magma mixing in boninite sequence of chichijima bonin islands abstract boninite from chichijima can be divided into two major types a porphyritic type phenocryst mode exceeds 1 and b aphyric type phenocryst mode is less than 1 both types are further subdivided by means of microphenocryst assemblages into five subtypes i olivine ol clinoenstatite cen bronzite brz ii ol brz iii brz iv brz pigeonite pig augite aug v cen brz there is a systematic correlation between these rock types and the bulk composition of aphyric boninites the most magnesian boninites are of type i and the least magnesian ones are of types iii and v type ii is slightly higher in cao than type i at a given feo mgo ratio type iv is distinct from the others for its high cao and low incompatible element contents at a given feo mgo ratio type iv cannot be derived from the others by crystallization differentiation compositional data for the other types of boninite define rectilinear compositional variations which lie on a tie line connecting fe rich bronzite and primitive boninite microphenocrysts in equilibrium with boninite magma are more magnesian so it would appear that the fe rich bronzite was in equilibrium not with boninite magma but with some evolved descendant e g bronzite andesite magma the variations of aphyric boninites are then best explained by a combination of mixing of primitive boninite and evolved descendants together with equilibrium crystallization processes in order to account for this combination of processes a density stratified open system magma chamber of the type proposed for oceanic ridge systems seems appropriate the upper part of the magma chamber is composed of differentiated bronzite andesite magma and the lower part consists of hot dense and primitive boninite magma crystallization and sporadic overturn and mixing give rise to the spectrum of magma types erupted at the surface

late cenozoic structure and tectonics of the northern mojave desert in the fort irwin region of the northern mojave desert late cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral thrust slip on northwest striking moderately dipping segments at the east ends of the blocks crustal blocks 7 10 km wide by 50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block faulting initiated after 11 ma and quaternary deposits are faulted and folded we document a minimum of 13 km cumulative sinistral offset in a north south transect from south of the bicycle lake fault to north of the drinkwater lake fault paleomagnetic results from 50 sites reveal two direction groups in early and middle miocene rocks the north to northwest declinations of the first group are close to the middle miocene reference pole however rock magnetic studies suggest that both primary and remagnetized directions are present in this group the northeast declinations of the second group are interpreted as primary and 63 5 7 6 clockwise from the reference pole and suggest net post middle miocene clockwise rotation of several of the east trending blocks in the northeast mojave domain the jurassic independence dike swarm in fort irwin may be rotated 25 80 clockwise relative to the swarm north of the garlock fault thus supporting the inference of clockwise rotation using a simple shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks we predict 23 clockwise rotation using the observed fault slip or one third that inferred from the paleomagnetic results the discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks where most of our paleomagnetic sites are located however at least 25 40 of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible rigid body rotation of the region evidenced by clockwise bending of northwest striking domain bounding faults our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the eastern california shear zone over the last 10 m y occurred within the northeast mojave domain the remainder must be accommodated in adjacent structural domains e g east of the avawatz mountains and west of the goldstone lake fault supporting appendices 1 and 2 are available on diskette or via anonymous ftp from kosmos agu org directory apend username anonymous password guest diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 or by phone at 800 966 2481 15 00 payment must accompany order

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

trends of sr and nd isotopes through time near the japan sea in northeastern japan nd and sr isotopic analyses were made on selected volcanic rocks of known stratigraphic age from a limited area in the green tuff region northeastern japan and in adjacent areas two ultramafic inclusions from ichinomegata were also analyzed the volcanic rocks range in age from 27 to 6 ma and vary in e nd from 0 9 to 5 7 and in e sr from 14 1 to 16 3 the isotopic data plot generally along the region of the mantle array when plotted against time the isotopic ratios are found to be a regular function of geologic age the results indicate that 1 the volcanic rocks in the area are part of a related sequence or evolutionary pattern over a time period of 27 ma 2 the evolutionary pattern goes from an initially enriched source to that of a very depleted source the shift in isotopic composition appears to reflect the ongoing lining of magma conduits or the decreasing extraction of lower crustal contributions with increasing infiltration of oceanic magma types in an environment where the crust is thinning based on the geotectonic development of this area due to the back arc spreading of the japan sea we identify two different stages the pre opening stage 15 ma and the post opening stage 15 ma the volcanic rocks of the pre opening stage are characterized by an enriched isotopic signature and those of the post opening era by a depleted signature the host rocks of the kuroko ore deposit lie within this evolutionary trend and contain a substantial amount of continentally derived materials processed through an oceanic environment in general we infer that the degree of continental crust involvement is high during the pre opening stages of a back arc basin when it is associated with a rifted continental mass

the dynamics of magma mixing in a rising magma batch the conditions under which two magmas can become mixed within a rising magma batch are investigated by scaling analyses and fluid dynamical experiments the results of scaling analyses show that the fluid behaviours in a squeezed conduit are determined mainly by the dimensionless number i mu 1 u g delta rho r 2 where 1 is the viscosity of the fluid u is the velocity g is the acceleration due to gravity is the density difference between the two fluids and r is the radius of the tube the parameter i represents a balance between the viscous effects in the uppermost magma which prevent it from being moved off the conduit walls and the buoyancy forces which tend to keep the interface horizontal the experiments are carried out using fluid pairs of various density and viscosity contrasts in a squeezed vinyl tube they show that overturning of the initial density stratification and mixing occur when i order 10 1 the two fluids remain stratified when i order 10 3 transitional states are observed when 10 3 i 10 1 these results are nearly independent of reynolds number re 1 rho 1 ur mu 1 and viscosity ratio mu 1 mu 2 in the range of 10 2 mu 1 mu 2 10 3 and re 1 300 applying these results to magmas shows that silicic to intermediate magmas overlying mafic magma will be prone to mixing in a rising magma batch this mechanism can explain some occurrences of small volume mixed lava flows

a model for mixing basaltic and dacitic magmas as deduced from experimental data it has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours the presence of phenocrysts larger than 0 5 mm increased considerably the efficiency of mixing flow patterns in the experimental system were visualized using pt spheres which indicated that convection occurs in basalt melt but not in dacite melt the reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10 3 and 10 6 respectively mixing proceeds initially by mechanical mixing of the two magmas in a large scale but later by coupling interfacial convection and mutual diffusion thus depending on the depth where vesiculation and following disruption of the magma occurs banded pumice homogeneous pumice and homogeneous andesite lava are erupted the observed textures of mixed rocks of plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model

a new calibration point for the late miocene section of the geomagnetic polarity time scale 40ar 39ar dating of lava flows from akaroa volcano new zealand recent dating of geomagnetic field reversals over the past 2 ma by both the radiometric and astrochronologic techniques indicates that the k ar derived ages for these reversals are too young we report herein on 40ar 39ar incremental heating study of two geomagnetic field reversals trapped in lavas on akaroa volcano new zealand that demonstrate that the k ar derived ages for field reversals at 10 9 ma are too young by about 7 the new age for the termination of chron 5 a critical tie point on the geomagnetic polarity time scale for late miocene time is 9 67 0 11 ma

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

textural and compositional evidence for magma mixing and its mechanism abu volcano group southwestern japan quaternary basalts andesites and dacites from the abu monogenetic volcano group sw japan composed of more than 40 monogenetic volcanoes show two distinct chemical trends especially on the feo mgo vs sio2 diagram one trend is characterized by feo mgo enrichment with a slight increase in sio2 content fe type trend whereas the other shows a marked sio2 enrichment with relatively constant feo mgo ratios si type trend the fe type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma rocks of the si type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and or fine grained basaltic inclusions they are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz sodic plagioclase and hornblende phenocrysts petrographic variation as well as chemical variation from basalt to dacite of the si type trend is accounted for by various mixing ratios of basalt and dacite magmas pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing olivine and spinel and quartz sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas respectively before the mixing within a lava flow the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow and the number of resorbed phenocrysts varies inversely suggesting zonation in the magma chamber

paleomagnetism of middle miocene volcanic rocks in the mojave sonora desert region of western arizona and southeastern california paleomagnetic directions have been obtained from 190 early to middle miocene 12 20 ma mafic volcanic flows in 16 mountain ranges in the mojave sonora desert region of western arizona and southeastern california these flows generally postdate early miocene tectonic deformation accommodated by low angle normal faults but predate high angle normal faulting in the region after detailed demagnetization experiments 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations trm because of the episodic nature of basaltic volcanism in this region the 179 flows yielded only 65 time distinct virtual geomagnetic poles vgps the angular dispersion of the 65 vgps is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation the paleomagnetic pole calculated from the 65 cooling unit vgps is located at 85 5 n 108 9 within a 4 4 circle of 95 confidence this pole is statistically indistinguishable at 95 confidence from reference poles calculated from rocks of similar age in stable north america and from a paleomagnetic pole calculated from rocks of similar age in baja california the coincidence of paleomagnetic poles from the mojave sonora desert region with reference poles from the stable continental interior indicates that 1 significant vertical axis net tectonic rotations have not accompanied post middle miocene high angle normal faulting in this region 2 there has been no detectable post middle miocene latitudinal transport of the region and 3 long term nondipole components of the middle miocene geomagnetic field probably were no larger than those of the recent 0 5 ma geomagnetic field in contrast paleomagnetic data indicate vertical axis rotations of similar age rocks in the transverse ranges the eastern transverse ranges and the mojave block we speculate that a major structural discontinuity in the vicinity of the southeastward projection of the death valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations

along strike magma mixing beneath mid ocean ridges effects on isotopic ratios abstract the effects of mixing processes on the isotopic variability of mid ocean ridge basalts are studied the processes considered are porous flow dispersion and convective mixing in magma chambers porous flow dispersion is capable of mixing magmas over distances of only a few tens of meters convective mixing on the other hand is found to produce mixing over scales of kilometers to hundreds of kilometers calculations of convective mixing are carried out for continuous magma chambers where mixing is limited by convective processes and for discontinuous chambers where mixing is limited by chamber size preliminary comparison of our calculations with observations along the mid ocean ridges shows that the calculations are consistent with the existence of a correlation between bathymetry and isotopic ratio at long but not at short wavelengths they are also capable of explaining a decrease in isotopic variability with increasing spreading rate

