melt in the partially crystallized host resulting from residual melt migration and or melt interdiffusion across the pipe host interface such processes between partially crystallized magmas may in part provide an explanation for textural mineralogical and chemical compositional features commonly exhibited by mafic enclaves within granites

counterclockwise p t t paths from amphibolites franciscan complex california relics from the early stages of subduction zone metamorphism overprinting mineral relationships in amphibolites from the franciscan complex california suggest metamorphic evolution from high temperature amphibolite facies to the blueschist facies with increasing p t ratio a counterclockwise p t t path early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are in turn cut or rimmed by sodic amphiboles some amphibolites are overprinted successively by eclogite and blueschist facies assemblages clinopyroxenes are zoned with increasing jadeite component from core to rim multiple generations of phengites are present in the same sample and are more si rich with each successive generation of growth garnets in eclogite overprinted amphibolites are zoned with an initial mgo increase away from the core to a maximum mgo zone followed by mgo decrease toward the rim garnets in other amphibolites show continually increasing mgo from core to rim several samples representative of the variety of franciscan amphibolites are describ

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

petrogenesis of a basalt rhyolite tephra from the west central afar ethiopia the cindery tuff is an unusual tephra fall deposit that contains evidence for the mixing of basaltic and rhyolitic liquids prior to eruption it contains clear rhyolitic glass shards together with brown basaltic glass spheres and a broadly bimodal phenocryst assemblage brown glasses are ferrobasaltic in composition and are similar to the voluminous pliocene tholeiites of the surrounding west central afar volcanic field both are enriched in the light rare earth and incompatible elements and possess higher 87sr 86sr and lower 143nd 144nd than morb rhyolitic glasses are subalkaline and compared to the basaltic glasses are strongly depleted in the compatible elements and enriched in the incompatible elements both glass types have similar incompatible element and isotopic ratios and with the rhyolite glass showing a 2 fold parallel enrichment in rare earth element abundances over the basaltic glass these observations suggest that the two glasses are genetically related

strontium neodymium and lead isotopic compositions of deep crustal xenoliths from the snake river plain evidence for archean basement abstract xenoliths of intermediate to felsic granulites found in evolved lavas from the snake river plain have been analyzed for nd pb and sr isotopic compositions and related trace element contents overall they exhibit wide ranges in present day values of 87 sr 86 sr 0 70235 0 83011 143 nd 144 nd 0 51023 0 51148 and 206 pb 204 pb 13 43 24 65 the rb sr u pb th pb and sm nd decay schemes have been variably affected by granulite facies metamorphism and possibly by transport in the host magmas however pb pb and sm nd isochron systematics seemingly are preserved in many of the xenoliths and indicate essentially concordant metamorphic ages of about 2 8 ga nd model ages are significantly older ca 3 1 3 4 ga for many of the xenoliths precambrian metasediments exposed along the southern margin of the snake river plain have sm nd systematics similar to those of the xenoliths these results suggest that at least two significant thermal events occurred during archean evolution of the crust in this region a early ca 3 1 3 4 ga additions of mantle derived magmas to the crust and b regional metamorphism at 2 8 ga accompanied at least locally by magmatism concordance between the sm nd and pb pb ages suggests that these isotopic systems are little affected by high grade metamorphism in this case the distribution of xenoliths in snake river plain lavas supports the presence of archean crust beneath much of southern idaho although such rocks rarely are exposed at the surface thus nd and pb isotopic studies of crustal xenoliths can provide a useful means of determining the extent of crustal age provinces where surface exposures are lacking

petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the solea graben northern troodos ophiolite cyprus hydrothermal mineral zonations and o isotope patterns of the northern troodos complex do not parallel the ophiolite pseudostratigraphy but reflect the convective geometry of an upper cretaceous seawater hydrothermal system large areas of the sheeted intrusive complex sic including the subaxial region of the solea graben are composed of 18o rich subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge other areas of the sic are recrystallized to distinctive epidosite rocks granular high variance assemblages of epidote quartz chlorite that are depleted in 18o al2o3 na2o k2o zr cu and zn and are enriched in cao and sr compared with other mafic volcanic and dike rocks of the solea graben epidosite alteration occurred at temperatures of 310 370 c and involved fluids with 18o values and salinities similar to those of upper cretaceous seawater the epidosite zones are discordant with earlier mineral o isotope zonations and with the axis of spreading in the solea graben suggesting a postspreading off axis origin the seawater hydrothermal system responsible for solea graben massive sulfide deposits was probably driven by hypabyssal intrusions not exposed emplaced in a terminal failed spreading episode the geometries of o isotope surfaces within the solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits depletions in base metals show that epidosite alteration liberated cu and zn to mineralizing fluids within the fossil upflow zone

geologic consequences of plate reorganization an example from the eocene southern alaska fore arc observation of relative timing of deformation metamorphism and plutonism in a high temperature low pressure metamorphic belt in the eastern chugach mountains of alaska leads to a model of ridge subduction followed by plate reorganization to account for the abnormally high geothermal gradients in the fore arc between 56 and 53 ma a change in the direction of the kula farallon spreading halted the previous southward migration of the kula farallon north american triple junction this forced the triple junction to migrate back north along the plate margin enlarging the slab window beneath the accretionary margin the expanded slab window produced a large scale thermal manifestation now recognized as the chugach metamorphic complex the accretionary complex responded to plate reorganization by orogen parallel extension associated with oblique subduction of the kula plate contraction began again following passage of the triple junction and subduction of the farallon plate

a laboratory model of a replenished magma chamber abstract it has recently been suggested that periodic influxes of hot but heavy magma into the base of a basaltic magma chamber can remain isolated from the rest of the chamber while the new magma cools and crystallization proceeds when thermal equilibrium is almost complete the suspended crystals settle out and the residual less dense liquid can then mix with the fluid above in the present paper the basic fluid dynamical processes underlying this model have been investigated in laboratory experiments using aqueous solutions the lower layer was hot kno3 solution for which saturated solutions become less dense as the temperature decreases with a cold deeper layer of less dense nano3 or k2co3 above the lower layer there was strong convective transfer of heat through a sharp interface separating the layers at a rate which is predicted here drawing on previous studies carried out with oceanographic applications in mind once crystallization began non equilibrium effects became important and the observed temperatures differ somewhat from those predicted in the experiments crystals grew mainly from the bottom rather than while in suspension but this is not an essential aspect of the model the important fact is that the density of the residual liquid in the lower layer decreased until it became equal to that of the upper layer and then the interface broke down so that the two layers mixed thoroughly together leaving a layer of kno3 crystals at the base no crystallization at all occurred when the hot input liquid was forced to mix initially with the cold solution already in the chamber

interaction between coeval mafic and felsic melts in the st francois terrane of missouri usa abstract in the eastern st francois mountains of missouri a 1 48 ga subvolcanic ring pluton known as the silvermine granite hosts swarms of dark pillow shaped enclaves exhibiting classical features of mafic melt globules trapped in granitic magma the pillows formed by turbulent injection of near liquidus mafic melt into a resident visco plastic granitic magma exchange of crystals fluids and ions between the two melts produced a hybrid tonalite granite enclave series and modified host granite composition several lines of evidence suggest interaction between granitic melt that produced the granophyric cap of the pluton and basaltic melt that formed dikes in the area rapid cooling of the pillows re melted the host creating back veins in many enclaves mineral chemistry indicates the hybrid rocks have undergone extensive subsolidus re equilibration

p t paths from high temperature shear zones beneath ophiolites mctamorphic rocks of the st anthony complex of north western newfoundland are best interpreted in terms of a high temperature shear zone formed between down going continental margin rocks and overriding oceanic lithosphere in a subduction zone high grade rocks immediately beneath the oceanic lithosphere peridotite display retrograde meta morphism in high strain zones whereas lower grade rocks near the base of the metamorphic complex display prograde metamorphism in high strain zones mylonite zones in meta basitcs at all levels in the complex contain the assemblage epidote hornblende albite sodic oligoclase these observations suggest that the inverted metamorphic gradient within the st anthony complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone the degree of re equilibration is strongly dependent on the degree of strain and is best achieved in synmetamorphic mylonite zones this interpretation of the st anthony complex can be extended to other sub ophiolite metamorphic sheets which show very similar relationships it is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones the p t paths of which preserve records of burial and exhumation in subduction zones

