prolonged magmatism juvenile nature and tectonic evolution of the chinese altai nw china evidence from zircon u pb and hf isotopic study of paleozoic granitoids abstract paleozoic granitoid magmatism played an important role in the tectonic evolution of the chinese altai and zircon u pb and hf isotopic compositions have been determined for samples from eleven granitic plutons batholiths the jiadengyu gneissic granitic pluton yielded a zircon u pb age of 479 ma and thus it does not represent precambrian basement as suggested previously our results and published data demonstrate that voluminous granitoids were continuously emplaced over more than 30 area of the chinese altai during the period from 447 ma to 368 ma with a climax at ca 400 ma ages for zircon overgrowth rims demonstrate additional thermal events at ca 360 and 280 ma respectively positive e hf t values 0 to 9 of normal magmatic zircons suggest that the granitoid magmas were derived from juvenile sources xenocrystic zircon cores are 543 421 ma old and also give positive e hf t values 2 5 to 12 suggesting their origin as early crystallized minerals in the magma chambers or as inherited cores from newly accreted meterials the strong magmatism at ca 400 ma significantly changed the hf isotopic composition of the magma source by substantial input of juvenile material in a relatively short period geophysical geological and geochemical data support that ridge subduction was a possible mechanism for the strong magmatism ca 400 ma and the above mentioned change of hf isotopic composition in the magma source

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

petrogenetic and tectonic inferences from the study of the mt cer pluton west serbia the mt cer pluton serbia is a complex laccolith like intrusion 60 km 2 situated along the junction between the southern pannonian basin and northern dinarides it intrudes palaeozoic metamorphic rocks causing weak to strong thermal effects based on modal and chemical compositions four rock types can be distinguished 1 metaluminous i type quartz monzonite quartz monzodiorite qmzd 2 peraluminous s type two mica granite tmg which intrudes qmzd 3 stra anica granodiorite quartz monzonite gds and 4 isolated mafic enclaves me found only in qmzd 40 k 39 ar dating and geological constraints indicate that the main quartz monzonite quartz monzodiorite body of mt cer was emplaced not later than 21 ma whereas the emplacement ages of the stra anica granodiorite quartz monzonite and two mica granites are estimated at around 18 and 16 ma respectively the mt cer pluton is similar to the mt bukulja pluton some 80 km southwestwards genesis of qmzd cannot be interpreted by fractional crystallization coupled with mixing or assimilation it is best explained by a convection diffusion process between mantle derived minette leucominette magmas and gds like magmas followed by two end member magma mixing the composition of gds rocks suggests that gds like magmas could have formed by melting of lower crustal lithologies similar to amphibolite metabasalts the geochemistry of tmg is reproduced by an assimilation fractional crystallization model with a ratio of rate of assimilation to rate of fractional crystallization of 0 4 using the compositions of the least evolved tmg of the bukulja pluton and adjacent metamorphic rocks as proxies for the parental magma and contaminant respectively the origin and evolution of the mt cer and adjacent mt bukulja plutons provide new constraints on the tertiary geodynamics of the northern dinarides southern pannonian region the quartz monzonite quartz monzodiorite is interpreted as a result of the oligocene post collisional dinaride orogen collapse which included a limited lithosphere delamination small scale mantle upwelling and melting of the lower crust by contrast the two mica granite magmas formed through melting in shallower crustal levels during the extensional collapse in the pannonian area

a case of simple magma mixing in the columbia river basalt group the wilbur creek lapwai and asotin flows saddle mountains formation three small sequential saddle mountains flows occupying similar areas on the columbia plateau were erupted over a short interval of time in the lewiston basin area the middle flow of the trio lapwai has intermediate mineralogy and lies on a straight mixing line between the other two flows for virtually all twenty five elements analyzed systematic changes in the ratios of incompatible elements demonstrate that these relationships are a result of magma mixing rather than either crystal fractionation or variable degrees of partial melting the lapwai flow has a composition approximately midway between the two homogeneous end members and is itself relatively homogeneous this implies efficient mixing between equal amounts of asotin and wilbur creek magmas and suggests that mixing was completed in a magma reservoir prior to eruption the wilbur creek and asotin end members have isotopic features which are believed to result from different degrees of assimilation of crustal material by magma derived from an enriched mantle source carlson 1984 the mixing processes described here cannot be related to that earlier mantle crust mixing process

damped regional scale stress inversions methodology and examples for southern california and the coalinga aftershock sequence 1 we present a new focal mechanism stress inversion technique to produce regional scale models of stress orientation containing the minimum complexity necessary to fit the data current practice is to divide a region into small subareas and to independently fit a stress tensor to the focal mechanisms of each subarea this procedure may lead to apparent spatial variability that is actually an artifact of overfitting noisy data or nonuniquely fitting data that does not completely constrain the stress tensor to remove these artifacts while retaining any stress variations that are strongly required by the data we devise a damped inversion method to simultaneously invert for stress in all subareas while minimizing the difference in stress between adjacent subareas this method is conceptually similar to other geophysical inverse techniques that incorporate damping such as seismic tomography in checkerboard tests the damped inversion removes the stress rotation artifacts exhibited by an undamped inversion while resolving sharper true stress rotations than a simple smoothed model or a moving window inversion we show an example of a spatially damped stress field for southern california the methodology can also be used to study temporal stress changes and an example for the coalinga california aftershock sequence is shown we recommend use of the damped inversion technique for any study examining spatial or temporal variations in the stress field