is plagioclase removal responsible for the negative eu anomaly in the source regions of mare basalts the nearly ubiquitous presence of a negative eu anomaly in the mare basalts has been suggested to indicate prior separation and flotation of plagioclase from the basalt source region during its crystallization from a lunar magma ocean lmo are there any mare basalts derived from a mantle source which did not experience prior plagioclase separation crystal chemical rationale for ree substitution in pyroxene suggests that the combination of ree size and charge m2 site characteristics of pyroxene fo2 magma chemistry and temperature may account for the negative eu anomaly in the source region of some types of primitive low tio2 mare basalts this origin for the negative eu anomaly does not preclude the possibility of the lmo as many mare basalts still require prior plagioclase crystallization and separation and or hybridization involving a kreep component

an ion microprobe study of the intra crystalline behavior of ree and selected trace elements in pyroxene from mare basalts with different cooling and crystallization histories abstract to study the effects of crystallization sequence and rate on trace element zoning characteristics of pyroxenes we used combined electron microprobe ion microprobe techniques on four nearly isochemical apollo 12 and 15 pigeonite basalts with different cooling rates and crystallization histories major and minor element zoning characteristics are nearly identical to those reported in the literature all the pyroxenes have similar chondrite normalized ree patterns negative eu anomalies positive slopes as defined by yb ce and slopes of ree patterns from ce to sm much steeper than from gd to yb the characteristics of the ree patterns of pigeonite and augite can be rationalized in terms of ree size and charge and m2 site characteristics numerous ree and other trace element v sr zr na sc cr zoning characteristics can be correlated with quadrilateral composition the total ree abundances increase with increasing ca in the m2 site and indicate that the distribution coefficients of ree between pyroxene and melt vary systematically with the wo component the total ree abundances also increase with decreasing mg mg fe and increasing al prior to plagioclase crystallization the eu anomaly in pyroxene is not substantially affected by plagioclase crystallization because eu3 is effectively partitioned into pyroxene relative to eu2 eu 2 eu 3 03 in pigeonite and the plagioclase effect is diluted by the low eu distribution coefficient for the more abundant pyroxene the cr and v components behave similarly in that cr and v decrease with increasing ti al in basalt sample 15058 and decrease with crystallization and at similar ti al na and sc increase in abundance from pigeonite to augite with similar ti al in both 15058 and 15499 in 15058 na decreases coherently with increasing ti al whereas sc shows a limited variability sr and zr in 15058 are somewhat more enriched in the augite with low ti al relative to pigeonite the augite does not become enriched in zr or sr until the pyroxene has high ti al and low mg mg fe in 15499 sr and zr show slight enrichment in augite relative to pigeonite these trace element zoning characteristics in pyroxene and the partitioning of trace elements between pyroxene and the melt are intimately related to the interplay among the efficiency of the crystallization process the kinetics at the crystal melt interface the kinetics of plagioclase nucleation and the characteristics of the crystal chemical substitutions within both the pyroxene and the associated crystallizing phases i e plagioclase

a strontium isotopic investigation of the bjerkreim sokndal layered instrusion southwest norway the 230 km 2 proterozoic bjerkreim sokndal layered instrusion has a monzonoritic bulk composition and comprises a 6 km thick broadly leuconoritic layered series ls overlain by unlayered mangerite and quartz mangerite in the bjerkreim lobe the ls comprises six megacyclic units mcu 0 ia ib ii 11 and iv in a syncline this lobe is surrounded by migmatitic gneisses the roof to the instrusion is missing the mg number opx an and whole rock initial 87 sr 86 sr isotope ratios sr 0 display broadly parallel trends through mcus ii and iii and into mcu iv with decreasing mg number 75 58 and an 50 40 accompanying increasing sr 0 0 7050 0 7085 this correlation breaks down in the upper part of mcu iv and mangerite the quartz mangerite has sr 0 values of 0 7085 abrupt reversals occur across the mcu boundaries the ls crystallized on the floor of a periodically replenished magma chamber that was continually assimilating country rock gneisses strong compositional zoning of the magma developed as a result of repeated replenishments with relatively dense primitive ferrobasaltic magma sr 0 0 7049 along the floor and the development of extensive buoyant roof melts assimilation took place on a massive scale up to 50 in the upper parts of the chamber

magma mixing processes in volcanic contexts a thermodynamic approach with the examples of st vincent soufriere volcano west lndies and cerro chiquito guatemala summary different modalities and thermodynamic conditions of magma mixing are compared heterogeneity and chemistry temperature and water pressure volatile water content and water solubility density and viscosity such data are somewhat scarce in the literature two case studies have been studied in detail to highlight this review 1 st vincent soufriere west indies magma mixing may have been a major process in triggering the 1979 explosive phase all products whole rock analyses are basaltic andesite 52 3 56 2 wt sio2 but dacitic 60 64 wt sio2 glass occurs as micropumice or inclusions in crystals 2 cerro chiquito guatemala a quaternary extinct dome consisting of dacite 62 5 67 1 wt sio2 enclosing 20 30 in volume of basaltic andesitic 54 9 55 3 wt sio2 enclaves the nearly thorough mixing process of case 1 strongly differs from the commingling process of case 2 the main phases may be mafic case 1 or acidic case 2 thermodynamic parameters are calculated using classical geothermometers and formulae the range of values and errors are estimated 60 80 c in case of temperature in addition other processes such as crystallization of mineralo gical phases and fluid vesiculation may modify thermodynamical conditions difference of magma temperature ranges from about 60 c case 1 up to about 130 c case 2 water concentrations of 2 5 wt in mafic magmas and 4 wt in acidic magmas are always lower than the upper limit of water solubility at depths where contrasting magmas were mixed the difference between the viscosities of the two magmas varies from one order of magnitude up to five various mixing scenarios may occur each of them being linked to different petrological processes including effusive or explosive volcanism as well as plutonism

evidence for pressure dependence of the peak position in the ree mineral melt partition patterns of clinopyroxene abstract major element compositions ree abundances and strontium and neodymium isotope ratios were measured for three megacrysts of garnet five of clinopyroxene one of amphibole and the host alkali basalts from two volcanoes in northeastern china strontium and neodymium isotopic data and major elements fe and mg indicate a cognate origin of megacrysts and host lavas logarithmic values of ree partition coefficients for these minerals were found to vary parabolically with ionic radius and hence the rees in these three minerals are considered to substitute primarily into only one lattice site of each mineral the ionic radius optimum radius defined by the position of peak top of the parabola is variable based on the ree partition coefficient data for both experimental and naturally existing clinopyroxene melt pairs the change of the optimum radius may be interpreted to be mainly pressure dependent

abrupt change in magma liquidus temperature because of volatile loss or magma mixing effects on nucleation crystal growth and thermal history of the magma the liquidus temperature of magma that reaches shallow levels introduction during its ascent may change abruptly as a result of the release of on its way from the source region into the earth s volatiles or through magma mixing or a combination of both crust or even onto the earth s surface magma travelling immediately after this abrupt change of the liquidus temperature through the mantle and crust evolves and one of the occurs a disequilibrium is observed in the melt and here it is shown goals in petrology is to characterize the various processes that the melt rapidly re equilibrates to its thermodynamic equilibrium that occur during this passage of special interest in the a quantitative model for cooling and crystallization of a simple framework of this study are shallow level processes such two component model melt is used to investigate such re equilibration as magma crust interaction magma mixing deprocesses the relaxation time defined as the time required for the volatilization and depressurization that change the geosystem to re equilibrate after varying degrees of liquidus temperature chemical signature of the melt its crystal content its perturbations is found to be 3 or significantly less than the liquidus temperature its flow behaviour and its erupttime it takes to crystallize 40 of the melt regardless of the ability whereas crustal contamination of magma can be amplitude of the perturbation associated with the re equilibration traced through for example trace element and isotope process is an increase in crystal fraction that can easily reach studies e g wilson 1989 and is discussed extensively 10 depending on the amplitude of the perturbation during the in the other papers of this special volume cooling compensation of the liquidus temperature perturbation the temdepressurization and possibly magma mixing strongly perature of the melt remains nearly constant or may even increase slightly depending on the latent heat budget possibly heat of mixing affect the thermal history of magma rising in the mantle and heat absorbed during volatile exsolution which suggests and crust for example any pressure decrease in volatilecrystallization of the melt without cooling saturated systems not uncommon in natural systems especially for more evolved compositions will lead to volatile exsolution and consequent increased liquidus temperature and change in the melting range one of the major consequences of such behaviour is isothermal crystallization which was recognized almost 40 years ago by tuttle bowen 1958 pp 67 and 68 and the

evidence for two stage mixing in magmatic inclusions and rhyolitic lava domes on niijima island japan abstract fine grained inclusions are found in the achiyama and the mukaiyama rhyolitic lava domes on niijima island japan the existence of fine grained rims in the inclusions indicates that these inclusions are a quenched mafic magma incorporated into their rhyolite host the chemical composition and the phenocryst assemblage of the inclusions in the achiyama dome are identical with those of the effusive basaltic lava on niijima island wakago basalt the inclusions of the mukaiyama dome are dacitic and have a disequilibrium phenocryst assemblage olivine clinopyroxene orthopyroxene hornblende biotite plagioclase quartz and opaque minerals whole rock compositions lie on a mixing line between the host rhyolite and wakago basalt the dacitic magma is therefore a product of mixing of the basaltic and rhyolitic magmas the occurrence of two different inclusions basaltic and dacitic in two different domes can be explained by a two stage magma mixing model the dacite is formed in the first stage by mixing in a vertically stratified magma chamber caused by an interfacial instability between rhyolitic and basaltic layers a small amount of rhyolitic magma is entrained into the lower basaltic magma layer and two magmas can mix essentially in a liquid state the mixed dacitic magma is formed in the lower layer the second stage is mixing caused by forced convection during the ascent through a conduit just before eruption the stably stratified silicic and mafic magmas in a magma chamber can turn over and efficiently mix because of the formation of an unstable flow in a conduit under this condition the system reaches thermal equilibrium immediately after the mafic magma is disaggregated and dispersed in the silicic host the mafic magma solidifies when the silicic endmember is predominant in the mixture hence the equilibrium temperature is below the solidus of the mafic magma mafic inclusions are formed at this stage whether the inclusions are basaltic or dacitic depends on whether the final ascent through a conduit eruption occurred before or after the formation of the layer of a mixed dacitic magma in a magma chamber