interactions between basaltic and granitic magmas in a svecofennian postorogenic granitoid intrusion aland southwest finland abstract the lemland intrusion is one of four postorogenic 1815 1770 ma granitoid intrusions occurring in southwestern finland the intrusions are dominated by porphyritic granite and basic intermediate rocks they show features of a bimodal magmatism causing net veining and formation of mixed rocks the current investigation of the southern part of the lemland intrusion reveals that the basic intermediate rocks form a ring like structure in which the rocks can be divided in three tentative units 1 fine grained mafic pillows in porphyritic granite 2 mixed rocks with mafic inclusions in a hybrid matrix and 3 medium grained mafic rock it is suggested that the ring like structure with the different units was emplaced from a disrupted zoned magma chamber and the different rock units were created due to differences in chemical and physical properties of the magmas the process started with the intrusion of basic magma into overlying acid magma creating basic pillows within it during the upward movement the basic magma left behind an area where the basic and the acid magma were intimately commingled and mixed this mixed magma intruded into the area inside the mafic pillows which formed an isolated barrier against the cooler acid magma the medium grained mafic rock in the middle of the ring structure is regarded as the same rock as the fine grained mafic rock the coarser grain size depending on the longer time for crystallization this rock was back veined and brecciated by the mixed magma creating a mega breccia when this process ended the acid magma was still active and formed cutting dykes in the mafic and mixed rocks

interlayered eclogites blueschists and epidote amphibolites from ne oman a record of protolith compositional control and limited fluid infiltration abbreviations ab albite ac acmite act actinolite alm aleclogites blueschists clinopyroxene rich rocks and epidote ammandine aln allanite ap apatite aug augite bar barroisite bt biotite phibolites occur interlayered on a millimetre to decimetre scale in a cc calcite chl chlorite cpx clinopyroxene crs crossite czo clinozoisite mafic unit interbedded with calcareous and quartz mica schists in dol dolomite ep epidote fe ox fe oxides gln glaucophane gross ne oman these rocks constitute part of the continental margin grossularite gt garnet hm haematite jd jadeite kln kaolinite kspar of oman subducted during the cretaceous petrological data indicate k feldspar mgt magnetite oxychl oxychlorite pg paragonite ph that they evolved through a clockwise p t path and attained peak phengite qz quartz pyrp pyrope rt rutile spess spessartine tc talc conditions at t 560 c p 12 kbar where the mineral ttn titanite win winchite zrn zircon assemblages of the blueschists eclogites and clinopyroxene rich rocks were all stable whole rock major trace and rare earth element data indicate that the mineralogical differences between these rock types resulted from the metamorphism of interlayered basaltic flows introduction and tuffs of variable compositions possibly reflecting different degrees interlayered blueschists eclogites and or greenschists are of magmatic differentiation the interlayered epidote amphibolites common in several areas such as cazadero california are chemically similar to the blueschists and appear to have formed e g oh et al 1991 the shuksan suite washington from the eclogites and blueschists during unroofing aided by fluid state dungan et al 1983 owen 1989 the tauern infiltration oxygen isotope data for minerals and whole rocks window eastern alps e g selverstone et al 1992 and indicate low fluid rock ratios during metamorphism and unroofing several cycladic islands e g okrusch brocker 1990 the data also suggest that different layers may have interacted with this interlayering has been attributed to 1 differences fluids of different isotopic signatures throughout their history these in the p t conditions of equilibration in the different results are similar to data reported for other areas of continent layers e g reinsch 1979 ridley 1984 2 original continent collision or continental crust subduction differences in the chemical compositions of the layers before metamorphism e g dungan et al 1983 oh et al 1991 or 3 differences in the extent of retrograde overprinting as a result of variable fluid infiltration e g schliestedt matthews 1987 brocker 1990 barrientos selverstone 1993

origin of hybrid ferrolatite lavas from magic reservoir eruptive center snake river plain idaho the mineralogy and geochemical characteristics of intermediate composition ferrolatites and related lavas from the magic reservoir eruptive center central snake river plain have been investigated to evaluate the origin and petrologic significance of these hybrid lavas the ferrolatites are chemically uniform but contain a disequilibrium phenocryst xenocryst assemblage derived in part from mixed rhyolitic and basaltic magmas that are closely represented by extrusive units in the area the hybrid lavas also contain xenoliths of archean granulites and have high 87sr 86sr and low 143nd 144nd ratios all of which suggest significant magma crust interaction quantitative models including magma mixing minor crystal fractionation and crustal contamination very closely reproduce the observed compositions of these ferrolatites closed system fractionation and or simple bulk contamination models are not as successful and can be ruled out it appears that preexisting mafic and silicic magmas from distinct sources e g mantle and crust encounter one another in crustal level magma chambers under conditions where intimate mixing may occur despite wide differences in the physical properties of these liquids

mafic replenishments into floored silicic magma chambers commingling between contemporaneous mafic and felsic magmas is now widely recognized in a broad range of intrusions and intrusive complexes these interactions are important features for two main of reasons 1 the rapidly chilled margins of mafic magma against silicic magma commonly preserve the compositions of mafic liquids and 2 because the mafic magma solidifies rapidly the resulting final configurations of mafic and felsic magmas can provide insights into physical processes and changing viscosity contrasts and rheologies of magmas and felsic crystal mush during crystallization of the mafic magma mingling of contrasted magmas was first recognized in the 1950s wider recognition of interactions between mafic and silicic liquids led to concepts of net veining in the 1960s intramagmatic flows chilled basaltic layers separated by felsic cumulates in the 1970s and in the 1990s to mafic silicic layered intrusions masli which could be as much as a few kilometers thick and more than 100 km2 in area it was quickly appreciated that these masli preserved stratigraphic records of mafic replenishments into silicic magma chambers floored by felsic crystal mush volcanic studies had anticipated the occurrence of this last type of intrusion on the grounds that extensive ponding of basaltic magmas beneath silicic chambers was seen to be essential to keep large silicic systems like yellowstone active for millions of years this paper looks at the history of changing perceptions and interpretations of magma mingling and whether or not sill complexes are distinct from mafic silicic layered intrusions the stratigraphy of mafic silicic layered intrusions records changing magmatic compositions events and processes in a temporal framework comparable to that provided by coeval volcanic rocks as a result careful study of masli has great potential for linking plutonic and volcanic processes and events

factor analysis and direct optimization of the amounts and properties of volcanic soil constituents abstract the variation of fourteen soil chemical and physical properties of twenty soil samples from andosols was decomposed into the contributions of seven soil constituents or end members the samples were from the slopes of the andesitic turrialba volcano in costa rica factor analysis of the data explained 98 of the variance by six orthogonal factors we replaced the factors by seven non orthogonal soil constituents and their mass fractions through direct fitting although the physical constraints of real world soil constituents decreased the explained variance to 92 the mean relative deviation between experimental and recalculated data improved from 11 to 8 we tentatively identify the soil constituents found through this decomposition as pure organic matter om volcanic ash a sesquioxide mixture with 6 om an association of allophane with iron silicate and 12 om and three different associates of about 57 mineral material and 43 om as dry end members the last three associations contribute to the low bulk densities and high moisture contents of these volcanic soils these seven constituents appear to be realistic for the studied soils so it appears possible to describe the complexity of a soil system with the aid of associates instead of pure constituents

sub ophiolite metamorphic rocks from nw anatolia turkey the metamorphic rocks from near kutahya in north west anatolia record different stages in the history of closure of the neo tethyan izmir ankara erzincan ocean sub ophiolite metamorphic rocks within the tavsanli zone are a tectonically composite sequence of quartz mica schists amphibole schists amphibolites and garnet amphibolites they show increasing metamorphic grade towards the base of the ophiolite a first metamorphic event typical of sub ophiolite metamorphic sole rocks was characterized by high grade assemblages and followed by retrograde metamorphism a second event was marked by a medium to high pressure overprint of the first stage metamorphic assemblages with assemblages indicating a transition between the blueschist and greenschist facies the chemistry of the sub ophiolite metamorphic rocks indicates an ocean island basalt origin and ar ar dating indicates a high temperature metamorphic event at 93 2 ma counter clockwise p t t paths recorded by the sub ophiolite metamorphic rocks are interpreted to result from intra oceanic thrusting during the closure of the izmir ankara erzincan ocean initiating subduction which formed the high temperature assemblages further subduction then produced the widespread blueschists of the tavsanli zone during the late cretaceous later cold thrusting obducted the ophiolite with the metamorphic sole welded to its base ophiolitic melanges and blueschists onto the anatolide passive margin in the latest cretaceous all these events pre date the final anatolide pontide continent continent collision