petrogenesis and magmatic hydrothermal evolution time limitation of kelumute no 112 pegmatite in altay northwestern china evidence from zircon upb and hf isotopes abstract granitic pegmatites and associated rare metal deposits unique features in the altay orogen are located in a key tectonic position of the southern central asian orogenic belt caob and document the tectonic evolution of the paleo asian ocean the kelumute no 112 pegmatite which intruded into the jideke two mica granites hosts abundant rare metal e g li be nb ta ore deposits and ranks only second to the koktokay no 3 pegmatite in both size and reserves to explore the evolution time limitation of magmatic magmatic hydrothermal and hydrothermal stages in the pegmatite magma system the tectonic setting and the genetic relationship between pegmatites and granites this study presents zircon u pb and hf isotopic compositions of the kelumute no 112 pegmatite and the wall rocks two mica granite and biotite granite as determined by la icpms and la mc icpms the weighted mean 206 pb 238 u ages of the internal textural zones of the pegmatite including zones i ii iii v and vi are 238 3 2 0 ma 233 5 3 7 ma 188 3 1 7 ma 218 8 1 9 ma and 210 7 1 6 ma respectively wall rocks of two mica granite and biotite granite are dated at 445 6 4 3 ma and 455 6 5 4 ma respectively zircons from the kelumute no 112 pegmatite have lower positive e hf t values 0 03 to 2 35 with t dm model ages of 1112 1225 ma wall rocks show similar zircon e hf t values 1 41 to 4 13 and t dm model ages 1172 to 1515 ma however three xenocrystic zircons from the granites are characterized by larger negative ehf t values 5 85 to 9 83 and older t dm model ages 1839 to 2090 ma thus the following conclusions can be drawn 1 the kelumute no 112 pegmatite and its wall rocks jideke two mica granite and biotite granite have no genetic relationship as indicated by the large gaps between their formation ages however they did originate from a common source composed of ancient crust and mantle derived materials under two different tectonic settings 2 the magmatic magmatic hydrothermal transition and hydrothermal stages of the no 112 pegmatite magma lasted for 5 ma 23 ma and 22 ma respectively and 3 the no 112 pegmatite magma was most likely formed in a post collision tectonic setting indicating that block amalgamation and collisional orogeny of the caob continued into the triassic

crystallization and hybridism in microgranitoid enclave magmas microstructural evidence microstructural evidence indicates that many microgranitoid enclaves mafic inclusions in granitoids crystallize from globules of magma that have undergone previous magma mixing hybridism the evidence includes 1 rounded xenocrysts ocelli of quartz rimmed with fine grained aggregates of early formed minerals especially mafic minerals 2 k feldspar megacrysts commonly rimmed with plagioclase rapakivi structure and 3 corroded cores dendritic overgrowths and calcic spikes in plagioclase initial quenching of a magma globule in felsic magma involves rapid temperature equilibration which causes a high rate of nucleation of minerals precipitating from the enclave magma at the time followed by a period of slow cooling during which the nucleation and growth rates of later crystallizing minerals are similar to the much slower rates prevailing in the granitic melt this may result in markedly poikilitic microstructures involving inclusion of the earlier formed finer grained minerals in quartz and or k feldspar which are common in microgranitoid enclaves

compression directions in southern california from santa barbara to los angeles basin obtained from borehole breakouts borehole elongation in 71 drill holes was used to infer breakout orientation and directions of maximum horizontal principal stress s h for six areas west of the san andreas fault in southern california santa barbara ojai central ventura basin east ventura basin west los angeles basin and east los angeles basin breakouts were determined from analysis of oriented four arm caliper data the breakouts form at the position of the maximum compressive stress on the borehole wall if the borehole is vertical and parallel to one of the principal stress directions the breakouts will form parallel to the minimum horizontal principal stress s h orthogonal to the maximum horizontal principal stress s h observations from deviated boreholes permit some constraints on the relative magnitudes of the principal stresses in most cases the data permit either a thrust faulting s v s h s h or strike slip faulting s h s v s h stress regime with ne to nw directions of s h these results are broadly consistent with results from focal mechanism studies hauksson 1990 li 1996 and with breakout and focal mechanism data present in the world stress map database zoback 1992 however we find systematic variations in s h directions suggestive of strong heterogeneity in the stress field at shallow depths similar to that present in the cajon pass borehole shamir and zoback 1992 anomalous nw directions of s h in the san fernando valley region and near the whittier fault may be related to structural complexities and or lateral ramps in nearby fault systems

crustal evolution of the hercynian belt of western europe evidence from lower crustal granulitic xenoliths french massif central abstract granulite facies lower crustal xenoliths scavenged by the tertiary basaltic pipes of bournac and roche pointue massif central france have been analysed for major and trace elements and radiogenic sr nd isotope ratios they display wide variations in major elements rare earth elements and isotopes 87 sr 86 sr 0 7041 0 7216 nd 11 9 to 1 5 which tend to correlate with the three different rock types encountered basic and acidic meta igneous and metasediments the basic liquids are of calc alkaline affinity and cumulates are abundant there is a gap in silica content between the basic and acid meta igneous xenoliths the isotopic compositions suggest mixing of old pre existing continental crust represented by the metasediments and mantle derived basic melts which represent juvenile addition to the crust during the hercynian orogeny the basic meta igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid producing the hercynian granitoids which have nd and sr initial ratios identical to those of the lower crustal xenoliths the residue from melting may be represented by the acid meta igneous rocks some of which have positive eu anomalies these results indicate that an important basaltic underplating event occurred during the evolution of hercynian orogenic belt although the exact timing of the underplating requires further constraint