structural geology and 40ar 39ar geochronology of the goldstone lane mountain area mojave desert california metasedimentary rocks near goldstone california consist in ascending structural order of 1 a calc silicate hornfels unit 2 a quartzite schist and radiolarian chert unit and 3 a calcareous and pelitic sequence of rocks that contains minor basaltic or andesitic composition dikes sills and flows unit 1 is of unknown but probably pre mesozoic age unit 2 is lower paleozoic in part ordovician and is correlative to lower paleozoic eugeoclinal rocks in the el paso mountains north of the garlock fault unit 3 is probably correlative to upper paleozoic permian basinal sequences in the el paso mountains these rocks were strongly deformed during a progressive deformational event that caused isoclinal to tight folding about northwest southeast trending fold axes and northeast dipping axial planes tectonic transport during this deformation was in a southwesterly direction similarity of structural trends to those in the el paso mountains suggests that the deformation in these two areas is the same age and predates intrusion of permo triassic plutons in the el paso mountains regional relationships suggest that deep water metasedimentary rocks in the northwestern mojave desert and in the el paso mountains are allochthonous with respect to miogeoclinal and cratonal paleozoic rocks elsewhere in the mojave the emplacement of the allochthonous rocks postdates deposition of permian strata and predates permo triassic to earliest triassic deformation and plutonism this deformation involved both eugeoclinal and miogeoclinal sequences and took place within an andean arc setting the subsequent mesozoic history of these two parts of the mojave desert has been the same 40 ar 39 ar data on hornblende biotite and muscovite in the goldstone and lane mountain quadrangles indicate that a plutonic complex consisting of gabbro quartz diorite and tonalite was intruded during the late jurassic probably slightly earlier than 148 m y ago voluminous granitic rocks were intruded during the late cretaceous about 85 90 m y ago these younger plutonic rocks caused variable resetting of all k ar systems in older rocks cooling of late cretaceous plutons to about 280 40 c occurred by about 78 4 m y regional uplift of these rocks to a crustal level where no further diffusive loss of argon took place occurred at about 45 m y ago

floating lids on large mid ocean spreading center magma chambers a discussion rosencrantz and nelson 1982 have concisely presented arguments in favor of the hypothesis that lids overlying ocean ridge magma chambers are supported by flotation of the lid on magma of equal or higher density i concur that given the highly fractured nature of oceanic crust the lids to oceanic magma chambers can only be supported by flotation regardless of the width of the magma chamber in their quantitative analysis rosencrantz and nelson implicitly made two assumptions on the shape of the magma chamber and therefore on the nature and density of the lid and on the nature and therefore the density of the contents of the magma chamber it is important to realize that in a situation in which either one of these assumptions is thought not to hold the acceptance of the flotation hypothesis places severe constraints on other properties of the situation the problem is raised in the light of the archean ocean ridge model of sleep and windley 1982 fig 3 in the same issue of journal of geology but is of general relevance given our ignorance concerning magma chambers there have been several suggestions for the shape of ocean ridge magma chambers in situations where permanent magma chambers are thought to exist sleep 1975 rosendahl 1976 and dewey and kidd 1977 envisaged a magma chamber narrowing upwards with a triangular cross section and a relatively flat base hereafter this shape is referred to as an inverted funnel from their work on the samail ophiolite in oman pallis

experimental study of trace element partitioning between lunar orthopyroxene and anhydrous silicate melt effects of lithium and iron abstract orthopyroxene opx was present during a significant portion of crystallisation of the lunar magma ocean but its influence on co existing melt trace element contents is not well quantified we performed high pressure p 1 1 to 3 2 gpa high temperature t 1400 to 1600 c experiments on synthetic fe rich compositions at reducing conditions relevant to the lunar mantle to constrain trace element partitioning between opx and anhydrous silicate melts opx melt partition coefficients d opx melt for a wide range of trace elements lile li ba ree la ce nd sm dy eu er tm y yb lu hfse zr nb hf ta th u and transition metals sc v mn co mo show only very minor variations across the considered p t range ree partition coefficients increase from d la opx melt 0 0014 0 0008 to d lu opx melt 0 051 0 007 d values for highly charged elements vary from d th opx melt 0 0013 0 0008 through d nb opx melt 0 0018 0 0006 and d u opx melt 0 0015 0 0006 to d ti opx melt 0 068 0 0010 d lu opx melt d hf opx melt values of 6 3 2 4 are at the high end of reported values for minerals that played a role during crystallisation of the lunar magma ocean and higher than previously reported for opx under identical oxygen fugacity conditions implying opx rich cumulates in the lunar mantle have highly superchondritic lu hf ratios lattice strain modelling of our ree partitioning data suggests that varying the concentration of divalent fe in opx very slightly decreases the ideal cation radius for m 3 elements entering the m2 site r 0 m2 whereas the partitioning of m 3 elements entering the m1 site is unaffected a subtle increase in the maximum partition coefficient for m 3 elements entering both the m1 and m2 sites with decreasing t is identified when experiments carried out at similar reducing oxygen fugacities are considered the presence of li at concentrations of up to 350 ppm does not have a measureable effect on the opx melt partitioning behaviour of ree or any other element showing that charge balancing of m 3 m 4 and m 5 elements in opx is likely dominated by a vacancy mechanism

sediment entrainment in viscous fluids can crystals be erupted from magma chamber floors the hypothesis that unconsolidated cumulus olivine crystals can be entrained into a venting magma is investigated from a physical standpoint entrainment is predicted to occur when the viscous stress acting over the surface of crystals immediately beneath the chamber outlet overcomes their weight for magma of kinematic viscosity v and density p erupting at a rate g crystals of radius a and density rho delta rho will be entrained if the outlet is closer to the cumulus crystal pile than the critical entrainment height h c qyl g a m where c is a constant and g g delta rho rho laboratory experiments using glycerol solutions and glass spheres verify this result and yield c 1 14 for the particular experimental conditions parameters appropriate to olivine accumulative magmas predict entrainment heights of less than a few tens of meters this implies that if olivine entrainment is to occur it should only do so when the chamber is tapped by an outlet in the wall positioned

petrology and geochemistry of late stage intrusions of the a type mid proterozoic pikes peak batholith central colorado usa implications for petrogenetic models abstract the 1 08 ga anorogenic a type pikes peak batholith front range central colorado is dominated by coarse grained biotite amphibole syenogranites and minor monzogranites collectively referred to as pikes peak granite ppg the batholith is also host to numerous small late stage plutons that have been subdivided into two groups e g wobus 1976 studies in colorado field geology colorado school of mines professional contributions colorado 1 a sodic series sio 2 44 78 wt k na 0 32 1 36 composed of gabbro diabase syenite quartz syenite and fayalite and sodic amphibole granite and 2 a potassic series sio 2 70 77 wt k na 0 95 2 05 composed of biotite granite and minor quartz monzonite differences in major and trace element and nd isotopic characteristics for the two series indicate different petrogenetic histories potassic granites of the late stage intrusions appear to represent crustal anatectic melts derived from tonalite sources based on comparison of their major element compositions with experimental melt products in addition nd isotopic characteristics of the potassic granites e nd 1 08 ga 0 2 to 2 7 overlap with those for tonalites granodiorites ca 1 7 ga boulder creek intrusions e nd 1 08 ga 2 4 to 3 6 exposed in the region some of the partial melts evolved by fractionation dominated by feldspar the late stage potassic granites share geochemical characteristics with most of the ppg which is also interpreted to have an anatectic origin involving tonalitic crust the origin of monzogranites associated with the ppg remains unclear but mixing between granitic and mafic or intermediate magmas is a possibility syenites and granites of the sodic series cannot be explained as crustal melts but are interpreted as fractionation products of mantle derived mafic magmas with minor crustal input high temperature and low oxygen fugacity estimates e g frost et al 1988 american mineralogist 73 727 740 support a basalt fractionation origin as do e nd values for sodic granitoids e nd 1 08 ga 2 2 to 0 7 which are higher than e nd values for colorado crust at 1 08 ga ca 1 0 to 4 0 enrichments in incompatible elements e g rare earth elements rb y and depletions in compatible elements e g cr sr ba in the sodic granitoids compared to coeval mafic rocks are also consistent with fractionation accessory mineral fractionation release of fluorine rich volatiles and or removal of pegmatitic fluids could have modified abundances of ce nb zr and y in some sodic granitoid magmas gabbros and mafic dikes associated with the sodic granitoids have e nd 1 08 ga of 3 0 to 3 5 which are lower than depleted mantle at 1 08 ga and their trace element characteristics suggest derivation from mantle sources that were previously affected by subduction related processes however it is difficult to characterize the mantle component in these magmas because assimilation of crust during magma ascent could also result in their observed geochemical features the pikes peak batholith is composed of at least two petrogenetically different granite types both of which exhibit geochemical characteristics typical of a type granites models proposed for the petrogenesis of the granitoids imply the existence of mafic rocks at depth and addition of juvenile material to the crust in central colorado at 1 1 ga

magma mixing in mantle xenolith bearing calc alkalic ejecta ichinomegata volcano northeastern japan abstract the latest pleistocene ejecta from the ichinomegata volcano ne japan composed of three maars show a wide compositional variation ranging from basalt 51 sio 2 to dacite 65 sio 2 these have the same phenocryst assemblage of olivine augite hornblende biotite quartz plagioclase and titanomagnetite upper mantle peridotite xenoliths are found not only in basalts but also in andesite whole rock major chemistry and modal compositions indicate that the chemical and modal variations have been caused by the mixing of three magmatic components primitive high alumina basalt sio 2 51 wt mgo 8 5 wt with olivine fo 88 and augite phenocrysts differentiated basaltic andesite sio 2 54 wt mgo 5 wt which has the same but slightly fe rich phenocryst phases as the primitive basalt and porphyritic dacite sio 2 65 wt mgo 2 0 wt with phenocrysts of hornblende biotite quartz plagioclase and titanomagnetite the upper mantle peridotite bearing andesite may not be derived directly from the upper mantle and its peridotite xenoliths may have been carried by the primitive basaltic magma the estimated density for three magmatic end components requires a high flow velocity of basaltic magma and a small volume 5 m 3 of the dacite magma body to facilitate breaking of the stratification in the dacite magma body this is consistent with the retention of peridotite xenoliths in the mixed magma and the preserved disequilibrium petrographic features

petrological and geochemical relationships between pyroxene megacrysts and associated alkali basalts from massif central france major trace element and ree analyses as well as sr isotopic ratios have been obtained on twelve clinopyene megacrysts and phenocrysts and their alkali basalt hosts from the french massif central equilibrium between crystals and host was examined based on petrographic and geochemical data two types of pyroxenes are recognized the acmite bearing clinopyroxenes rich in incompatible elements and the salitic clinopyroxenes poor in incompatible elements 87sr 86sr isotopic data reveal no significant difference between clinopyroxenes and host lavas they are in apparent isotopic equilibrium the sr isotopic ratios of the two types of pyroxenes are also quite similar however pyroxene crystals from the first group are not in equilibrium with their host they have crystallized at high pressure from differentiated alkali lavas and have been incorporated in a more primitive magma pyroxene crystals from the second group are in apparent equilibrium with their host lava they have crystallized at various pressures for the latter distribution coefficients are proposed for compatible elements trace elements and ree