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

magma mixing in alkaline magmas an example from tenerife canary islands abstract the central region of tenerife canary islands offers a wide variety of alkaline rocks of intermediate composition between basalts and more differentiated magmas a recent less than 0 1 ma old eruption on the southwestern slopes of teide volcano produced separate felsic sio 2 59 and mafic sio 2 47 rocks as well as abundant hybrid inclusions sio 2 50 57 that show a wide range of geochemical and mineralogical variation between the end members mineralogical evidence reverse zoning skeletal and patchy feldspars incompatible assemblages mineral disequilibrium and geochemical data characteristic trends of major and trace elements ree ratios major and trace mass balance calculations support the generation of a mixing process we present a model in which basaltic magma entered a trachyte magma chamber which resulted in a stratified chamber of trachytic above more mafic hawaiitic melts the formation of hybrid magmas between these two end members is believed to involve some chemical diffusion across the interface between the two magmas before mechanical mixing a lowering of temperature due to the cooling effect of the trachytic magma and consequently a decrease in density by crystallization of plagioclase permitted the basaltic magma to penetrate through the trachyte which caused the eruption this natural example is in good agreement with the results obtained in experimental work on interdiffusion of silicate melts

origin of the amphibolite sole of the josephine ophiolite emplacement of a cold ophiolite over a hot arc the amphibolite sole that occurs along the basal thrust of the josephine ophiolite madstone cabin thrust is distinct from those described for ophiolites such as the bay of islands and oman antigorite bearing serpentinite mylonites occur directly above the amphibolite and the presence of antigorite brucite indicates formation between 300 and 400 c the antigorite mylonites also contain relict lizardite chrysotile suggesting formation of antigorite by prograde metamorphism along the base of the ophiolite further evidence for prethrusting low temperature serpentinization comes from a lizardite chrysotile shear zone within fresh peridotite directly above the thrust that is cut by undeformed diabase dikes having geochemical affinities to the upper pillow lavas of the josephine ophiolite the prograde metamorphism of the serpentinite at the base of the ophiolite was apparently synchronous with greenschist facies retrogressive metamorphism of the underlying amphibolite the amphibolite is chemically diverse and has been divided into two groups group 1 amphibolite appears to be derived from mafic plutonic rocks and has immobile trace element abundances indicating affinities to volcanic arc magmas group 2 amphibolite is metavolcanic contains some quartzite and has magmatic affinities to mid ocean ridge basalt and within plate basalt an association that is distinctive of older basement rocks elsewhere in the western klamath terrane we suggest the amphibolite was derived from the footwall of the madstone cabin thrust including country rocks for the chetco complex group 2 and possibly older intrusive phases of the chetco complex itself group 1 previous workers have shown that the base of the amphibolite is intruded by gneissic gabbro of the chetco complex that intrusion of the chetco complex overlapped in time with deformation of the amphibolite and that during and after emplacement the josephine ophiolite and overlying flysch were intruded by abundant arc related dikes and plutons emplacement of the josephine ophiolite appears to have occurred by underthrusting of a cold ophiolite by an active magmatic arc

soret separation of mid ocean ridge basalt magma chemical differentiation of an initially homogeneous mid ocean ridge basalt morb liquid has been experimentally observed in a temperature gradient above the liquidus the magnitude of this effect in producing differences in composition is comparable to that of crystal liquid fractionation for a given temperature difference however the chemical changes produced by the two processes soret and crystal liquid fractionation are different soret separations resemble those observed in a third process silicate liquid immiscibility this similarity is a reflection of the fact that the soret separable components are the same network former network modifier structural components which segregate during silicate liquid immiscibility

counterclockwise p t t trajectory from the metamorphic sole of a neo tethyan ophiolite turkey abstract the kiziltepe ophiolitic thrust sheet in the bolkar mountains of turkey occurs between two subparallel ophiolite belts bounding the tauride carbonate platform and represents a remnant of the cretaceous neo tethyan oceanic lithosphere it is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra oceanic subduction zone in the neo tethys 92 90 ma blueschist facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing p t with cooling regional tectonic constraints suggest a maastrichtian age for the timing of this blueschist facies metamorphism sodic amphibole rich veins and crossite mg riebeckite rims on hornblende suggest that growth of blueschist facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries we infer a counterclockwise p t t trajectory during which metamorphism was accompanied succeeded by rapid uplift along the northern edge of the tauride belt in late cretaceous early tertiary time

composite minor intrusions as windows into subvolcanic magma reservoir processes mineralogical and geochemical evidence for complex magmatic plumbing systems in the british tertiary igneous province the rudh a chromain sill is a composite minor intrusion emplaced into jurassic sandstones on the south coast of the ross of mull nw scotland and is part of a suite of basic silicic sheets associated with the early development of the 58 56 ma mull central igneous complex new whole rock major and trace element data combined with sr nd pb isotope data indicate that two distinct magma end members were involved in the formation of the sill a tholeiitic basaltic andesite magma generated by contamination and fractional crystallization of regionally available olivine tholeiite basaltic magma and a rhyolitic magma produced predominantly through crustal melting of basement metasediments intermediate compositions at the gradational boundaries between the basic margins and silicic interior of the sill formed by high temperature diffusive hybridization within a compositionally zoned magma reservoir prior to sheet emplacement the basic portions of the sheet are replete with a large variety of crustal xenoliths as well as numerous gabbroic and noritic cumulate fragments the rudh a chromain sill therefore preserves evidence for the complex interplay between a number of magmatic processes which when combined with data from the remainder of the suite suggests that the magmatic plumbing system which fed the sill complex was not simply one large long lived magma chamber but rather a plexus of variably connected sheet like magma reservoirs

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

magmatic evolution of the skye igneous centre western scotland modelling of assimilation recharge and fractional crystallization the skye igneous centre forming part of the british tertiary magmatic province developed over a 7 myr period 61 54 ma and is characterized by a complex suite of lavas hypabyssal and intrusive rocks of picritic to granitic composition the intrusion of magma from mantle to crust at 2 10 3 km 3 yr 6 mt yr advected magmatic heat of roughly 0 2 gw averaged over the period of magmatism supporting an excess heat flux of about 130 mw m 2 or about twice the present day average continental heat flow the volume of new crust generated at skye 15 000 km 3 spread over the present day area of skye corresponds to 9 km of new crust the geochemical evolution of the skye magmatic system is constrained using the energy constrained recharge assimilation and fractional crystallization ecrafc model to understand variations in the sr and pb isotopic and sr trace element composition of the exposed magmatic rocks with time the character composition and specific enthalpy of both assimilant and recharge magma appears to change systematically up section suggesting that the magma reservoirs migrated to progressively shallower levels as the system matured the model of the magma transport system that emerges is one in which magma batches are stored initially at lower crustal levels where they undergo rafc evolution residual magma from this stage then migrates to shallower levels where mid crustal wall rock is assimilated the recharge magma at this level is characterized by an increasingly crustal signature for some of the stratigraphically youngest rocks the data suggest that the magma reservoirs ascended into and interacted with upper crustal torridonian metasediments

the loch scridain xenolithic sill complex isle of mull scotland fractional crystallization assimilation magma mixing and crustal anatexis in subvolcanic conduits