new precise timing constraint for the keketuohai no 3 pegmatite in xinjiang china and identification of its parental pluton abstract the keketuohai pegmatite field is one of the pegmatite rich regions in the altay orogen with abundant pegmatites and voluminous granite magmatism the keketuohai no 3 pegmatite is renowned for its deposits of rare metals but despite a lot of in depth research the geochronology of the pegmatite remains the subject of dispute and there is a lack of precise geochronological evidence to support the view that the aral granite represents the parental magma of the pegmatite to further our understanding of this pegmatite field one sample was collected from the aral granite in the northern part of the ore district and six molybdenite samples were taken from the marginal zone of the keketuohai no 3 pegmatite the geological context of the molybdenite samples indicates an origin by hydrothermal metasomatism in the zone between the margin of the pegmatite and the wall rock and the time of its formation should coincide with or immediately follow the formation of the pegmatite margin this allows the lower time limit of the pegmatite formation to be set la mc icp ms dating of zircons from the aral granite gives 206 pb 238 u ages that range from 209 0 to 215 6 ma from 16 spot analyses and the plotted data are mainly distributed on or around the concordant line the weighted mean age is 211 4 0 8 ma mswd 1 4 which may represent the crystallization age of the granite the re os isotopic model ages for the six molybdenite samples range from 207 8 to 212 2 ma the weighted mean age of 209 9 1 3 ma mswd 0 69 and the re os isochron age of 208 8 2 4 ma mswd 0 89 are consistent within error if decoupling of the re os isotope system in molybdenite is neglected this consistency in the age data may indicate the time at which molybdenite was deposited from fluids after researching in detail the previously published chronological data we propose that earlier views on the ages of the keketuohai no 3 pegmatite and the aral granite may be incorrect and we now suggest that the pegmatite started to form around 209 210 ma in the late triassic the agreement between the newly determined u pb ages for the aral granite and the re os ages of the molybdenite in the pegmatite strongly indicates a close relationship between the crystallization of the aral granite and the formation of the pegmatite and its rare metals deposits it also points to the aral granite being the parent of the no 3 pegmatite

crystallization of accessory phases in magmas by local saturation adjacent to phenocrysts abstract accessory minerals commonly occur attached to or included in the major crystalline phases of felsic and some intermediate igneous rocks apatite is particularly common as inclusions but fe ti oxides pyrrhotite zircon monazite chevkinite and xenotime are also known from silicic rocks accessories may nucleate near the host crystal liquid interface as a result of local saturation owing to formation of a differentiated chemical boundary layer in which accessory mineral solubility would be lower than in the surrounding liquid differentiation of this boundary layer would be greatest adjacent to ferromagnesian phenocrysts especially fe ti oxides it is with oxides that accessories are most commonly associated in rocks a boundary layer may develop if the crystal grows more rapidly than diffusion can transport incorporated and rejected elements to and from the phenocryst diffusion must dominate over convection as a mode of mass transfer near the advancing crystal liquid interface in order for a boundary layer to exist accumulation of essential structural constituent elements of accessory minerals owing to their slow diffusion in evolved silicate melt also may force local saturation but this is not a process that applies to all cases local saturation is an attractive mechanism for enhancing fractionation during crystallization differentiation if accessory minerals attached to or included in phenocrysts formed because of local saturation their host phenocrysts must have grown rapidly when accessories nucleated in comparison to lifetimes of magma reservoirs some inconsistencies remain in a local saturation origin for accessory phases that cannot be evaluated without additional information

gis based hazard mapping and zonation of debris flows in xiaojiang basin southwestern china debris flow sites were identified at 140 locations in the xiaojiang basin in yunnan province southwestern china their spatial distribution and catchment characteristics are described in detail on the basis of previous research air photo interpretation field investigation and mapping using geographical information systems gis using a statistical approach a quantitative model of hazards assessment and zonation was developed through synthesis analysis of basin areas gradients and the relative reliefs of these debris flow sites in terms of debris flow hazard assessment areas within the xiaojiang basin can be classified as severe heavy and light hazard regions

spatial heterogeneity of tectonic stress and friction in the crust the complex geometry of faults seismicity and diversity of earthquake mechanisms suggest that the stress and strength in earth s crust are spatially heterogeneous we investigated the degree of heterogeneity using the following two end member models in one end member model we assumed that the orientation of stress is uniform in the crust as is assumed in many stress inversion studies in this model the variability of earthquake mechanisms means that friction during faulting must vary for each event we computed friction from the ratio of the resolved shear stress to the effective normal stress on the fault plane with the assumption of hydrostatic pore pressure the values of vary over a large range from 0 to 1 5 in the other extreme model we assumed optimally oriented slip and a constant 0 6 as is suggested by byerlee s law for all the earthquakes and determined the local stress orientation for each earthquake the orientation of the stress changes drastically from one earthquake to another and the assumption of uniform stress field commonly used in stress inversion is not warranted an important conclusion is that a regionally uniform stress field and constant friction on optimally oriented faults are mutually exclusive the actual situation in the crust is most likely to be intermediate between these two end member models from the existing data alone we cannot determine the degree of heterogeneity uniquely but both and the local stress field near earthquake faults are likely to vary substantially and studies on earthquake rupture dynamics must take these heterogeneities into consideration