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges

magma mixing versus xenocryst assimilation the genesis of trachyandesites in sancy volcano massif central france sr and nd isotopic compositions have been determined on basaltic and acid trachyandesites bta ata from the sancy volcano mont dore massif france these represent more than 80 of the lavas erupted during its activity between 0 9 and 0 2 ma these lavas have been recently interpreted as the result of two component magma mixing during and after repeated injections of basaltic magmas in trachytic reservoirs magmatic heterogeneities in the ata s large to small enclaves banded lavas megacrysts testify to the mingling event complete mixing is supposed to have been achieved in the hybrid bta s which contain sanidine plagioclase and clinopyroxene megacrysts in disequilibrium with their host the megacrysts are interpreted as relicts of the trachytic end member isotopic data on basic comagmatic enclaves and host ata matrix samples from three different cycles of mingling succession of heterogeneous pyroclastics heterogeneous ata lava flows or domes and occasionally homogeneous bta lava flows are not incompatible with two component mixing but could just reflect the heterogeneity of the analysed samples however the bta s have sr contents and sr isotopic ratios which are too high to be simple binary mixing products between the postulated end members three hypotheses are considered to explain this discrepancy 1 the analysed end members are not those involved in bta genesis 2 some crustal contamination occurred during and after the mixing event 3 sr rich sanidine xenocrysts with radiogenic 87sr86sr have been assimilated and digested in the bta s in this third hypothesis that i favour it is not necessary to resort to magma mixing to explain the genesis of the bta s assimilation of xenocrysts by basaltic hawaiitic or mugearitic magmas accounts for both mineralogical disequilibria and isotopic characteristics of these lavas

the origin of mount st helens andesites abstract mount st helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants the mafic lavas include a transitional olivine plagioclase basalts with low k 2 o and incompatible trace element abundances b calc alkaline olivine plagioclase clinopyroxene basalts enriched in k 2 o tio 2 and incompatible trace elements and c calc alkaline olivine plagioclase basaltic andesites with incompatible trace element contents transitional between the two basalt types intermediate lavas include a tholeiitic two pyroxene andesites b calc alkaline plagioclase two pyroxenes olivine amphibole mafic andesites 56 59 wt sio 2 and c calc alkaline plagioclase two pyroxenes amphibole high silica andesites 61 62 wt sio 2 eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches it is unlikely that any of the andesites or dacites are derived by fractional crystallization of the recognized basaltic variants formation of the andesites simply by contamination or assimilation fractional crystallization of basaltic magma is also improbable more plausibly the andesites represent mixing between basaltic and dacitic end member magmas each of which may be somewhat heterogeneous or vary in composition with time in this model efficient mixing must occur in some parts of the magma plumbing system while some conduits or storage reservoirs must be effectively isolated

an apparent shear zone trending north northwest across the mojave desert into owens valley eastern california strain rates measured at four geodetic networks in eastern california situated between northern owens valley and transverse ranges along a small circle drawn about the pacific north america pole of rotation are remarkably consistent each exhibits 0 14 mu rad yr simple right lateral engineering shear strain accumulation across the local vertical plane tangent to the small circle local faults e g owens valley garlock helendale traversing these networks are not as closely aligned with the vertical planes of maximum shear strain accumulation as is the local tangent to the small circle a fifth network slightly east of the small circle shows no significant strain accumulation thus a shear zone trending n35 degree w from near the eastern end of the big bend of the san andreas fault to northern owens valley is indicated by these data this corresponds to the eastern california shear zone proposed on geological evidence by dokka and travis the shear zone carries approximately 8 mm yr of the pacific north america relative plate motion from the san andreas fault north northwest across the mojave desert into owens valley and the northern basin and range province the shear zone observed at the surface may be a manifestation of a through going subcrustal fault

finger structures in the lille kufjord layered intrusion finnmark northern norway highly irregular contacts are developed between peridotite and troctolite in layers forming the uppermost part of a transition zone between two of the cyclic units of the lille kufjord intrusion the upwardly directed peridotite fingers crosscut both the feldspar lamination and the feldspathic xenoliths in the troctolite and are interpreted as the result of the replacement of troctolite by peridotite similar structures are developed in the rhum ultrabasic pluton replacement may have been caused by the migration of a more hydrous and magnesian magma trapped initially in olivine or olivine clinopyroxene cumulates into plagioclase olivine cumulates precipitated from a basaltic liquid

the mixing of magmas in plutonic and volcanic environments analogies and differences abstract magma mixing processes have been widely recognized in both the plutonic and volcanic environments but the quality and quantity of information that can be extracted from the two environments are substantially different understanding the advantages and limits associated with the study of plutonic and volcanic rocks is essential to establish precise methodological approaches to build the most complete conceptual model by merging information from these two complementary igneous environments in this work we review magmatic interaction processes in the plutonic and volcanic environments by considering several aspects of these geological phenomena in particular we first briefly report on the structural and geochemical evidence for magma mixing in both plutonic and volcanic rocks with the aim to provide a general picture of this natural phenomenon successively we discuss some recent results about magma mixing achieved using the concepts from chaos theory and discuss their potential impact on magma differentiation finally we attempt to build a general picture of this igneous process by merging present day information from both the plutonic and volcanic environments it emerges from the general picture that the time spent by the magmatic system in the molten or partially molten state is the crucial factor for the preservation of the fingerprints of magma mixing in the two environments we propose a conceptual model that may be useful to understand what kind of information we can obtain from volcanic and plutonic rocks and ultimately to maximize our knowledge about magma mixing

mineral scale sr isotope constraints on magma evolution and chamber dynamics in the rum layered intrusion scotland sr isotopic zoning within single plagioclase crystals from rocks from unit 9 of the rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination the 87sr 86sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying unit 10 peridotite is approached models of near solidus interaction of the cumulate with a fluid or melt or large scale textural re equilibration cannot easily account for the systematic differences in 87sr 86sr between small crystals and the rims of larger crystals

the crustal magma chamber of the katla volcano in south iceland revealed by 2 d seismic undershooting summary results of a 2 d seismic undershooting experiment on the katla central volcano in south iceland are reported large localized traveltime anomalies 0 4 s are observed on an array within the katla caldera the traveltimes are forward modelled using a wavefront tracker developed in appendix a thus non linear effects encountered in traveltime tomography are avoided as well as common problems with ray tracing in the presence of strong lateral heterogeneity an extreme variation in compressional velocity is required to extend over a significant volume in order to model the data the resulting model is not unique but constraints on the allowable range of velocities 2 5 6 0 km s render the basic features well constrained a clear s wave shadow is closely associated with delays in traveltime due to a shallow slow anomaly low amplitude p waves go hand in hand with early arrivals due to thin structural features flanking the slow anomaly the model is interpreted in terms of a magma chamber containing extensively molten rock the magma chamber is shallow with a bottom at a depth of about 1 5 km below sea level 3 0 km below surface and measures about 5 km across the depth of the chamber is roughly at the level of buoyant equilibrium for basaltic melt in the crust owing to poor vertical resolution at shallow depths in the undershooting geometry the top of this shallow magma chamber is not well resolved on the other hand the bottom of the chamber is well resolved the chamber is underlain by rocks of average or high velocity for that depth the magma chamber is a persistent feature big enough 10 km3 to supply magma for large eruptions and to supply heat to permit remelting of hydrated basaltic crust to produce silicic magmas at shallow levels the chamber is fed by magma fracturing from below the model agrees with phenomenological models of magma chambers in iceland based on geological observations and provides a quantification of those models in terms of depth and size on the other hand it is fundamentally different from recent models of magma chambers at mid ocean ridges which may be more akin to the pervasive region of partial melt at depth beneath iceland this underlines the important effect of the icelandic hotspot on tectonics and volcanism in iceland and implies a substantially different crustal and thermal structure in iceland from that of normal mid ocean ridges

grain boundary enrichment of incompatible elements in some mantle peridotites electron microprobe analyses sensitive to 15 40 ppm at 3 confidence have been made for minerals in a spinel dunite from san carlos arizona u s a and a spinel lherzolite from ichinomegata akita ken japan whereas both rocks contain an appreciable amount of k2o p2o5 and tio2 the constituent minerals contain only trace amounts of these elements microprobe analyses across the grain boundaries on polished sections and on mineral surfaces before and after acid leaching show that most of the incompatible elements in the rocks are contained within a surface film al k emission spectroscopy suggests that the surface film is neither an aggregate of fine grained intergranular minerals nor a quenched glass it could be an adsorbate on the irregular sites of crystal surfaces and was probably formed by any of diffusion from interiors of minerals i e segregation mantle metasomatism and partial melting the contribution of incompatible elements from the grain boundaries into partial melts has been documented by heating experiments on peridotite chips under controlled fo2 the spinel dunite yields at 1300 c a 4 5 vol partial melt with about 1 k2o 1 6 na2o 0 14 p2o5 0 8 tio2 210 ppm sr 140 ppm ba and 40 ppm ce along the grain boundaries grain boundaries are an important storage of incompatible elements in peridotites and they in turn can be the dominant source of these elements in magmas during partial melting of the upper mantle