holocene environmental change in eastern spain reconstructed through the multiproxy study of a pedo sedimentary sequence from les alcusses valencia spain abstract we present a multiproxy characterization of a complex polycyclic soil sequence from les alcusses moixent valencia se spain the area has abundant settlements dating from early neolithic to roman times the sequence comprises six main units dating back to 8 7 8 5 ka cal bp we integrated mineralogy inorganic ph grain size elemental composition and organic chemistry pyrolysis gas chromatography mass spectrometry pyrolysis gc ms micro charcoal pollen and non pollen palynomorph npp data all data is contextualized within a framework of archaeological information and radiocarbon dating with the aim of deciphering the holocene environmental changes we infer a shift from wetter and warmer in the early mid holocene to drier and cooler climatic conditions with increased fire occurrence in the mid late holocene particularly after 5 3 ka cal bp although the pollen record indicates a rapid forest retreat and expansion of grasslands for the neolithic period local proxies molecular soil om indicators and npp point to the presence of pine forest in the studied location well after the regional decline started the same proxies suggest a sharp forest decline after about 7 0 ka cal bp strong soil erosion likely linked both to climatic instability and the intensification of human exploitation resulted in a hiatus from the late neolithic to the roman period a certain climate amelioration wetter conditions and land abandonment or at least lower human pressure are suggested for the late roman period early middle ages direct evidence of changes in soil properties due to agriculture was not detected in the sequence except for the upper soil unit studied which reflects intense agricultural management with the possible use of agrochemicals

the pan african toro complex northern nigeria magmatic interactions and structures in a bimodal intrusion the toro complex is one of the pan african older granites of nigeria first described as a reversely zoned pluton made of a central dioritic mass surrounded by a broad granitic rim it has been thoroughly reinvestigated both from the petrographic and structural points of view with the help of systematic anisotropy of magnetic susceptibility ams measurements the granite main body is a hornblende biotite porphyritic monzogranite characterized by an early submagmatic fabric displaying a concentric pattern of foliations and west plunging lineations stage 1 this fabric is overprinted by a later one due to solid state strain along north south subvertical dextral shear zones stage 2 in the vicinity of the diorite an evengrained granite displays magmatic structures that are contemporaneous with this strike slip event the diorite granite contact is a complex zone where field petrographic and geochemical data enable recognition of the effects of mixing and mingling between a mafic and a felsic magma tonalites cropping out within this contact zone are interpreted as hybrid rocks the reverse zonation of the diorite itself is also the result of some hybridization process magmatic interactions mainly resulted from in situ infiltration of granitic liquid into the dioritic mass the detailed history of this bimodal intrusion began with the emplacement of the granitic magma acquiring a first stage fabric before full crystallization of the granitic core intrusion of the dioritic magma permitted reheating of the granitic magma that then crystallized with specific structural characters the second stage structures whether characterized by magmatic fabric near the diorite or by solid state strain features in north south shear zones elsewhere in the granite are related to late pan african dextral strike slip tectonics in the basement of northern nigeria the bimodal toro complex is therefore considered as a late pan african syntectonic pluton

u pb sm nd and rb sr geochronological and isotopic study of northern sierra nevada ophiolitic assemblages california distinct ophiolitic assemblages occur as oceanic basement within three of the four regional tectonic belts of the northern sierra nevada new u pb zircon sm nd and rb sr data are presented for each assemblage providing critical geochronological and isotopic constraints on the petrogenesis and tectonic evolution of the ophiolitic and associated ensimatic assemblages ophiolitic assemblages include from west to east the smartville complex central belt and feather river belt the smartville complex represents an island arc volcanic plutonic sequence with a major late stage sheeted dike swarm the sm nd systems from a wide compositional spectrum of rocks record a 178 21 ma petrogenetic age and an nd t 9 2 0 6 zircon u pb systems on an uppermost dacite yield a 164 2 ma age and on a number of plagiogranite screens and dikes from the sheeted complex 162 1 ma ages the central and feather river belts are structurally complex polygenetic assemblages the u pb zircon and sm nd systems record major 205 ma and 315 ma petrogenetic events respectively both involving depleted mantle derived magmas such magmatism probably occurred in marginal basin transform systems developed within an older oceanic depleted mantle basement regime both sm nd and u pb zircon systems show local components of proterozoic sialic material the sialic contaminants were probably introduced into the system as craton derived detritus it is doubtful that any of the ophiolitic assemblages studied represent genetically related crust upper mantle sequences generated during the development of new oceanic lithosphere integration of the geochronological data with geological relations reveals a pattern of petrogenesis and tectonics whereby progressively younger ensimatic terranes were added to the continental margin through time by plate convergence and were ultimately welded into north american sial by a crosscutting batholithic belt this accretionary pattern is reflected in both the protolith ages and deformation metamorphic ages of each of the regional belts which progressively young westward crustal components of the accreted ensimatic terranes grew by mainly basaltic igneous activity within island arc marginal basin and leaky transform systems adjacent to the continent edge prior to final tectonic accretion such complexities are suggested to be typical of cordilleran type ophiolites and representative of the circum pacific erogenic style

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

evolution of the northern sierra nevada metamorphic belt petrological structural and ar ar constraints the sierra nevada metamorphic belt constitutes an important record of the growth of continental crust from essentially oceanic materials in the northern sierra the central part of the belt is made up of volcanoplutonic arcs and sediment dominated units inferred to be accretionary wedges or closed ocean basins the latter are broken formation and melange composed of radiolarian chert lava and volcanogenic and continental turbidites sedimentary detritus in the largest of these units can be plausibly linked to sources farther east in the sierra suggesting that deposition occurred near the eastern sierran arc isoclinal folds steeply dipping foliations and steeply plunging down dip lineations are characteristics structures the westernmost unit is only feebly recrystallized and deformation was accomplished principally by stress solution and local redeposition in veins more easterly inboard units are compositionally similar but they recrystallized at pumpellyite actinolite and blueschist facies conditions and deformed via solution transfer and dislocation creep phengite silica contents the degree of quartz veining and the locations of pseudo isograds support an eastward increase in metamorphic pressure and temperature metamorphic conditions during the growth of pumpellyite and actinolite ranged from approximately 150 350 degrees c and 200 400 mpa compatible with recrystallization and deformation in subduction zones or the deeper levels of magmatic arcs ar armore ages of volcanisclastic rocks and crosscutting plutons constrain the age of deformation and metamorphism in the western part of the region to 174 165 ma deformation and recrystallization in more easterly units may have been coeval or begun as early as triassic time 58 refs 14 figs 4 tabs less

sr isotope variations in a composite sill crystal liquid processes and the origin of the skye granites sr isotope data from the rubh an eireannaich sill one of a suite of composite mafic silicic minor intrusions associated with the 59 ma skye intrusive centre nw scotland indicate that both crystal liquid fractionation and magma mixing processes have played important roles in its genesis the data indicate that two distinct magmas were involved in the formation of the sill a ferrobasaltic andesite magma generated by contamination and fractional crystallization of regionally available basaltic magma and a rhyolite magma which has a significant crustal component hybrid intermediate compositions at the gradational boundaries between the mafic and silicic components of the sill formed by mixing prior to or during emplacement whilst fractional crystallization of the unmixed silicic magma occurred almost exclusively after mixing the identification of a significant crustal component within the silicic portion of the sill suggests that the compositionally related granites of the skye intrusive centre have a similar origin

two contrasting pressure temperature time paths in the villa de cura blueschist belt venezuela possible evidence for late cretaceous initiation of subduction in the caribbean the villa de cura blueschist belt is one of several east west trending allochthonous belts comprising the caribbean mountain system of northern venezuela this blueschist belt consists of four structurally coherent subbelts that also trend east west from north to south these are characterized by 1 pumpellyite actinolite 2 glaucophane lawsonite 3 glaucophane epidote and 4 barroisite the retrograde pressure temperature p t path of the northern three subbelts generally parallels their prograde path such p t paths are typical for franciscan style subduction settings and are characterized by relatively low geothermal gradients indicative of refrigeration during subduction zone parallel ascent and exhumation of these rocks the barroisite subbelt formed at high pressures similar to those of the glaucophane epidote subbelt but at substantially higher temperatures and followed a counterclockwise p t path new 40 ar 39 ar ages record peak metamorphism at 96 3 0 4 ma for the barroisite subbelt and 79 8 0 4 ma for the northern three subbelts the caribbean plate is thought to have been a fragment of the farallon plate which together with the great arc of the caribbean greater antilles aves ridge lesser antilles leeward antilles migrated northeastward after a subduction polarity reversal and overrode the young proto caribbean lithosphere that had formed by spreading between north america and south america the more silicic barroisite subbelt may have been part of the arc that was subducted immediately after polarity reversal whereas the other three belts formed much later when the geothermal gradient had decreased substantially the villa de cura belt was exhumed in two stages first by late cretaceous arc parallel extension and second by miocene southward thrusting onto the south american continent