the generation of a stratiform skarn and volcanic exhalative pb zn deposit sawusi in the southern chinese altay mountains the constraints from petrography mineral assemblage and chemistry abstract numerous pb zn and fe ore deposits occur in the volcanic sedimentary basins along the southern chinese altay mountains the mineralization of these deposits is hosted by early middle devonian volcanics and sedimentary rocks and is directly associated with skarns the genesis of these deposits is uncertain or hotly debated this study considers the sawusi pb zn deposit from the southern chinese altay the host rock of the sawusi deposit is rhyolitic crystal tuff with intercalated bands of fe mn calcic carbonates and a small amount of rhyolite within the upper section of the early devonian kangbutiebao formation on the basis of a field investigation the petrographic features of the skarn were described detailed electron microprobe analysis was performed on the skarn minerals and the o h and c isotopic compositions of quartz and its fluid inclusions were determined petrographic observation revealed a progressively intensifying alteration from the tuff and its intercalated bands to the stratiform skarn characterized by alternate bands of dark garnet biotite skarn and light grunerite biotite skarn mineral chemical analysis shows that the stratiform skarn of the sawusi deposit consists mainly of spessartine almandine mn rich grunerite biotite and fe mn calcic carbonate minerals a mass balance consideration of the cations in the skarn minerals suggests that the stratiform skarn formed through dimetasomatism between the tuff and intercalated bands of sedimentary fe mn calcic carbonates however the scale of the metasomatism is limited to a few centimeters laser 40 ar 39 ar dating was performed on the skarn grunerite yielding an isochron age of 412 15 ma the 40 ar 39 ar age is identical within analytical error to the zircon u pb age 401 2 7 ma of the rhyolite from the kangbutiebao formation that hosts the skarn and pb zn mineralization and the age 401 3 1 ma of the biesisala porphyry sawusi deposit the o h and c isotopic compositions show that the ore forming fluid includes two components one is h 2 o co 2 which has volatilized from carbonate and dehydrated from tuff and the other is meteoric derived surface water carrying minor meteoric co 2 or organic matter derived co 2 therefore the stratiform skarn and pb zn mineralization of the sawusi deposit are quite different from the contact metasomatic skarn and related deposits and suggest a volcanic exhalation sedimentation origin instead this study has broad relevance to the pb zn and fe deposits along the southern altay mountains in the areas of ore genesis study and ore prospecting

la icp ms u pb zircon columbite tantalite and 40ar 39ar muscovite age constraints for the rare element pegmatite dykes in the altai orogenic belt nw china the chinese altai is renowned for its rich rare element resources nine representative rare element rel pegmatites were dated using la icp ms and 40ar 39ar methods the columbite grains yield a weighted mean 206pb 238u age of 239 6 3 8 ma for the dakalasu be nb ta pegmatite and concordia u pb ages of 258 1 3 1 ma and 262 3 2 5 ma for the xiaokalasu li nb ta pegmatite the zircons display a weighted mean 206pb 238u age of 198 5 2 5 ma for the husite be pegmatite and concordia u pb ages of 194 3 1 6 ma and 248 2 2 2 ma for the qunkuer be and taerlang barren pegmatites the muscovite 40ar 39ar dating gives plateau ages of 286 4 1 6 ma 297 0 2 6 ma 265 2 1 5 ma 178 8 1 0 ma 162 2 0 9 ma 237 7 1 3 ma 237 4 1 2 ma and 231 9 1 2 ma for the talate li be nb ta baicheng nb ta kangmunagong barren husite be qunkuer be nb ta xiaokalasu li nb ta weizigou be and taerlang barren pegmatites respectively these new ages coupled with previous geochronological work suggest that the rel pegmatites in the chinese altai formed during early permain late jurassic time the rel pegmatites located in the central altaishan terrane are younger than those in the qiongkuer abagong terrane showing a correlation with the coeval and adjacent granites the formation of the rel pegmatites and these granites indicates frequent and strong magmatic activity in the post orogenic and anorogenic setting the spatial and temporal distribution of pegmatites and granites reveals a magmatism path from the se of age early middle permian to the nw middle permian middle triassic and finally to the central part middle triassic jurassic of the chinese altai

microgranitoid enclaves in granites globules of hybrid magma quenched in a plutonic environment microgranitoid enclaves autoliths cognate xenoliths mafic inclusions are common in high level granitoid plutons they have relatively fine grained igneous microgranitoid microstructures1 and many show evidence of having flowed in a magmatic condition2 these features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3 5 or restite6 11 an alternative interpretation is that the enclaves represent globules of mafic magma that have mingled commingled and quenched in the granitoid host magma12 16 a detailed review of the literature occurrence morphology and composition of micro granitoid enclaves strongly supports this view hut indicates that the enclave magmas range in composition from mafic to felsic and that magma mixing may be involved in their formation17 i summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid

stress rotation during the coalinga aftershock sequence this study considers spatial and temporal changes of the stress regime during the 1983 coalinga aftershock sequence in both cases the observed changes manifest themselves as rotations of the most compressive stress axis inversion of 165 m 3 aftershocks shows that in the southern aftershock zone the azimuth of the most compressional stress axis is oriented 24 e of north while in the northern and central zones it is oriented 51 e of north statistical analysis by use of non parametric bootstrap resampling shows that this rotation is significant at above the 95 confidence level inverting the focal mechanisms of 122 m 3 aftershocks in the central and northern aftershock zones reveals a rotation of the principal stress axes during the first 500 days after the main shock the compressional axis rotates from an azimuth of 62 e of north to 47 e of north statistical analysis by use of nonparametric bootstrap resampling shows that this rotation is just significantly different from zero at the 95 confidence level subject to the assumption that 80 of the fault planes can be correctly picked from the two nodal planes as the number of fault planes picked correctly varies from 50 random choices to 100 the significance of the rotation varies from 87 to 98 further investigation fails to find the signature of the stress changes predicted by dislocation theory in the focal mechanisms of the aftershocks active folds in the area suggest that the pre main shock stress regime had a compressional axis with a ne sw orientation thus the rotation during the aftershock sequence is thought to be a rebound from a stress anomaly induced by the main shock this stress anomaly may be causally related to the occurrence of the aftershocks

rayleigh fractionation with continuous removal of liquid abstract in its simple form the rayleigh surface equilibrium fractionation equation is only valid for a closed system in which solid separates from liquid and no liquid is lost many natural systems particularly magma chambers lose liquid continuously as they fractionate here it is shown that evolution in such a system can if loss of liquid proceeds in a constant ratio to precipitation of solid be modelled by a modified rayleigh equation where the solid liquid distribution coefficient d is replaced by d q d 1 q 1 where q is the ratio of solid precipitated to liquid extracted this can be visualised as treating the liquid removed as an additional phase extracted during fractionation for which d 1 the effect of this is particularly important for incompatible elements where as d 0 d 1 q 1 so that when q 1 the most incompatible element gives d 0 5

dating the lower crust by ion microprobe ion microprobe uthpb ages of zircons from granulite facies lower crustal xenoliths from north queensland australia correlate well with the ages of major orogenic episodes manifest at the earth s surface about half of the xenoliths contain proterozoic zircons which are similar in age to the episodes of high grade metamorphism of the older surface rocks all the xenoliths contain late paleozoic zircons which show a real 100 ma range in206pb238 u ages from 320 to 220 ma which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust the paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash flows and intrusion of high level granites in this region 320 270 ma mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths and coupled with the age information can be used to infer the geological processes operating in the lower crust the zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two stage history for these rocks involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt some of the zircons from the felsic xenoliths contain co2 rich fluid inclusions indicating that those zircons grew during high grade metamorphism the isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic mantle derived lithologies the major orogenic event responsible for the formation of that crust occurred in the late paleozoic when proterozoic supracrustal rocks were emplaced into the lower crust possibly along thin skinned thrust slices this was accompanied by intrusion of high temperature mantle derived melts which caused partial melting of pre existing crust the most likely setting for such tectonism is a continental margin subduction zone

constraining fault constitutive behavior with slip and stress heterogeneity 1 we study how enforcing self consistency in the statistical properties of the preshear and postshear stress on a fault can be used to constrain fault constitutive behavior beyond that required to produce a desired spatial and temporal evolution of slip in a single event we explore features of rupture dynamics that 1 lead to slip heterogeneity in earthquake ruptures and 2 maintain these conditions following rupture so that the stress field is compatible with the generation of aftershocks and facilitates heterogeneous slip in subsequent events our three dimensional finite element simulations of magnitude 7 events on a vertical planar strike slip fault show that the conditions that lead to slip heterogeneity remain in place after large events when the dynamic stress drop initial shear stress and breakdown work fracture energy are spatially heterogeneous in these models the breakdown work is on the order of mj m 2 which is comparable to the radiated energy these conditions producing slip heterogeneity also tend to produce narrower slip pulses independent of a slip rate dependence in the fault constitutive model an alternative mechanism for generating these confined slip pulses appears to be fault constitutive models that have a stronger rate dependence which also makes them difficult to implement in numerical models we hypothesize that self consistent ruptures could also be produced by very narrow slip pulses propagating in a self sustaining heterogeneous stress field with breakdown work comparable to fracture energy estimates of kj m 2

magma mixing in granitic rocks of the central sierra nevada california abstract the el capitan alaskite exposed in the north american wall yosemite national park was intruded by two sets of mafic dikes that interacted thermally and chemically with the host alaskite comparisons of petrographic and compositional data for these dikes and alaskite with published data for sierra nevada plutons lead us to suggest that mafic magmas were important in the generation of the sierra nevada batholith specifically we conclude that 1 intrusion of mafic magmas in the lower crust caused partial melting and generation of alaskite rhyolitic magmas 2 interaction between the mafic and felsic magmas lead to the observed linear variation diagrams for major elements 3 most mafic inclusions in sierra nevada plutons represent chilled pillows of mafic magmas related by fractional crystallization and granitoid assimilation that dissolve into their felsic host and contaminate it to intermediate granodioritic compositions 4 vesiculation of hydrous mafic magma upon chilling may allow buoyant mafic inclusions and their disaggregation products to collect beneath a pluton s domed ceiling causing the zoning mafic margins to felsic core that these plutons exhibit