convection and crystal settling in sills it has been advocated that convective and crystal settling processes play significant and perhaps crucial roles in magmatic differentiation the fluid dynamics of magma chambers have been extensively studied in recent years both theoretically and experimentally but there is disagreement over the nature and scale of the convection over its bearing on fractionation and possibly over whether it occurs at all the differential distribution of modal olivine with height in differentiated alkaline basic sills provides critical evidence to resolve this controversy at least for small to medium large magma chambers our own and others published data for such sills show that irrespective of overall olivine content modal olivine contents tend to increase in a roughly symmetrical manner inwards from the upper and lower margins of the sill i e the distribution patterns are more often approximately d shaped rather than the classic s shape generally ascribed to gravity settling we concur with the majority of other authors that this is an original feature of the filling process which has survived more or less unchanged since emplacement we therefore conclude that the magmas have not undergone turbulent convection and that gravity settling has usually played only a minor modifying role since the intrusion of these sills we offer a possible explanation for the apparent contradiction between fluid dynamical theory and the petrological evidence by suggesting that such sills rarely fill by the rapid injection of a single pulse of magma rather they form from a series of pulses or a continuous pulsed influx over a protracted interval during which marginal cooling severely limits the potential for thermal convection

major element variations in basalts and diabases from the north arm mountain massif bay of islands ophiolite implications for magma chamber processes at mid ocean ridges abstract the major element compositions of aphyric diabases from the north arm mountain massif range from primitive basalts to highly evolved ferrobasalts from modeling of these compositional variations it is clear that the diabases have undergone highly variable 10 to 90 increments of crystallization at low pressures the dominant minerals inferred to have crystallized from the liquids that formed these diabases are olivine plagioclase and clinopyroxene the same minerals that dominate the underlying cumulate gabbroic and ultramafic rocks the projection of these compositions onto pseudo liquidus phase diagrams suggests that the primary magma s from which this north arm suite was produced were generated by melting at 10 25 kbar or greater pressure there is no indication in the major element variations of these diabases of the operation of high pressure crystal factionation processes which is not surprising because low pressure fractionation processes will effectively mask evidence for earlier high pressure processes it is suggested that the tapping of intercumulus liquid from crystal mush areas and the tapping of discrete fractionated magmas within the roof zone of magma chambers is a method of producing a wide variety of magma compositions from a single magma chamber in which the majority of the chamber may be actively convecting and therefore remain relatively uniform in composition

evolution of magma mixing in an alkaline suite the grande cascade sequence monts dore french massif central geochemical modelling abstract trachyandesitic lavas from an alkaline suite grande cascade sequence monts dore french massif central have been investigated for evidence of mechanical magma mixing a wide variety of textures are present in the sequence as pillow like enclaves bandings and emulsified facies the grande cascade flows and pyroclasts range from basalt to trachyte 44 67 wt sio 2 pillow like enclaves intricate commingling and local diffusive mixing at the mafic felsic interface in emulsified types show that mafic inclusions were incorporated in their trachytic host as blobs of magma such inclusions were probably dispersed throughout the trachytic magma by mechanical mixing different stages of the evolution of a mechanical mixing process were recognized in relation with the stratigraphy and chronology of the products the most heterogeneous features enclaves bandings are restricted to the bottom of the sequence where basalt and trachyte coexist at the top texturally homogeneous trachyandesites occur relict phenocrysts are observed in both basaltic enclaves oligoclase fe rich clinopyroxene sanidine of the trachyte and trachytic hosts mg rich olivine mg rich clinopyroxene ca rich plagioclase of basalt the intermediate rocks basic trachyandesites contain xenocrysts of mg rich olivine clinopyroxenes oligoclase sanidine together with reversely zoned plagioclase and pyroxene phenocrysts this assemblage is presumed to result from the incorporation of xenocrysts related both to mafic and felsic endmembers all mineral assemblages are out of equilibrium reversed compositional zoning profiles in plagioclase and clinopyroxene crystals were produced by the mixing event we are able to identify the compositional characteristics of the components that were mixed a primary alkaline basalt mg olivine labrador mg clinopyroxene magnetite and a silicic trachyte oligoclase fe clinopyroxene sanidine are the endmembers of mixing observed chemical variations in major and trace elements can be modelled as pure binary mixtures between basaltic and trachytic endmembers using the best mixing line method provost and allegre 1979 the calculations emphasize that trachyandesites are the result of complete hybridization between basalt and trachyte the data on mesostasis show that melt mixing and not only crystal transfers has produced the intermediate lavas of the grande cascade sequence textural criteria petrography mineral chemistry and geochemical data major and trace elements support the magma mixing model

block rotations along the strike slip finlay ingenika fault north central british columbia implications for paleomagnetic and tectonic studies structure developed in association with oblique motions on the dextral strike slip finlay ingenika fault fif in the mcconnell creek area north central british columbia is dominated by subvertical to vertical strike slip faults some of the faults cut the area into discrete fault bounded blocks several kilometres wide stress tensors for 24 sites were inverted from the regional cleavage which predates the block bounding faults the site mean stress tensors indicate that the fault bounded blocks were rotated clockwise about a subvertical axis during progressive oblique motions on the fif rotation varies systematically being maximal 58 7 3 3 close to the fif and minimal 0 0 1 6 about 20 km away paleomagnetic samples were collected from 13 sites in the widespread early jurassic to cretaceous plutonic rocks interpretable magnetic components of presumed late cretaceous age were obtained from six sites the observed paleopole is significantly different from the late cretaceous reference pole for cratonal north america cna after corrections for the local block rotations the precision is much improved k increased from 48 to 383 and the paleopole moves closer to the reference pole but is not coincident with it rotation about the eulerian pole for the best fitting small circle to the tintina trench and northern rocky mountain trench fault zone however brings the observed pole into coincidence with that for cna and requires 670 km of dextral displacement on the fault zone it is evident therefore that local structures associated with large dextral strike slip faults could account for at least part of the paleomagnetic disparity between some western parts of the canadian cordillera and cna

chemistry morphology and origin of magmatic reaction chromite stringers associated with anorthosite in the upper critical zone at winnaarshoek eastern limb of the bushveld complex

magmatic residence times of zoned phenocrysts introduction and application of the binary element diffusion modelling bedm technique this paper describes a general technique binary element diffusion modelling bedm for determining single crystal residence times in magmas that relies on modelling the diffusion of two or more elements in the crystal bedm has the advantage over other diffusion based models in that it does not need a precisely defined initial compositional profile for the crystal at zero time and instead requires that the concentrations of the two elements are correlated during crystallisation any differences subsequently observed between the two elements are caused by intracrystalline diffusion during residence in hot magma these differences are removed by artificially ageing the slower diffusing of the two elements and the amount of time taken to undo the difference between the elements is simply related to the crystal residence time decoupling time at high temperatures the bedm principle is demonstrated using artificial data and is then applied to literature data for sr and ba in a zoned sanidine crystal from the bishop tuff anderson et al in j petrol 41 3 449 473 2000 for this crystal the method gives a residence time estimate of 114 ka at 800 c which is then compared with estimates from other methods in theory the method can be further expanded for use as a geothermometer as well as geochronometer however this is not easily possible with the diffusivity data currently available

neogene clockwise tectonic rotation of the eastern transverse ranges california suggested by paleomagnetic vectors results from a paleomagnetic study of neogene volcanic rocks situated within and around the pinto basin region of the eastern transverse ranges suggest that the ranges have undergone a clockwise tectonic rotation of 41 4 7 7 since late miocene time both normal and reversed polarities are observed and inclination data indicate no significant flattening the rotation was accompanied by progressive sinistral slip on the several east west trending faults which bound and transect the eastern transverse ranges a computation using the observed amount of cumulative left slip on the east west trending faults of the eastern transverse ranges 50 km and the spacing between the pinto mountain and salton creek faults which bound the region 70 km indicates that 35 of clockwise rotation is required which roughly agrees with the paleomagnetic data removal of the eastern transverse ranges rotation in a reconstruction shows that the distributed dextral shear across the mojave desert region to the north must have been on the order of 100 km since late miocene time from the results of this study we infer that crustal rotation of the eastern transverse ranges began later and is of less magnitude than that of the western transverse ranges

an experimental study on the effects of phenocrysts on convection in magmas abstract the effect of phenocrysts on convection in magma chambers is investigated experimentally using small heavy particles in convecting fluids the particles are initially uniformly distributed in a fluid which is either heated from below or cooled from above the system is allowed to evolve and temperature and particle concentration profiles are measured as functions of time when the concentration of particles is sufficiently small convection is basically unaffected by their presence when the concentration is above a critical value however the convective motion is considerably altered the effect of particles on the subsequent fluid behaviour is different in the cases of heating from below and cooling from above in the former case there are strong convective motions confined to a sedimentary layer of decreasing thickness beneath a clear layer which displays rather weak convective motions with time the destabilizing increase of temperature in the lower layer overcomes the stabilizing contribution to the bulk density due to the particles and the layer overturns quite suddenly in the situation of cooling from above a critical condition separates a case of continual overturn from a case of no overturn at all with the sedimentary layer falling unimpeded to the bottom theoretical analysis suggests that the critical value is determined primarily by the ratio of the contribution to the bulk density of the suspension due to particles to the change in fluid density due to the thermal effect the size distribution of the particles can also modify the fluid behaviour applying our general results to geological situations we suggest that the presence of relatively small concentrations of phenocrysts can critically influence the mode of convection in magmas