hot and deep rock record of subduction initiation and exhumation of high temperature high pressure metamorphic rocks feather river ultramafic belt california abstract studies of a 10 to 300 m thick unit of high grade metamorphic rock external schists that crops out along the western border of the feather river ultramafic belt frb northern california yield new insights into subduction initiation and ophiolite emplacement processes the high temperature t foliation of the external schists dip moderately to steeply eastward beneath the ultramafic rocks of the frb a 150 km long slab of suboceanic upper mantle and the high t fabric shows a tops to the west frb side up sense of shear the structurally highest external schists record peak metamorphic conditions of 650 760 c at 1 3 2 2 gpa in contrast sheeted dikes of the devil s gate ophiolite that overlie the ultramafic rocks yield metamorphic conditions of 710 730 c at about 0 3 0 7 gpa a km scale lens of amphibolite within ultramafic rocks yields somewhat lower pressures than the structurally highest external schist as do the structurally lower rocks within the external schists significant exhumation of the external schists relative to the structurally overlying ophiolitic rocks occurred along at least two major zones and the most significant exhumation was accommodated at least 1 5 km structurally above the ultramafic external schist contact based on available geochronology intraoceanic subduction may have initiated at approximately 240 ma and exposure of the external schist occurred prior to the deposition of rocks in the structurally highest part of the calaveras complex minimum 177 ma a subduction complex that structurally underlies the external schists high t metamorphism of the devil s gate ophiolite may have resulted from partial failed ridge subduction

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

the role of deformation in the formation of metamorphic gradients ridge subduction beneath the oman ophiolite two tectonic scenarios have been proposed for genesis and emplacement of the oman ophiolite one suggests that the ophiolite was generated at a spreading center the other suggests generation within an intraoceanic arc an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during early stages of the emplacement of the oman ophiolite was developed to test these two possibilities the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating heat advection by flowing rock mantle heat flow and radioactive heating the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship field observations in oman can be satisfied with the spreading center model but not with the arc model moreover simulations indicate that the ophiolite was probably 2 m y old at the time of intraoceanic thrusting and that shear stresses of 100 mpa were attained during thrusting in addition to satisfying currently available field constraints these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen future work should be directed toward 1 using thermobarometry and thermochronology to constrain the spatial variation in ptt paths of the metamorphic sole 2 dating the igneous genesis of the ophiolite and 3 using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting these simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients in contractional fault zones metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient and adjacent fault zone rocks may display crossing pt paths unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting cooling footwall

age origin and significance of an unconformity that predates island arc volcanism in the northern sierra nevada the paleogeographic position of rocks of the northern sierra nevada relative to north america has long been the topic of debate a detailed study of the lakes basin region of the northern sierra nevada substantiates that the shoo fly complex of early paleozoic age was folded and unconformably overlain by an island arc sequence in late paleozoic time microfossils recovered during the study represent the first in situ fauna ever collected from the shoo fly complex and provide an ordovician silurian lower limit on the early deformation late jurassic nevadan deformation strongly affected all rocks in the region regional considerations suggest that mid paleozoic deformation may have been widespread in the sierra nevada and could indicate an early phase of an eastward prograding suture that ultimately resulted in emplacement of the roberts mountain allochthon of the great basin

age relationships and depositional environments of paleozoic strata northern sierra nevada california upper paleozoic volcanic rocks in the northern sierra nevada rest unconformably on the lower paleozoic shoo fly complex late ordovician conodonts recovered from an exotic limestone block in shoo fly melange have north american affinities the complex may have traveled little from its original location magmatism may have begun as early as early devonian and persisted into early mississippian time the volcanic rocks constitute a longitudinal section through a relatively immature volcanic arc dominated by tholeiitic magmas two lithologically distinct members of the overlying peale formation are given formation status the predominantly volcanogenic lower member is renamed the keddie ridge formation the name peale formation is retained for the upper chert member two new late mississippian early pennsylvanian radiolarian localities in the peale chert at the north end of the paleozoic outcrop belt correlate well with a previously described assemblage at the south end and thus rule out time transgressive deposition early permian fusilinids in a chert pebble conglomerate only 10 m upsection from an early pennsylvanian radiolarian locality tightly constrain a regional depositional hiatus the peale chert displays common soft sediment deformation slump structures intraformational breccias and abrupt termination of thick sequences along strike locally the chert is intruded by tholeiitic basalt these features suggest deposition in small tectonically active basins possibly in an extensional back arc regime the overlying permian sequence of arc tholeiites is truncated by mesozoic strata in most areas but appears to grade into triassic shale and limestone in the extreme northwest

chemical trends in a perhumid soil catena on the turrialba volcano costa rica abstract the variation in chemical composition of soil samples xrfs data from a soil catena of 14 soil profiles on the northern slope and foot slope of the andesitic turrialba volcano 3300 m has been analysed to test the two hypotheses which underlie our interpretation that this catena is a weathering sequence the first hypothesis is that all parent material is chemically homogeneous and the second hypothesis is that development of chemical variation is the net result of progression of weathering and of diversity of weathering which both depend on altitude via gradients in temperature and precipitation the aim of this purely chemical approach is to find out if trace elements are significant tracers for different weathering paths leading to different amorphous or crystalline products for example imogolite allophane halloysite gibbsite and ferrihydrite the results show that volcanic ash is sufficiently homogeneous to detect the well known dominance of a general weathering trend with relative depletion of mobile elements mg k si etc and relative enrichment of less mobile elements ti fe al ce cr v zr etc the least mobile elements were ti v and zr from their contents a weathering index for all samples has been derived that describes 95 of their total variance hypothesis 1 in addition to this general trend ti v and zr showed systematic deviations that could be defined as a chemical differentiation factor in combination with the general weathering trend this explained 99 of the total variation of ti v and zr hypothesis 2 the contents of other major minor and trace elements used can only partially be described by the ti v zr derived weathering index and chemical differentiation the level of explanation decreases roughly with increasing mobility of the chemical element in the system triangular plots of element ratios suggest that contents of mobile elements are influenced by too many weathering processes trace elements did not show a more specific chemical differentiation than major and minor oxides the leaching pattern of mg indicates the presence of three different processes 1 short term preferential leaching of basic cations at high altitude 2 subsequent allophane formation between 660 and 1220 m altitude and 3 long term loss of basic oxides and not of silica upon ageing of fluvio volcanic sediments in the foot plain below 240 m fe shows anomalous depletion redistribution due to oxidation and reduction and al shows extra depletion in two of the allophanic profiles

metastability of granulites and processes of eclogitisation in the uhp region of western norway the flatraket complex a granulite facies low strain enclave within the western gneiss region provides an excellent example of metastability of plagioclase bearing assemblages under eclogite facies conditions coesite eclogites are found 28 kbar unprecedented metastability of plagioclase and quartz is implied alternatively a tectonic break exists between the flatraket complex and uhp eclogites supporting the concept of a tectonic boundary to the uhp zone of the western gneiss region the distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability and is a function of the protolith

geochemical evolution of the tertiary mull volcano western scotland the early tertiary mull volcano western scotland is one of the most dissected and best exposed igneous complexes of the north atlantic province the new and published geochemical data enable us to chart the magmatic evolution of the mull volcano from the oldest lavas through the intrusive rocks of three overlapping igneous centres to the youngest dykes in this study we identify four successive magma types within the remnant volcano the earliest type the mull plateau group comprises mildly alkaline basaltic rocks with steep chondrite normalized rare earth element ree patterns this type is succeeded within the lava succession and dyke swarm by the coire gorm magma type with essentially flat chondrite normalized ree patterns a third magma type represented within the lava and dykes the central mull tholeiites is more depleted in incompatible trace elements than the preceding types and has flat to lree depleted chondrite normalized patterns the major intrusions and cone sheets of mull centre 1 and early centre 2 belong to this magma type midway through the igneous activity associated with centre 2 the magma type changed to become more alkalic and more enriched in incompatible trace elements this magma type the late mull type is found to persist through the cone sheets and major intrusions of centre 3 to the youngest dykes these changes in magma composition were related to variations in the mantle source and depth of partial melting beneath mull and or differences in the efficiency of melt pooling before ascent through the lithosphere with the exception of the early staffa magma sub type part of the mull plateau group the location of magma chambers in which the bulk of contamination occurred changed with time from deep lower crustal lewisian gneiss to shallow upper crustal moine schist intermediate members of the plateau group and the late mull magma type are enriched in fe ti and p relative to the central mull tholeiites we attribute this difference to the more alkalic nature of these suites lower fo2 and the formation of fe3 p complexes in the magma the intermediate rocks were important in magma mixing processes with two types of mixing identified on mull 1 cryptic mixing between basalts and low fe intermediate magmas typified by lavas and early basic cone sheets of the central mull tholeiite magma type 2 observable mingling between rhyolitic magmas and high fe intermediate magmas of the late mull type shown by the mixed magma bodies of the glen more and loch ba ring dykes the main factor in determining which type of mixing occurred appears to have been the density contrast between the various magmas it may safely be maintained that mull includes the most complicated igneous centre as yet accorded detailed examination anywhere in the world bailey et al 1924 memoir of the geological survey of great britain scotland hmso edinburgh 1924