constraining earthquake source inversions with gps data 1 resolution based removal of artifacts 1 we present a resolution analysis of an inversion of gps data from the 2004 mw 6 0 parkfield earthquake this earthquake was recorded at thirteen 1 hz gps receivers which provides for a truly coseismic data set that can be used to infer the static slip field we find that the resolution of our inverted slip model is poor at depth and near the edges of the modeled fault plane that are far from gps receivers the spatial heterogeneity of the model resolution in the static field inversion leads to artifacts in poorly resolved areas of the fault plane these artifacts look qualitatively similar to asperities commonly seen in the final slip models of earthquake source inversions but in this inversion they are caused by a surplus of free parameters the location of the artifacts depends on the station geometry and the assumed velocity structure we demonstrate that a nonuniform gridding of model parameters on the fault can remove these artifacts from the inversion we generate a nonuniform grid with a grid spacing that matches the local resolution length on the fault and show that it outperforms uniform grids which either generate spurious structure in poorly resolved regions or lose recoverable information in well resolved areas of the fault in a synthetic test the nonuniform grid correctly averages slip in poorly resolved areas of the fault while recovering small scale structure near the surface finally we present an inversion of the parkfield gps data set on the nonuniform grid and analyze the errors in the final model

the nd and sr isotopic composition of i type microgranitoid enclaves and their host rocks from the swifts creek pluton southeast australia abstract initial nd and sr isotopic compositions have been determined for microgranitoid enclaves and their host rocks from the siluro devonian swifts creek pluton within the lachlan fold belt of southeastern australia enclaves are of quartz dioritic to tonalitic composition and their textures support an igneous origin host rocks range from tonalite to granite the isotopic data form an array in nd 87 sr 86 sr space with enclaves systematically displaced towards higher nd and lower 87 sr 86 sr values enclaves range in nd t 405 ma from 7 3 to 3 1 with a narrow range in 87 sr 86 sr from 0 70613 to 0 70718 whereas host rocks range from 9 0 to 6 4 and 0 70761 to 0 70946 in nd and 87 sr 86 sr respectively the isotopic relations rule out simple crystal liquid fractionation or restite unmixing alone as the major genetic link between enclaves and host rocks the data are also incompatible with an origin of the enclaves by crystal accumulation or liquid immiscibility instead mixing between a mafic mantle derived and a crustal derived end member to generate the host rock enclave spectrum is compatible with the data because the initial isotopic ratios of enclaves and host rocks are well within the range of crustal rocks we favour the alternative view that both enclave and host rock magmas are of crustal origin consequently we avoid extrapolations to hypothetical end members we suggest that the enclaves from the swifts creek pluton are derived by partial melting of a mafic lower crust and mixed at or near the site of magma generation with a chemically less primitive and isotopically more evolved component to generate the enclave and host rock spectrum the isotopic heterogeneities within the enclave and host rocks survived the superimposed effects of crystal liquid fractionation model ages tdm for enclaves and host rocks are almost identical averaging 1 7 and 1 6 ga respectively these model ages indicate that both the enclave and host rock protoliths were derived from a mantle reservoir during the proterozoic

complex trace element effects of mixing fractional crystallization composite processes applications to the alaer granite pluton altay mountains xinjiang northwestern china abstract complex patterns of intersection of linear with power function curvilinear data point arrays are observed on ratio ratio correlation plots of trace elements in the alaer palaeozoic granite pluton altay mts xinjiang northwestern china data points in the linear array are relatively lower in sio 2 content and differentiation index d i and they become progressively decreased rightwards along the array in abundances of zr and hf which could be better explained by mixing between zircon rich restite and the granite melt data points in curvilinear arrays are higher in sio 2 content and d i value and therefore have presumably suffered more extensive fractional crystallization the complex patterns cannot be resolved with the classical models two models for the complex trace element effects of mixing fractional crystallization are developed in the paper with which the patterns can be resolved in all models we begin with a linear data array formed by simple two component mixing and then superpose fractional crystallization trends to generate a family of curves model 1 d 1 d 3 d 2 d 3 or d 1 d 3 d 2 d 3 in the general situation fractional crystallization curves assume a cluster of concave up or convex up power function curves which intersect the initial mixing line sloping positively special cases 1 3 d 1 d 2 d 3 or d 1 d 2 d 3 or d 2 d 1 d 3 the fractional crystallization curve is transformed into a straight line special case 4 d 1 d 2 d 3 the fractional crystallization line coincides with the initial mixing line model 2 d 1 d 3 d 2 or d 1 d 3 d 2 fractional crystallization curves assume the form of hyperbolic curves which intersect the initial mixing line sloping negatively in all of the two models the iso fraction lines are uniformly steeper d 1 d 2 or more gentle d 1 d 2 than the initial mixing line iso fraction lines increase d 1 d 2 or decrease d 1 d 2 progressively in slope with the decrease of residual liquid mass fraction f through modelling of the data point arrays on ratio ratio correlation plots of trace elements one can discern and distinguish between simple mixing pure fractional crystallization and mixing fractional crystallization composite processes initial compositions of the magma and the mixing modes can also be constrained through modelling the functional form concavity or convexity and the power index of the fractional crystallization curves the relative incompatibility of trace elements can be estimated semi quantitatively and the incompatibility sequence can be established it is demonstrated through modelling that the alaer granite pluton experienced mixing fractional crystallization composite processes the mixing mode is probably described by the restite model the incompatibility sequence of the 7 concerned trace elements is d th d la d y d nb d ta d hf d zr this incompatibility sequence may be applicable to the anatectic granites