evolution of icelandic central volcanoes evidence from the austurhorn intrusion southeastern iceland the austurhorn intrusive complex in southeastern iceland represents an exhumed tertiary central volcano the geometry of the intrusion and geochemistry of the mafic and felsic rocks indicate austurhorn was a volcanic center analogous to eyjafjallajokull and torfajokull in iceland s eastern neovolcanic zone evz early transitional tholeiitic basalt magmatism at austurhorn formed a shallow crustal chamber 5 km in diameter apparent rhythmic modal layering of and intrusive contacts within the gabbro indicate the mafic chamber was replenished frequently as it cooled and crystallized felsic activity postdated near solidification of the gabbro numerous granitic magmas intruded along gabbro margins and within the adjacent crust field relations indicate that infrequent felsic replenishment prevented convective mixing of the austurhorn chamber during this time although commingled mafic and felsic magmas are observed in an extensive net veined complex late stage mafic dikes intrude the entire complex suggesting that magmatic heat was abundantly available throughout the evolution of the austurhorn system plagioclase and clinopyroxene compositions in mafic through felsic rocks including gabbros support a model of progressive differentiation field relations constrain the felsic magmas to originate at p 1 kbar presumably by fractional crystallization the structure and geochemistry of the austurhorn intrusive complex suggest formation in an immature rift environment similar to the modern evz the proposed rift segment was parallel to the western and eastern neovolcanic zones and probably resulted from a reorganization of plate boundaries 7 ma saemundsson 1979 helgason 1985 jancin et al 1985 triggered by activity of the iceland mantle plume

a model of reverse differentiation at dikii greben volcano kamchatka progressive basic magma vesiculation in a silicic magma chamber dikii greben volcano is the largest modern volcano with silicic rocks in the kurile kamchatka island arc it consists of many domes and lava flows of rhyodacite dacite and andesite which were erupted in a reverse differentiation sequence non equilibrium phenocryst assemblages quartz mg rich olivine an rich an poor plagioclase etc abundance of chilled mafic pillows in the dacites and andesites and linear variations of rock compositions in binary plots are considered as mineralogical textural and geochemical evidence for mixing mafic pillows in volcanics have a lower density because of high porosity and contain the same non equilibrium phenocryst assemblages as the host rocks their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water rich basaltic magma at depth they are considered as supercooled vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling the lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption the rhyodacitic and basic end members of the mixing series cannot be linked by low p fractionation though high p amphibole rich fractionation is not excluded

constraints on the formation of submarine lava flows from numerical model calculations abstract to constrain life time and flow rates in a submarine lava tube pillow 3d finite element calculations were carried out for a simplified fixed geometry model tube the principal parameters included into the model are a temperature dependent newtonian viscosity of the lava whereby viscosity is also dependent on composition temperature and crystallinity of the lava cooling by seawater and the release of latent heat of crystallization time dependent model runs suggest the longevity of the tube flow is constrained by the competition of advective heat flow along the tube axis and the conduction of heat towards the margins of the flow in order to keep the lava flowing a minimum critical pressure gradient along the tube is required otherwise the amount of thermal energy brought in by the influx of hot lava is less than the amount lost at the outer boundaries and the tube will solidify providing an exponentially decaying driving pressure at the tube inlet this critical pressure gradient has been calculated for different tube lengths and diameters and is compared to critical pressure values resulting from a more simplistic solution assuming constant viscosity and pure conductive or convective cooling an analysis of the difference between pressure gradients obtained through the numerical and simplified model is used to derive an appropriate power law for the critical pressure gradient in bodies cooled by convection and conduction including the effect of temperature dependent viscosity that gradient scales with the tube radius r as r3 85 this law is valid for tube style lava flows and lava viscosities in the basaltic range

basalt convection and plagioclase retention a model for the generation of high alumina arc basalt recent investigations of crystal settling in convecting magma have suggested that crystals may be spatially sorted according to both size and mineralogy for a basalt magma simultaneously crystallizing plagioclase clinopyroxene and olivine a possible result is the selective removal of mafic minerals and the preferential retention of plagioclase crystals along the intrusion margin this represents a possible mechanism for generating plagioclase phyric magmas of high alumina basalt composition in this study a combined basalt crystallization and convection model has been developed to assess the extent to which crystallization and convection leads to mineralogic sorting and preferential plagioclase retention in a primitive mantle derived calc alkaline basalt intrusion the model assumes a simple rectangular chamber geometry steady state non turbulent single cell convection and spatially uniform crystallization the results indicate that over a narrow range of convective velocities 1 crystal sorting

multiple merensky reef of the bushveld complex south africa the merensky reef mr of the bushveld complex is known for containing the lion s share of the world s reserves of platinum group elements pge it is commonly described as a single relatively thin layer or package of chromite and sulphide bearing orthopyroxenite hosted by almost barren norite and or anorthosite here we present a drill core section that contains however four separate mr orthopyroxenite layers hosted by mottled anorthosites each of these orthopyroxenites is distinguished through occurrence absence of marginal chromitite seams grain size petrographic composition and pge abundance we interpret them as multiple undercutting mr that occurs as sill like apophyses extending laterally from a pothole margin into the footwall the undercutting mr is attributed to in situ crystallization within sheet like melt filled cavities at potholes margins which are produced through thermal and or chemical erosion of the footwall rocks by replenishing magmas this finding indicates that the mr forming event involved several distinct pulses of replenishing magmas whose crystallization products were mostly eroded away in the stratiform portions but remained locally preserved in apophyses within footwall rocks the mineralized mr formed a few meters below a temporary chamber floor is indicative of strong thermal compositional convection in the basal layer of the bushveld s chamber the convection was able to deliver large volumes of fresh magma for the scavenging of pge by sulphide and chromite growing within the sheet like cavities of the mr potholes

petrology of the younger andesites and dacites of iztaccihuatl volcano mexico i disequilibrium phenocryst assemblages as indicators of magma chamber processes

strontium isotopic resolution of magma dynamics in a layered intrusion kalka is one of about 14 major layered mafic ultramafic intrusions forming the giles complex in central australia1 the stratigraphical section of the kalka intrusion passes from a basal pyroxenite zone 450 m through norite and olivine gabbro zones 3 500 m to an uppermost anorthosite zone 800 m 2 further resolution into 21 cyclic units is derived from repeated mineral crystallization sequences and cyclical variation in plagioclase and olivine compositions a conventional interpretation would have a basaltic magma progressively fractionating as crystallization proceeded from mafic base to leucocratic top with periodic resetting to less evolved states by fresh incursions of primary magma or by convectional overturn within a sealed magma chamber however the reconnaissance isotopic data reported here indicate development of a more complex open system high initial 87sr 86sr ratios in conjunction with normal 143nd 144nd ratios demonstrate massive contamination by country rock within the main part of the intrusion3 furthermore great variation in sr initial ratios from 0 7049 to 0 7088 suggests substantial changes in magma composition beyond those induced by fractionation similar isotopic heterogeneity in the newer gabbros of scotland4 and the bushveld complex5 has been attributed to variable contamination or the emplacement of magma batches of differing composition

mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab slab derived fluids play an important role in heat and material transfer in subduction zones dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow nevertheless direct observations of the fluid chemistry and pressure temperature conditions of fluids are few this report describes co2 bearing saline fluid inclusions in spinel harzburgite xenoliths collected from the 1991 pinatubo pumice deposits the fluid inclusions are filled with saline solutions with 5 1 1 0 wt nacl equivalent magnesite crystals co2 bearing vapor bubbles and a talc and or chrysotile layer on the walls the xenoliths contain tremolite amphibole which is stable in temperatures lower than 830 c at the uppermost mantle the pinatubo volcano is located at the volcanic front of the luzon arc associated with subduction of warm oceanic plate the present observation suggests hydration of forearc mantle and the uppermost mantle by slab derived co2 bearing saline fluids dehydration and decarbonation take place and seawater like saline fluids migrate from the subducting slab to the mantle wedge the presence of saline fluids is important because they can dissolve more metals than pure h2o and affect the chemical evolution of the mantle wedge

the intrusion of fluid mechanics into geology during the last few years a number of fluid dynamicists often in collaboration with geologists have been investigating problems suggested by geology and creating the new field of geological fluid mechanics the research has uncovered many new fluid mechanical phenomena and the studies have ranged widely over theoretical contributions laboratory experiments and confirmatory field observations this paper presents an overview of part of the field with a strong emphasis on the novel fluid mechanical concepts the presentation includes discussion of some of the effects of cooling and crystallization and the resulting convective motions the flow of gravity currents at both high and low reynolds number and the erosion of solid retaining walls by hot turbulent flows

chemical modification of enclave magma by post emplacement crystal fractionation diffusion and metasomatism hornblende bearing microgranitoid enclaves from the swifts creek pluton scp se australia display mineralogical and textural variations from their margins to their centers margins are fine grained and display quench textures and are enriched in amphibole biotite and in some cases magnetite relative to their coarser grained centers enclaves of this type and their adjacent granitoid host rocks have been sectioned into 0 3 to 1 cm thick slabs in order to determine the chemical variations associated with these mineralogical changes the fine grained margins are variably enriched in ti al mg fe mg k rb ba nb y sc v ni and the ree up to a factor of three relative to the enclave centers this enrichment is compensated by a depletion of si and zr elements like ca na and sr show less coherent variation from margin to center host rocks in immediate contact with these enclaves are depleted in mg na k rb and ba relative to host rocks that are not exposed to enclaves no one single process can account for all the variations instead we propose that the chemical variations are related to a combination of in situ crystal fractionation of isolated magma globules mass transfer by diffusion and metasomatic exchange

layered intrusions and traffic jams a wide range of explanations has been proposed for the origin of repetitive layering in mafic ultramafic and in per alkaline intrusions here we propose that the interaction of mineral grains that sink and float in the crystallizing magma is an alternative mechanism that can explain many of the features of layered intrusions without the need to invoke extrinsic factors similar to traffic jams on a motorway small perturbations in crystal density develop that impede further ascent or descent of buoyant or heavy minerals respectively these traffic jams separate layers of magma from the rest of the magma chamber the magma in the individual layers further evolves as a largely independent subsystem with gravitational sorting organizing the mineral distribution within each layer layering can develop in the intermediate range between full mineral separation in low viscosity or slowly cooling magma chambers and homogeneous crystallization in high viscosity or fast cooling chambers this self organization mechanism provides a novel explanation for the formation of rhythmic layering in low viscosity magmas for example in the ilimaussaq igneous complex in southwest greenland