pre eruptive magma mixing in ash flow deposits of the tertiary rum igneous centre scotland the northern marginal zone of the rum igneous centre is a remnant of an early caldera and its infill it is composed of intra caldera breccias and various small volume pyroclastic deposits overlain by prominent rhyodacite ash flow sheets of up to 100 m thickness the ash flows were fed from a feeder system near the caldera ring fault and intrusive rhyodacite can locally be seen grading into extrusive deposits a variety of features suggest that the ash flows were erupted from a magma chamber that contemporaneously hosted felsic and mafic magmas i chilled basaltic inclusions in rhyodacite ii formerly glassy basaltic to andesitic enclaves with fluid fluid relationships iii feldspars with thick reaction rims enclosed in the basaltic to andesitic inclusions yet with cores chemically resembling those of the rhyodacite iv trace element compositions of the rhyodacite and the mafic enclaves form a mixing line between the end member rhyodacite and basalt compositions additionally textural and chemical features in the rhyodacite feldspar phenocrysts are consistent with magma mixing v feldspars with resorption embayments cutting through internal zonation of the crystals vi complexly zoned crystals with sieve textured zones that are overgrown with euhedral zones vii oscillatory zonation of feldspar phenocrysts in the rhyodacite showing sharp increases in anorthite an 10 followed by gradual decrease in an content an 4 this evidence points to eruption of ash flows from a felsic magma chamber that was periodically replenished by injection of mafic magma diffusional mixing between the two magmas was permitted by temperature and compositional differences but was slow due to the contrast in viscosities and densities the fe ti p enriched basic magma that replenished the chamber was degassing on entering the lower temperature environment and soon equilibrated thermally followed by chemical exchange between the two end member magmas this process formed hybrid andesite enclaves enriched in trace elements beyond that caused by simple mixing implying trace element diffusion in addition to bulk mixing eruption was caused by replenishment with and degassing of the basic magma and the chamber partially evacuated while the process of hybridisation was underway the erupted products record magma mixing by chamber replenishment blending of two magmas and elemental exchange in the magma chamber and also physical mingling in the eruptive conduit

magma mixing the marsco suite isle of skye scotland magma mixing has been proposed as a major mechanism for the origin of a wide variety of rock suites in mid ocean basalts there are few obstacles to mixing because of the similarities in chemical and physical properties of the proposed end members however in calc alkaline rocks the proposed end members have disparate properties and these present obstacles to mixing

sedimentary and volcano tectonic processes in the british paleocene igneous province a review research on the british paleocene igneous province bpip has historically focused on the emplacement chemistry and chronology of its elaborate central intrusive complexes and lava fields however the bpip has also been dramatically shaped by numerous erosion sedimentation and volcano tectonic events the significance of which becomes ever clearer as localities in the bpip are re investigated and our understanding of volcano sedimentary processes advances the resultant deposits provide important palaeo environmental palaeo geographical and stratigraphical information and highlight the wide range of processes and events that occur in ancient volcanic settings such as the bpip in this paper we review the sedimentary and volcano tectonic processes that can be distinguished in the bpip and conceptualize them within a generalized framework model we identify and describe the sedimentary responses to four broadly chronological stages in the history of the bpip volcanoes 1 the development of the lava fields 2 early intrusion induced uplift 3 caldera collapse and 4 post volcano denudation and exhumation of central complexes we highlight and illustrate the range of sedimentary processes that were active in the bpip these operated on and helped shape a dynamic landscape of uplands and lowlands of alluvial fans braided rivers lakes and swamps and of volcanoes torn apart by catastrophic mass wasting events and or caldera collapse

chilled margins and commingling of magmas in the bressanone brixen hercynian granodiorites eastern alps northern italy abstract the brixen hercynian pluton is the result of a multiple intrusion of crustal anatectic magma granodiorite granites and two mica cordierite bearing leucogranites and subcrustal magma gabbro diorites and fayalite bearing leucogranites some adjacent granodioritic stocks show chilled margins containing globular inclusions in the size of several centimetres of a coarse grained leucogranite the granodiorite margins also show narrow zoning with more acidic external borders this phenomenon is correlated to differentiated processes such as filter pressing and the soret effect which were active during the rise of these stocks the strong petrographic and chemical analogies between the globular inclusions of some chilled margins and the fayalite leucogranites suggest their common subcrustal origin the granodiorite containing these inclusions is of crustal origin the occurrence of globular inclusions inside it thus indicates only mechanical mingling between crustal anatectic and subcrustal magmas as shown by the absence of hybrids and chemical interactions

replenished magma chambers effects of compositional zonation and input rates several recent publications have reported investigations of the dynamical effects and geological consequences of the replenishment of a homogeneous magma chamber by the rapidinjection of hot relatively dense fresh magma the new magmaforms a separate layer at the base of the chamber whereit cools and crystallizes until the density of the residual liquid in the lower layer becomes equal to that of the upperlayer at which time the fluid in the two layers becomes intimately mixed the fluid dynamical aspects of this modelwere confirmed by laboratory experiments using aqueous solutions this paper reports three experimental variations on the basic theme which examine the effects of the rate of injection into homogeneous and compositionally stratified chambers first the rapid injection of a hot dense layer beneath a fluid with a lower temperature and a vertical compositional gradient is considered a series of layers gradually builds up from the base of the upper gradient region while the lower layer cools crystallizes and finally overturns in this situation however the rise of the lower layer fluid is restrained by the density gradient and mixing takes place with only part of the upper fluid second the slow influx of a hot dense liquid beneath a cold less dense homogeneous layer is considered a considerable quantity of the input fluid coolsand crystallizes immediately releasing some lighter fluid to mix with the fluid in the homogeneous layer with time some dense input fluid spreads along the bottom so that locally the situation is the same as the one considered in the previous studies with a local overturning occurring when the density of the lower fluid has decreased sufficiently the deposition of crystals continues and some rather fine chimney like structures can develop which bear a resemblance to the black smokers recently discovered on the sea floor third a combination of the first two variations is studied by the consideration of the slow influx of hot dense liquid beneath a uniformly cooler fluid with a vertical compositional gradient the input fluid is constrained in its rise by the density gradient as the influx continues the additional thermal energy is also confined and causes the crystals to redissolve and build up a lower layer as was observed in the case of a rapid injection beneath a compositional gradient

calaveras reversed westward younging is indicated samples of limestone collected from strata in the southern part of the western metamorphic belt of the sierra nevada which traditionally have been assigned to the calaveras formation have yielded early triassic conodonts and samples of metavolcanic rock indicate an early jurassic rb sr age these ages together with previously published fossil and isotopic data are consistent with general westward younging of units rather than with eastward younging as has been assumed until recently by many workers chiefly on the basis of sparse observations of bedding facings the rocks are strongly deformed and the possibility that tectonism rather than stratigraphic succession accounts for the age pattern cannot be dismissed