nd and sr isotopic compositions of lower crustal xenoliths from north queensland australia implications for nd model ages and crustal growth processes abstract sr and nd isotopic analyses of granulite facies lower crustal xenoliths from northern queensland australia fall on a mixing array on a 87 sr 86 sr vs 143 nd 144 nd diagram at 300 ma this was a time of intense orogeny in northern queensland attended by granitic magmatism in the upper crust and granulite facies metamorphism in the lower crust ion microprobe zircon dating indicates that most of the xenoliths formed at this time the sr and nd isotopic compositions of these rocks are therefore interpreted to have resulted from large scale mixing in the lower crust between mantle derived basaltic melts and isotopically evolved early proterozoic continental crust both end members are represented in the xenolith suite depleted mantle nd model age tdm ages for meta igneous granulite xenoliths having appropriate 147 sm 144 nd i e between 0 12 and 0 15 range from 1 9 to 2 2 ga for mafic melt like xenoliths and 1 2 to 1 4 ga for felsic melt like xenoliths however zircon ages on these same samples show that their protoliths crystallized during the permo triassic orogeny thus the old tdm ages for the mafic xenoliths do not accurately reflect their crustal residence ages possibly due to crustal assimilation or their derivation from an enriched mantle source this is a common feature of mafic granulite xenoliths the world over and highlights the difficulty in determining the age of mafic underplates hence the timing of crustal growth from tdm ages because the felsic melt like xenoliths lie on the mixing array their mid proteozoic tdm ages are likely to have resulted from mixing and therefore represent an average crustal residence age the combined data suggest that significant crustal growth and reworking occurs in the lower crust during phanerozoic crustal orogenies

contribution of enclave studies to the understanding of origin and evolution of granitic magmas enclaves give important information on the origin and evolution of granitic magmas the presence in a granite of surmicaceous enclaves restites is good evidence of continental crust contribution to magma genesis the presence of dark microgranular enclaves is an indication of mantle contribution the joint presence in the same granite of the two types of enclaves indicates that its magma has probably more than one source dark microgranular enclaves and their host granites as well are hybridized rocks resulting from incomplete mixing or mingling of more acidic and basic components enclave studies allow one to specify mixing conditions when mixing occurs relatively early with respect to granite crystallization only earlier formed granitic minerals zircon apatite may be incorporated in the basic magma when mixing occurs later all granitic minerals may be involved when all the dark microgranular enclaves of a granite are similar the intervention of a single mixing event can be assumed on the contrary when several chemically and mineralogically contrasted varieties occur several mixing events can be assumed the order of these events may be deduced from the nature of the granitic minerals present in the enclaves early of late minerals or sometimes by observation of double enclaves enclave shapes bring information on the distance between the place of mixing and the place of observation enclaves with irregular or crenulated margins or with chilled margins as well always occur near mixing places the enclave transport in the magma makes these characteristics disappear in spite of hybridization processes the primitive composition of the basic component of the mixture may sometimes be recognized a systematic identification of this component in the different granites from the same orogen should allow to understand the space and time evolution of basic magmatism in an orogen magma mixing is a cause of heterogeneity in granites but it is also an important factor of homogenization as the thermal contribution of the basic magma produces new mingling of the components and sometimes new intrusive processes

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

shape and microstructure of microgranitoid enclaves indicators of magma mingling and flow abstract at tarandore point 328 km south of sydney new south wales australia two tonalite phases are in contact with a gabbroic diorite all belonging to the moruya batholith lobate to crenulate contacts pillowing of the more mafic phase in the more felsic phase net veining of the more mafic phase by the more felsic local chilling of the more mafic phase against the more felsic and an abundance of rounded to elongate microgranitoid enclaves all suggest mingling of magmas at the site of emplacement therefore all phases were emplaced before the others solidified and an order of intrusion cannot be determined the variety of microgranitoid enclaves suggests that repeated magma mingling has occurred linear geochemical trends indicate either magma mixing or restite unmixing as a prime cause of variation in the members of the moruya suite of granitoids direct evidence of mixing is not present but evidence that the magmas were substantially liquid at the time of emplacement sugests that mixing is possible restite unmixing is unlikely to be the prime process because near minimum melts which would occur in all granitoid phases according to that model should mix easily rather than mingling in fact mingling has occurred between tonalite phases of very similar composition which would not be expected if the liquid parts of enclaves were minimum melts available experimental and theoretical data on mixing in both magma chambers and conduits cannot be applied directly to the occurrence at tarandore point owing to inadequately constrained parameters but mixing generally should be favoured by higher temperatures which is consistent with our inference that relatively high proportions of melt were present in the granitoid phases at the time of emplacement conduit mixing may be reflected in a local zone of strong to extreme elongation of enclaves in which the distortion has been accomplished by magmatic flow without solid state deformation as evidenced by strong dimensional preferred orientations coupled with a lack of microstructural evidence of intragranular plastic strain this suggests that elongate enclaves should not be used as indicators of solid state deformation or as tectonic strain markers without microstructural evidence that suitably strong intracrystalline plastic deformation has taken place

southern california deformation southern california is one of the most seismically active places in the united states and has therefore been of long standing interest to geoscientists in their perspective jackson et al discuss recent work to collect geodetic data on the motion and earthquake potential of the faults in southern california

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

neodymium and strontium isotope content of microdiorite enclaves points to mantle input to granitoid production microdiorite enclaves are arguably the most poorly understood of the more prominent features of granitoids surprisingly they have been invoked as support for completely different models for the origin of chemical zonation in granitoids depending on whether they are interpreted as restites syn plutonic blobs of magma remobilized cumulate or basified country rock xenoliths here we present the first detailed neodymium and strontium isotope study of suites of such enclaves from two plutons in both cases we find relatively high nd values most readily explained if enclaves represent syn plutonic injections of mafic magma into the host granitoid magma as such they are the tangible remains of a very important process of mantle derived heat and mass transfer operating in the generation of granitic magmas