kinematic significance of morphological structures generated by mixing of magmas a case study from salina island southern italy morphological features of mixing mingling structures of heterogeneous juveniles from the 13 ka old upper pollara eruption salina island southern italy are studied these heterogeneous rocks result from the mixing between an andesitic and a rhyolitic magma concentration patterns generated by magma mixing are analyzed on digital images and results have been compared with those obtained from numerical simulations of mixing processes coupling chaotic advection and chemical diffusion results show that the upper pollara rocks were produced by mixing 24 49 of andesite and 76 51 of rhyolite multifractal analysis of the mixing structures is performed and the mixing intensity i e the degree of interaction between the two end member magmas has been deduced from the analysis of the singularity spectrum results show that narrower is the multifractal spectrum the higher is the degree of homogenization of magmas reynolds number re during mixing has been estimated from the geometrical analysis of mixing structures by using experimentally defined relationships between shape parameters and re results show that re is between ca 500 and 7000 there is a positive correlation between the estimated initial percentage of mafic magma in the different analyzed samples and re in agreement with the observation that the higher is the percentage of the mafic lower viscosity magma the higher is the turbulence of the mixing system in addition our analysis reveals an unexpected inverse relationship between the calculated re and the degree of magma homogeneity suggesting that energy dissipation may have had a major role in the controlling mixing process because dissipation is inversely proportional to the mixing efficiency results suggest that mixing processes between the andesitic and rhyolitic magmas mainly developed in the conduit it is also suggested that mixing occurred in a shear layer type or pipe type flow

spatial constraints on magma chamber replenishment events from textural observations of cumulates the rum layered intrusion scotland the eastern layered series of the rum layered suite is formed of 16 macro units each comprising a lower peridotite and an overlying feldspar rich layer the local term is allivalite the origin of the peridotite layers is unresolved with two contrasting models the earlier of the two is based on repeated replenishment of an opensystem magma chamber with deposition of fractionated material on the chamber floor the second is based on the early formation of a troctolitic complex which is then repeatedly intruded by sills of replenishing picritic magma to form the peridotite horizons the lack of resolution of this fundamental problem is a consequence of the reliance of previously published studies on field observations i present evidence to show that the clinopyroxene in the allivalites preserves information about the distribution of the last melt to solidify permitting determination of not only the extent of supersolidus textural equilibration but also the sub solidus history of the allivalite horizons comparison of profiles of clinopyroxene plagioclase plagioclase median dihedral angle across allivalite units demonstrates that it is possible to distinguish between those that were intruded by later picrite sills and those adjacent to peridotite horizons formed by replenishment and subsequent deposition of fractionated crystals above the pre existing pile in the region studied only the main peridotite body of unit 9 was intruded into a pre existing allivalitic mush

chasing the garlock a study of tectonic response to vertical axis rotation vertical axis clockwise block rotations in the northeast mojave block are well documented by numerous authors however the effects of these rotations on the crust to the north of the northeast mojave block have remained unexplored in this paper we present a model that results from mapping and geochronology conducted in the north and central owlshead mountains the model suggests that some or all of the transtension and rotation observed in the owlshead mountains results from tectonic response to a combination of clockwise block rotation in the northeast mojave block and basin and range extension the owlshead mountains are effectively an accommodation zone that buffers differential extension between the northeast mojave block and the basin and range in addition our model explores the complex interactions that occur between faults and fault blocks at the junction of the garlock brown mountain and owl lake faults we hypothesize that the bending of the garlock fault by rotation of the northeast mojave block resulted in a misorientation of the garlock that forced the owl lake fault to break in order to accommodate slip on the western garlock fault subsequent sinistral slip on the owl lake fault offset the garlock creating the now possibly inactive mule springs strand of the garlock fault dextral slip on the brown mountain fault then locked the owl lake fault forcing the active leach lake strand of the garlock fault to break

on the na2o content of convergent zone high alumina basalts abstract high alumina basalts in the convergent zone are classified into high medium and low na type the high na high alumina basalts occur in arcs without low alkali tholeiites where younger and warmer oceanic lithospheres are subducting the low na high alumina basalts occur in arcs where on the contrary older and cooler plates are descending and occur also in arcs with extensional tectonics even though younger lithospheres are underthrusting a good correlation is observed between the na2o content of high alumina basalts and the age of the subducting lithosphere this suggests that generation of high alumina basalt magma is controlled by the nature of the descending plate

dynamics of dikes versus cone sheets in volcanic systems igneous sheet intrusions of various shapes such as dikes and cone sheets coexist as parts of complex volcanic plumbing systems likely fed by common sources how they form is fundamental regarding volcanic hazards yet no dynamic model simulates and predicts satisfactorily their diversity here we present scaled laboratory experiments that reproduced dikes and cone sheets under controlled conditions our models show that their formation is governed by a dimensionless ratio 1 which describes the geometry of the magma source and a dynamic dimensionless ratio 2 which compares the viscous stresses in the flowing magma to the host rock strength plotting our experiments against these two numbers results in a phase diagram evidencing a dike and a cone sheet field separated by a sharp transition that fits a power law this result shows that dikes and cone sheets correspond to distinct physical regimes of magma emplacement in the crust for a given host rock strength cone sheets preferentially form when the source is shallow relative to its lateral extent orwhen the magma influx velocity or viscosity is high conversely dikes form when the source is deep compared to its size or when magma influx rate or viscosity is low both dikes and cone sheets may form fromthe same source the shift fromone regime to the other being then controlled by magma dynamics i e different values of 2 the extrapolated empirical dike to cone sheet transition is in good agreement with the occurrence of dikes and cone sheets in various natural volcanic settings

the relative roles of boundary layer fractionation and homogeneous fractionation in cooling basaltic magma chambers abstract in a cooling magma chamber magmatic differentiation can proceed both by fractionation of crystals from the main molten part of the magma body homogeneous fractionation and by mixing of the main magma with fractionated melt derived from low temperature mush zones boundary layer fractionation in this study the relative roles of boundary layer fractionation and homogeneous fractionation in basaltic magma bodies were examined using a thermodynamics based mass balance model model calculations show that boundary layer fractionation cannot be a dominant fractionation mechanism when magma chambers are located at low pressures 2 wt such as arc basalt and the magma chambers are located at depth 100 mpa because the melt derived from mush zones is enriched in alkalis and h 2 o crystallization from the main magma is suppressed by mixing with the mush melt as a consequence of depression of the liquidus temperature therefore homogeneous fractionation is more effectively suppressed in magma chambers in which boundary layer fractionation is more active if magmatic differentiation proceeds primarily by boundary layer fractionation magmas can remain free of crystals for long periods during magmatic evolution

strontium isotopic evidence against magma addition in the upper zone of the bushveld complex abstract sr isotopic data from the main and upper zones of the bushveld complex show that the evolution of the upper zone started with a large influx of magma close to the level of the pyroxenite marker a distinctive orthopyroxenite layer in otherwise relatively uniform gabbronorites whole rock samples which span the complete stratigraphic succession ca 2100 m above this layer fall on a single rb sr isochron 2066 58ma and hence have a common initial ratio of 0 7073 1 this ratio is significantly lower than those of the main zone ca 0 7085 below the level of the pyroxenite marker the entire upper zone crystallized from a mixed magma which was thoroughly blended before crystallization this magma had an isotopic ratio intermediate between that of the main zone and the added magma which had an initial ratio of ca 0 7067 further significant magma additions during crystallization are precluded unless they were of the same isotopic composition as the blended magma which is considered improbable hence the layering and mineralogical diversity of the upper zone was produced by internal processes and not produced by magma influxes during crystallization the lithological compositional and isotopic changes at the pyroxenite marker and the petrological coherence of all rocks above this horizon support the placing of the upper zone boundary at this point in the stratigraphy

paleomagnetic and structural evidence for localized tectonic rotation associated with fault drag in the northeastern mojave desert implications for the late cenozoic tectonic evolution of the eastern california shear zone abstract paleomagnetic data coupled with detailed geological analysis of the southeastern goldstone lake region indicate that lower miocene volcanic and epiclastic rocks of the pink canyon area have been folded faulted and tectonically rotated 28 4 9 0 clockwise about a vertical axis identical rocks lying to the west across the goldstone lake fault east branch have been rotated 9 6 7 4 the coyote canyon fault is locally folded about a vertical axis 25 in a clockwise sense in the pink canyon area timing relationships indicate that rotation is post early miocene regional relationships imply that deformation is late miocene to holocene in age these relationships imply that tectonic rotation is local rather than regional in extent as proposed by some tectonic models the results of this study are generally consistent with the dokka and travis 1 model and the subsequently revised dokka model 2 of strain partitioning in the northeastern mojave desert block

paleomagnetic results from the mogollon datil volcanic field southwestern new mexico and a refined mid tertiary reference pole for north america miocene oligocene rocks of the mogollon datil volcanic field in southwestern new mexico have yielded paleomagnetic data giving 61 site mean unit vgps and a paleomagnetic pole located at 81 9 n 143 6 e a95 4 5 k 17 6 this pole is indistinguishable from other mid tertiary reference poles for north america combining the mogollon datil data with vgp data from other studies of mid tertiary 20 40 ma rocks yields a paleomagnetic pole at 81 5 n 147 3 e n 239 a95 2 4 k 15 4 when divided into normal and reversed polarity subsets the data yield paleomagnetic poles that are statistically different at the 95 confidence level mcfadden and lowes 1981 the paleopole determined from reversely magnetized units located at 78 6 n 138 5 e n 125 a95 3 4 k 14 5 is 6 8 on the far side of the paleopole determined from normally magnetized units positioned at 84 2 n 166 4 e n 114 a95 3 3 k 17 0 when viewed from a common site location i e the western united states similar pole positions an differences are also seen in 10 ma subsets of this data set incomplete removal of a normal overprint on the reversely magnetized units appears unlikely to explain these results rather we suggest that the differences are consistent with the concept that differing harmonics may be associated with the two polarity states over the long term merrill and mcelhinny 1977 merrill et al 1979