hybridization between felsic and mafic magmas in calc alkaline granitoids a case study in northern sardinia italy abstract two examples of felsic mafic association resulting from a magma mingling mechanism have been studied in carboniferous calc alkaline granitoids from sardinia using mineralogical and geochemical techniques 1 the centimetric to decimetric magmatic mafic enclaves from the reversely zoned tonalitic santa restituta pluton this type of enclave which is common in calc alkaline granitoids represents blobs of mafic magma incorporated into a silicic host magma 2 the osidda mafic complex about one kilometre in diameter enclosed in the benettuti granodiorite this study intends to compare these two types of felsic mafic association in order 1 to clarify the nature of the interaction processes which occurred between the two magmatic components 2 to test the influence of the degree of fragmentation of the mafic component into the felsic matrix on the intensity of such processes the santa restituta enclaves sre and the osidda mafic complex omc exhibit the same general mineral assemblage plagioclase biotite hornblende quartz this assemblage is similar to that of the corresponding host granitoids however the sre and the omc are characterized by a greater abundance of biotite and hornblende the mineralogical study reveals the presence of two types of crystals the first type corresponds to minerals typical of the mafic or the felsic component in the mafic magma they are represented by remnants of an rich plagioclase by hornblende clots presumably pyroxene relicts and by orthopyroxene observed only in one facies of the omc these crystals represent remnants of phenocrysts present in the mafic component prior to the mingling event in the felsic component the first type of crystals is represented by quartz ocelli which are sometimes incorporated in the mafic magma as xenocrysts surrounded by a reaction rim of biotite and hornblende the second type comprises minerals which are found indiscriminantly in both magmatic components and which formed during the mingling event mingling minerals they include plagioclase an52 35 biotite hornblende quartz mineralogical data indicate that thermal equilibrium occurred between the mafic magma and the felsic matrix that equilibration was reached in the sre is shown by the compositional similarity of the mingling minerals of the enclaves and their host rocks however due to a lesser degree of fragmentation thermal equilibration was not reached in the omc geochemical data however do reveal close relationships between the two magmas which appear therefore to have both been hybridized by mutual chemical interaction a chemical comparison between the sre and the omc shows that in both cases the same qualitative interaction mechanisms operated between the host granitoids and either a voluminous mafic body osidda or a highly fragmented mafic component santa restituta enclaves however the intensity of hybridization is minor in the omc as compared with the sre it is suggested that hybridization of the host granitoids involves 1 mass to mass mixing due to disaggregation and dispersal of the mafic magma in the host granitoid resulting from mechanical stirring during the pluton s genesis from the initial meeting of the two magmas to the pluton s final emplacement 2 loss of certain elements si k rb ba li zr p nb ree which migrated towards the mafic magma through diffusion due to compositional gradients fluid transfer as a consequence the composition of the host granitoids is linked to that of their magmatic enclaves the enclaves with the lowest feo tot mgo ratios are enclosed in the least felsic granitoids and vice versa hybridization of the mafic component involves 1 mechanical transfer of crystals quartz from the host granitoid 2 enrichment in numerous elements migrating from the felsic component this is expressed mineralogically by the crystallization of numerous anhedral phases such as quartz sio2 biotite k rb h2o hornblende h2o apatite p and zircon zr 3 loss of ca and sr by the mafic component the nature of the processes of chemical migration are discussed the importance of the continuous renewal of the contact surface between the two components is emphasized the original compositions of the interacting mafic and granitic components become therefore highly modified this phenomenon can probably be generalized to apply to calc alkaline granitoids as a whole which are in fact characterized by the common occurrence of magma mingling

magma mixing and the production of compositional variation within granite suites evidence from the granites of southeastern australia granite suites are groups of plutons possessing characteristic features that are a result of their derivation from source material of a specific composition variation within suites has been ascribed to a variety of processes magma mixing or mingling is a popular hypothesis generally proposed in terms of blending between a crustal melt and mafic material from the mantle that caused that melting when the compositions of pain of suites from the bega batholith of southeastern australia are compared any differences seen at either end of the range in composition are also seen at the other limit so that both the most mafic and mostfelsic rocks show similar relative abundances of particular elements similar relationships are seen for other granites in the region these observations are not consistent with largescale magma mixing or mingling and although those processes may operate on a small scale they cannot have been responsible for the major compositional variations likewise assimilation of country rocks had no significant role in producing variation in the granites of southeastern australia the production of variation by differential separation of melt from residual solid source material or restite must be favoured for many of the granite suites of this region

integration of natural data within a numerical model of ablative subduction a possible interpretation for the alpine dynamics of the austroalpine crust a numerical modelling approach is used to validate the physical and geological reliability of the ablative subduction mechanism during alpine convergence in order to interpret the tectonic and metamorphic evolution of an inner portion of the alpine belt the austroalpine domain the model predictions and the natural data for the austroalpine of the western alps agree very well in terms of p t peak conditions relative chronology of peak and exhumation events p t t paths thermal gradients and the tectonic evolution of the continental rocks these findings suggest that a pre collisional evolution of this domain with the burial of the continental rocks induced by ablative subduction of the overriding adria plate and their exhumation driven by an upwelling flow generated in a hydrated mantle wedge could be a valid mechanism that reproduces the actual tectono metamorphic configuration of this part of the alps there is less agreement between the model predictions and the natural data for the austroalpine of the central eastern alps based on the natural data available in the literature a critical discussion of the other proposed mechanisms is presented and additional geological factors that should be considered within the numerical model are suggested to improve the fitting to the numerical results these factors include variations in the continental and or oceanic thickness variation of the subduction rate and or slab dip the initial thermal state of the passive margin the occurrence of continental collision and an oblique convergence

mixing between granitic and dioritic crystal mushes guernsey channel islands uk abstract the contact zone between the cobo granite and bordeaux diorite complex of guernsey channel islands uk displays numerous features which result from the interaction of these two penecontemporaneously emplaced intermediate to felsic magmas initial interaction resulted in the formation of chilled mafic enclaves in granite magma as thermal equilibrium was approached some physical mixing took place to produce a heterogeneous marginal facies continued interaction resulted in incorporation of previously mixed magma into intruding magma the early mixed material is locally preserved as enclaves but more commonly underwent disaggregation promoted by its incompletely crystallised nature the mineral components becoming distributed as xenocrysts and often as microenclaves or glomeroxenocrysts into surrounding magma further modification within the contact zone was brought about by the infiltration of melt through the interconnected pore space of the magma mushes on the scale of centimetres to hundreds of metres these processes produced geochemical profiles which do not exhibit perfect mixing trends the petrographic and geochemical features described not only demonstrate the efficacy of mixing between partially crystallised magma mushes of broadly similar composition but also provide criteria by which such interaction may be recognised elsewhere many features in particular mineral scale disequilibria and small scale modal heterogeneity bear striking similarities to those which occur widely in granite plutons where obvious evidence for magma mixing is absent as such it is possible that many granite bodies preserve a subtle record of hitherto overlooked mixing events

simulation of the metamorphic and deformational history of the metamorphic sole of the oman ophiolite emplacement of ophiolites onto less dense continental crust is an enigma the rates of emplacement the mechanisms of emplacement and the stress and temperature histories of the rocks are all incompletely understood the oman ophiolite is well exposed and has been studied extensively by field geologists an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during the early stages of the emplacement of the oman ophiolite is developed here to explain field relations and provide insight into the development of the metamorphic rocks at the base of the ophiolite the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating and heat advection by rock the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship in simulations involving young hot subducted lithosphere a wider portion of the subduction zone deforms by power law creep and deformation occurs at lower stresses and slower strain rates minimizing the deformational heating compared to colder older subducted lithosphere dehydration occurs at an accelerated rate in younger hotter lithosphere removing heat from the subducted plate at an earlier stage and causing the zone of brittle deformation to propagate downward more rapidly power law creep of subducted basaltic rocks is limited to temperatures greater than 500 c and occurs at differential stresses of the order of 100 mpa the simulations predict that metamorphic field gradients and the spatial distribution of rock types in the metamorphic sole of tethyan ophiolites might be used to infer the emplacement direction of the ophiolite