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

two disequilibrium quartz feldspar 18o 16o fractionations within the aral granite batholith altay mountains of china evidence for occurrence of two stages of o and h isotopic exchange of a heterogeneous granite system with aqueous fluids

field evidence for successive mixing and mingling between the piolard diorite and the saint julien la vetre monzogranite nord forez massif central france recent mapping of the nord forez region massif central france reveals a variety of complex field relationships along contacts between a large enclave of diorite and its enclosing monzogranite field relations hybrid rocks and a wide variety of mafic magmatic enclaves can be explained by a succession of processes between coexisting mafic and felsic magmas that either prevent or promote exchanges the large volume of mafic magma and consequently the large amount of heat it brought with it favored exchange processes because the distribution of heat varied with both time and place the nature and intensity of the compositional exchange processes also varied most of the features attributed to exchange are related to processes that occurred at the level of emplacement and after the beginning of crystallization of the two magmas these processes affected both the granitic and dioritic magmas already thoroughly hybridized by nearly similar processes that took place earlier at depth

aftershocks are well aligned with the background stress field contradicting the hypothesis of highly heterogeneous crustal stress 1 it has been proposed that the crustal stress field contains small length scale heterogeneity of much larger amplitude than the uniform background stress this model predicts that earthquake focal mechanisms should reflect the loading stress rather than the uniform background stress so if the heterogeneous stress hypothesis is correct focal mechanisms before and after a large earthquake should align with the tectonic loading and the earthquake induced static stress perturbation respectively however i show that the off fault triggered aftershocks of the 1992 m7 3 landers california earthquake align with the same stress field as the pre landers mechanisms the aftershocks occurred on faults that were well oriented for failure in the pre landers stress field and then loaded by the landers induced static stress change aftershocks in regions experiencing a 0 05 to 5 mpa coseismic differential stress change align with the modeled landers induced static stress change implying that they were triggered by the stress perturbation contrary to the heterogeneous stress hypothesis these triggered aftershocks are also well aligned with the pre landers stress field obtained from inverting the pre landers focal mechanisms therefore the inverted pre landers stress must represent the persistent background stress field earthquake focal mechanisms provide an unbiased sample of the spatially coherent background stress field which is large relative to any small scale stress heterogeneity the counterexample provided by the landers earthquake is strong evidence that the heterogeneous stress model is not widely applicable

geochemistry of the rare metal bearing pegmatite no 3 vein and related granites in the keketuohai region altay mountains northwest china abstract the chemical composition of a barren biotite granite a two mica granite and a rare metal bearing pegmatite the no 3 vein in keketuohai altay mountains northwest china along with mineral separates of apatite and mica were analyzed and compared in the present work bulk chemistries of biotite granite two mica granite and pegmatite rim are consistent with fractional crystallization trend from granites toward pegmatite changes in pegmatite composition can be explained mainly by fractional crystallization of biotite rb sr isochrons yield 248 8 7 5 ma for the biotite granite 247 8 6 3 ma for the two mica granite and 218 4 5 8 ma for the pegmatite rim similar geochemical characteristics including initial e nd t values ranging from 0 76 to 3 04 for apatites similar initial e nd t values for whole rock samples of studied granites and pegmatite ranged from 1 40 to 3 21 and for biotites from granites ranged from 2 75 to 3 15 suggest that these rocks were derived from a common magma source old continental crust may have played a significant role in magma generation based on these data we interpret the evolution of the keketuohai granite pegmatite by magma differentiation from a common source biotite granite was the first to crystallize followed by two mica granites the residual melt was probably stored in a granitic batholith the pegmatite veins were injected 30 m y later during another episodic tectono magmatic event in the altay mountains

k feldspar phenocrysts in microgranular magmatic enclaves a cathodoluminescence and geochemical study of crystal growth as a marker of magma mingling dynamics abstract the crystallization history of zoned k feldspar phenocrysts in microgranular magmatic enclaves in the karkonosze granite sw poland reveals that the crystals grew in stirred coeval magmas of contrasting compositions the growth mechanism and crystal compositions are investigated using cathodoluminescence and profiling by electron microprobe and laser ablation icp ms these methods provide insight into the crystallization process and the varying compositions of the host melt the phenocrysts show two types of growth patterns with or without resorption interfaces the trace element distribution heterogeneous across different zones and within single phenocryst zones reflects a dynamic process of crystal growth from compositionally heterogeneous magma hybridization is also reflected in the density of structural al o al defects features that relate to coupled ba al incorporation into the crystal structure differences in structural defect densities and crystal composition trace the degree of hybridization in the granitic magma during the growth of the k feldspar phenocrysts

regime and trace element evolution of open magma chambers o hara and co workers1 2 have recently summarized some of the evidence accumulated for open system cyclic magmatic systems and provided useful geochemical equations for the replenishment eruption cycles of an ideal magma reservoir i show here that o hara s theory fits the situation where the volcanic eruption taps the more differentiated magma before the mixing stage an alternative ideal model is discussed which is applicable to the more common process of hybrid magma eruption in mid ocean ridge basalts this new model may explain why large variations in incompatible elements coexist with moderate depletion of compatible elements