zircon inclusions in corundum megacrysts i trace element geochemistry and clues to the origin of corundum megacrysts in alkali basalts abstract zircons enclosed in corundum megacrysts from basalts have a short prismatic habit with the prism slightly more developed than the pyramidal faces the 110 prism is dominant compared with 100 indicating crystallisation from alkaline peralkaline environments rare zircon decomposition reactions suggest the possible presence of a primary sio 2 phase in the source paragenesis for corundum megacrysts the zircon inclusions are geochemically distinctive high y up to 1 2 u uo 2 up to 1 7 wt th tho 2 up to more than 2 wt hf up to 3 4 wt and ree ree up to 5000 ppm they form a cogenetic group that is characterised by extreme ree fractionation with preference for the hree varying from 30 100 chondritic sm to 5000 20000 chondritic lu ree partitioning between zircon and silicate liquid is evaluated the parental melts calculated for the zircon inclusions and other corundum related zircons using zircon melt partition coefficients derived in the present paper show concave down ree patterns with a maximum sm concentration close to 100 chondritic sm abundance such ree patterns represent highly evolved melts after extensive fractionation of low ca feldspar prior to and throughout the crystallisation of zircon the characteristics of zircon inclusions provide evidence that corundum megacrysts must have crystallised from alkaline and highly evolved melts under very reducing conditions implying that the corundum is unlikely to have formed as high pressure phenocrysts in the host mafic magmas

collapse of southwestern north america and the evolution of early miocene detachment faults metamorphic core complexes the sierra nevada orocline and the san andreas fault system plate reconstructions indicate that early miocene pacific north american relative motion was not parallel to the transform but was instead oblique and transtensional we propose that the western edge of the north american plate east of the transform gravitationally collapsed to the southwest in response to this divergence the northern and eastern edges of the collapsed region underwent extension between 24 and 20 ma whereas extension in southeastern california and southern arizona during this interval was coaxial with the overall s50 60 w collapse direction the extension direction along the northern oundary mojave extensional belt was north south and the result of the combined effects of the southwestward collapse and the independent northwestward translation of the region to the north collapse continued between 20 and 16 ma and extension persisted in southeastern california and southern arizona extension along the northern boundary waned owing to the apparent cessation of motion of the region to the north thus the kinematics of the northern boundary changed to dextral shear and extension along what is here named the trans mojave sierran shear zone this east west zone of mainly right shear oroclinally folded the southern tail of the sierra nevada and the central mojave desert rocks within this 83 110 km wide zone were rotated approx 40 60 clockwise about vertical axes finally as collapse moved the north american plate progressively west across the locus of transform shear its western edge became increasingly subject to the motion of the pacific plate over time the edge of north america was truncated by faults of the san andreas system and transferred to the pacific plate

peridotite xenoliths in silica rich potassic latite from the transition zone of the colorado plateau in north central arizona potassic latite in the transition zone of the colorado plateau near chino valley arizona contains abundant eclogite and amphibolite xenoliths and minor websterite and pyroxenite xenoliths one unit contains peridotite xenoliths analyzed samples have mg ratios of 68 and 71 58 and 63 wt sio2 and are enriched in potassium and other large ion lithophile lil elements rare earth element ree patterns are light ree enriched with la greater than 100 times chondritic abundance the peridotite xenoliths are partly to totally altered but contain remnant olivine orthopyroxene and clinopyroxene one harzburgite nodule also contains spinel mineral compositions from the xenoliths are relatively refractory and similar to those in other spinel peridotite xenoliths from the colorado plateau geothermometry on olivine spinel and two pyroxene pairs indicates equilibration temperatures of less than 800 c for the peridotite nodules the relatively low temperatures calculated from mineral equilibria are consistent with temperature estimates for other mantle nodules from under the colorado plateau

information about open system magma chambers derived from textures in magmatic enclaves the kameni islands santorini greece post caldera eruptions of santorini greece over the past 3000 years resulted in the formation of the kameni islands which comprise a series of compositionally similar dacitic lava flows each lava flow has a distinct population of partially crystalline mafic enclaves which we propose were derived from the break up of a layer of replenishing magma responsible for triggering the eruption five of the recent flows erupted in 1570 1939 1940 1941 and 1950 include enclaves of essentially identical andesitic bulk compositions which formed by crystallization of originally aphyric melts prior to eruption detailed examination of angles subtended at the junctions between pairs of plagioclase grains demonstrates that enclaves from each flow have a characteristic textural signature with distinct differences in the extent of quench related modification of the original population of dihedral angles formed by impingement of growing grains these variations suggest that the temperature difference between the host dacite and the replenishing andesite at the time of layer overturn and eruption differed between flows the uniformity of major element compositions of both the replenishing magma and the host dacitic lava flows demonstrates that the critical parameter in determining the timing of layer overturn is the pre eruptive h 2 o content of both host dacite and replenishing magma we suggest that the replenishing magma responsible for two of the three eruptions in the period 1939 1941 was significantly wetter than that responsible for the later 1950 eruption the enclaves with the least amount of quench related modification occur in the 1570 flow we suggest that in this case the intruding magma was relatively dry

peridotite sills and metasomatic gabbros in the eastern layered series of the rhum complex mapping of the eastern layered series els of the rhum ultrabasic complex on the northern flank of hallival shows that peridotite and allivalite troctolite or gabbro layers are laterally discontinuous and vary both in thickness and lithology peridotite generally has sharp upper and lower contacts against the allivalites which sometimes cut across the layering in the allivalite reaction dissolution and hybridization effects between peridotite and allivalite are developed locally some troctolite layers terminate as isolated fingered blocks in peridotite there are many small peridotite bodies which are clearly intrusive into allivalite and have previously been identified as distinct peridotite sheets and plugs they are petrographically almost identical to the major stratiform peridotites and in some cases are apophyses from them we propose that many of the peridotite layers in the els formed as thick sills of picritic magma emplaced into a partly solidified layered troctolite complex the stratiform gabbros of the els are heterogeneous layered rocks that commonly contain relicts of troctolite and anorthosite wavy metre scale contacts between gabbro and troctolite cut across pre existing grain size modal and rhythmic layering with little disruption these metasomatic gabbros mimic the textures grain size and rhythmic layering of their troctolitic protoliths we propose that many of the els gabbros formed as a result of interaction between porous troctolites and a low temperature basaltic melt residual basaltic melt segregated from solidifying peridotite may have caused this metasomatism

simple shear of southern california during neogene time suggested by paleomagnetic declinations we have studied the paleomagnetism of various neogene age rocks in southern california as a means of determining the amounts of oligocene and younger tectonic rotation and translation which has occurred in this region our results suggest that fully 25 of this area in particular the transverse ranges has undergone extreme clockwise rotation work in southeastern california implies that 40 of clockwise rotation has occurred here although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200 the crustal block bounded by the san gabriel and san andreas fault has undergone a net clockwise rotation of 35 although the data here can be interpreted to show that an original early miocene rotation of about 50 was followed by a late miocene or pliocene counterclockwise rotation of 15 paleomagnetic results from the offshore islands suggests that san clemente santa barbara and san nicolas islands have not rotated but santa catalina has undergone about 90 of clockwise rotation all of the northern channel islands including anacapa santa cruz santa rosa and san miguel are implied to have rotated 70 or 80 clockwise this result is also found for the santa monica mountains east of anacapa island in the santa ynez range north of the channel islands paleomagnetic study of the monterey formation also indicates large clockwise rotations of near 90 these data also suggest that the santa maria basin is not rotated and that the north boundary of the rotated region is the santa ynez river fault stratigraphic control on the paleomagnetic data from the monterey formation implies that the rotation began about 16 m y ago and may be continuing today in the western region paleomagnetic inclination data from our study show that the northern channel islands in particular may have translated 15 northward since middle miocene time however equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections non dipole magnetic field behavior and right offset on the san andreas fault system palinspastic reconstruction of southern california regions for the early miocene implies that parts of the transverse ranges structures were once aligned with north trending extensional structures in the southwestern united states we propose that pacific american plate interactions both rifted the continental crust to create this pattern and rotated the western most structures within a dextral simple shear zone which had a half width of about 400 km

size frequency distributions of rocks on mars and earth analog sites implications for future landed missions the size frequency distribution of rocks at the viking landing sites and a variety of rocky locations on the earth that formed from a number of geologic processes all have the general shape of simple exponential curves which have been combined with remote sensing data and models on rock abundance to predict the frequency of boulders potentially hazardous to future mars landers and rovers rock data from the near field of the viking landers where dimensions can be measured accurately in stereo images and estimates from the far field of viking 1 have convex up curved shapes on log log graphs of cumulative frequency per square meter or cumulative fractional area versus diameter the rock data show a sharp drop off at large diameters and a progressive approach to a plateau with decreasing diameter approaching the total rock coverage which can be fit well with simple exponential functions similar shaped size frequency distributions of rocks are found at a wide variety of rocky surfaces on the earth and can be fit well with simple exponential functions this distribution is compatible with fracture and fragmentation theory which provides a physical basis for its wide application a combined fit to rock area data at both viking sites was made with a general exponential function in which the pre exponential is the total area covered by rocks simple linear height versus diameter relationships were also derived from height diameter ratios at the viking sites which suggest that rockier areas on mars have higher standing rocks than less rocky areas height was then substituted into the general exponential function derived for diameter yielding the cumulative fractional area of rocks versus height for any given total rock coverage on mars results indicate that most of mars is rather benign with regard to hazards from landing on large rocks for total rock coverage of 8 equivalent to modal rock coverage on mars and the viking 1 site without the outcrops about 1 of the surface is covered by 20 cm or higher rocks a lander designed to accommodate landing on 0 5 m high boulders such as the mars pathfinder airbag system could land on a surface covered by about 20 rocks similar to the viking 2 site which is rockier than 95 of the planet with 1 of the surface covered by rocks of 0 5 m or higher

life time of a stratified magma chamber recorded in ultramafic xenoliths from ichinomegata volcano northeastern japan variations in major element chemistry and modal compositions of the mantle xenolith bearing calc alkalic ejecta from ichinomegata volcano are inferred to be due to mixing of three magmatic end members basalt i sio2 51 wt mgo 8 5 wt basalt ii sio2 54 wt mgo 5 wt and dacite sio2 65 wt mgo2 wt ultramafic xenoliths are found in mafic mixtures of dacite basalt i and dacite basalt ii the thermal histories of the xenoliths in both mixtures are compared with each other chemical compositions of olivine and orthopyroxene in xenoliths suggest that xenoliths in basalt i were equilibrated at about 800 c while those in basalt ii were also equilibrated originally at about 800 c but were subsequently annealed at about 1000 c for more than 102 3 years prior to the eruption