structure and petrology of tauride ophiolites and mafic dike intrusions turkey implications for the neotethyan ocean cretaceous neotethyan ophiolites occur in four east west trending subparallel zones within the tauride tectonic belt in southern turkey the ophiolites of the inner intermediate and outer zones tectonically overlie the mesozoic platform carbonates of the tauride belt and are commonly underlain by a cenomanian ophiolitic melange these ophiolites consist mainly of tectonized mantle rocks mafic ultramafic cumulates and gabbros and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence metamorphic soles that are several hundred meters thick occur as thrust faulted slices beneath these ophiolites and show well developed metamorphic field gradients ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit dike rocks are made of subalkalic basalt to andesite typical of evolved island arc tholeiites they display large compositional variations with sio 2 content between 50 and 60 wt and mgo between 8 and 4 wt and contain higher ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates the majority of the analyzed dike rocks show a slight depletion in light rare earth elements ree with low la smn ratios and are depleted in both high field strength hfs and heavy rees while enriched in large ion lithophile elements lile relative to normal mid ocean ridge basalt morb these characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by lile and light ree enriched fluids geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field or in a transitional zone between the garnet and spinel stability fields at about 60 km depth this asthenospheric window was probably created during subduction of a neotethyan ridge system magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere the new 40 ar 39 ar hornblende dates of 92 90 ma and 90 91 ma from the metamorphic soles and dike swarms respectively show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the inner tauride seaway these data suggest that the tauride ophiolites within the three zones to the north originated from the same root zone situated north of the tauride carbonate platform and that they constitute remnants of a single ophiolitic nappe sheet derived from the inner tauride seaway within the neotethyan ocean

mixing basaltic and dacitic magmas by forced convection although magma mixing is an old concept which has been applied to many magma types recent petrographic studies have revived interest in the theory by suggesting that it may account for the origin of calk alkaline magma1 6 and mid ocean ridge basalt7 9 there has been no previous experimental investigation into how two magmas with different properties can mix to form a homogeneous magma of andesitic composition or to produce textures such as observed in banded dacite we present here the results of experiments demonstrating that basaltic and dacitic magmas can be easily mixed by forced convection to form both banded dacite and homogeneous andesite in less than a few hours

rapid emplacement of young oceanic lithosphere argon geochronology of the oman ophiolite 40 ar 39 ar dates of emplacement related metamorphic rocks beneath the samail ophiolite in oman show that cooling to

petrogenesis of mesozoic granitoids in the dabie uhp complex central china trace element and nd sr isotope evidence post collisional magmatism in the dabie orogen is characterized by the near contemporaneous intrusion of voluminous granitoids and minor mafic magmas in the late mesozoic 135 110 ma the granitoids are represented by three major batholiths in northern dabie zhubuyuan baimajian and tiantangzhai the largest zhubuyuan batholith comprises hornblende bearing intermediate rocks penghe suite and hornblende free granitic rocks huangbai suite the huangbai granites show limited variation in chemical composition but are highly variable in textures ranging from medium to coarse grained they have rather homogeneous sr and nd isotopic compositions with 87sr 86sr i 0 708 0 709 and end t 19 to 24 the penghe intermediate suite covers a wide spectrum of rock types including diorite monzodiorite quartz monzonite to monzogranitic rocks and all of them contain hornblende and mafic enclaves the existence of compositional and textural disequilibrium e g calcic and resorbed cores of plagioclase and the nature of chemical variation in these rock types indicate that magma mixing between an evolved mafic and a granitic magma was involved in their genesis fractional crystallization fc could have coupled with the process of magma mixing but with minor contamination by the upper continental crust during magma ascent isotopically the intermediate rocks show a variation in end t values from 13 7 to 18 while their 87sr 86sr i ratios vary from 0 7074 to 0 7088 the genesis and evolution of the dabie post collisional magmatic rocks may be summarized as follows intrusion of mantle derived mafic ultramafic magmas into thickened continental crust induced dehydration melting of the lower crust rocks with intermediate compositions and subsequent production of large amounts of granitic melts huangbai suite extensive interaction between the crustal and mantle derived melts through mixing produced a hybrid parental magma which subsequently underwent a fc process en route to higher crustal levels producing a wide variety of rock types as shown by the penghe intermediate rock suite injection of successive pulses of basic magma into upward mobile magma chambers could have resulted in the formation of the mafic enclaves as well as mafic dykes

mingled silicic lavas in the mesoproterozoic gawler range volcanics south australia abstract interlayered and mingled plagioclase rhyolite vesicular rhyolite and quartz rhyolite lithofacies occur in a unit of the mesoproterozoic gawler range volcanics in the southern gawler ranges south australia the three rhyolite lithofacies have distinctive phenocryst assemblages groundmass textures and compositions mingling styles include isolated lenses or blobs of one lithofacies surrounded by another lithofacies mm to m scale compositional flow banding and swirled and contorted combinations of lithofacies areas where the plagioclase rhyolite vesicular rhyolite or quartz rhyolite lithofacies are volumetrically dominant have been mapped in all sections plagioclase rhyolite occurs at the base and is succeeded by variably mingled plagioclase rhyolite vesicular rhyolite and quartz rhyolite relatively homogeneous quartz rhyolite dominates the upper parts the compositionally heterogeneous interval is 100 200 m thick and extends 25 km along strike it is the topmost part of a far more extensive 180 km strike length and thicker 350 m plagioclase rhyolite unit eucarro dacite that is relatively homogeneous all three rhyolite types are evenly porphyritic and either massive or flow banded the compositional flow banding and mingling textures imply that the entire unit was emplaced as lava and also that the eruption style was fundamentally effusive or very weakly explosive fountaining geochemical data suggest that three rhyolitic magmas existed in a magma chamber that was compositionally heterogeneous after a large volume of plagioclase rhyolite magma had been withdrawn quartz rhyolite and or vesicular rhyolite were entrained into the flow mingling occurred during laminar flow in the conduit and continued during extrusion resulting in compositional flow banding and more irregular combinations

effects of volatiles on mixing in calc alkaline magma systems the intimate mixing between different magmas of disparate densities characteristic of some calc alkaline magma systems is explained by a new mechanism involving the emplacement of a layer of wet undersaturated mafic magma at the base of a magma chamber containing more differentiated magma turbulent transfer of heat between the mafic magma and the overlying magma leads to crystallization and exsolution of volatiles in the lower layer for an initial water content of a few per cent the bulk density of the mafic magma can become equal to that of the overlying magma causing overturning and intimate mixing of the magmas the mechanism is most probable in the low pressure environment of a high level magma chamber

terranes early faults and pre late jurassic amalgamation of the western sierra nevada metamorphic belt california the western sierra nevada metamorphic belt is subdivided into six tectonic terranes which are juxtaposed along mostly cryptic pre late jurassic faults the northern sierra terrane contains a basement of imbricated and locally chaotic lower paleozoic quartz sandstone mafic rocks chert argillite and other rocks overlain unconformably by upper devonian middle jurassic volcanic and siliceous strata the feather river terrane is a paleozoic possibly polygenetic ophiolite the red ant terrane is a small terrane with an early mesozoic blueschist facies overprint the calaveras terrane is a chaotic assemblage of upper paleozoic triassic argillite chert volcanic rock and limestone contained in two regionally mapped melange units the tuolumne river terrane is a stratigraphic assemblage of disrupted paleozoic ophiolite overlain by middle triassic lower jurassic siliceous and volcanic strata both intruded by 200 ma lower jurassic plutons the slate creek terrane is a 200 ma pseudostratigraphic sequence of cumulate ultramafic plutonic and volcanic rocks the northern sierra and calaveras terranes contain permian mccloud fossils whereas the tuolumne river terrane contains permian mccloud and tethyan fossils in olistoliths within lower mesozoic deposits these terranes are bounded mostly by steeply dipping faults of the late jurassic foothills fault system but locally are juxtaposed along earlier highly deformed low angle faults that are remnants of major terrane bounding faults a system of early mostly cryptic east dipping faults juxtapose in descending structural order the northern sierra feather river red ant calaveras and tuolumne river terranes each of the four boundaries that separate these five terranes have three or more independent pre late jurassic upper age brackets including crosscutting plutons sedimentary provenance and shared structural fabrics the slate creek terrane tectonically overlies the tuolumne river calaveras and red ant terranes along an isoclinally folded west dipping fault that is cut by a 165 ma pluton callovian kimmeridgian strata and coeval intrusive rocks including the 155 165 ma volcanic rocks sheeted dikes and zoned mafic plutons of the smartville complex are deposited on and intruded into the older previously juxtaposed terranes and constitute an overlap sequence the well known faults of the foothills fault system including the melones fault cut kimmeridgian strata of the overlap sequence and thus cannot include ancient sutures these faults reactivated many of the earlier terrane bounding faults pre late jurassic amalgamation of the sierra nevada metamorphic belt rules out previous models of late jurassic arc continent collision earlier collisions and or noncollisional tectonic processes must be invoked to explain strong contractional deformation in the western sierra nevada metamorphic belt