petrologic determination of ascent rates for the 1995 1997 soufriere hills volcano andesitic magma the recent eruption of the soufriere hills volcano in montserrat july 1995 to present october 1997 has produced a hornblende bearing andesitic lava dome it is possible to petrologically estimate changes in ascent rates of amphibole bearing magmas for certain rates of decompression a breakdown rim of fine grained anhydrous reaction products forms where amphibole is in contact with melt the thickness of the rim varies with ascent rate most of the amphibole phenocrysts in the magma storage region lack breakdown rims about 10 have 200 400 m thick coarse grained breakdown rims that are interpreted to be relicts of a past heating event study of a time series of new dome andesites showed that ascent rate increased from december 1995 0 001 m s to july 1996 0 008 m s while eruptive style remained extrusive ascent rate increased to 0 012 m s in august 1996 and the first major explosive eruption occurred on 17 18 september 1996

the 1995 1998 eruption of the soufriere hills volcano montserrat wi eruption of the soufriere hills volcano began on 18 july 1995 after three years of elevated seismic activity four months of increasingly vigorous phreatic activity culminated in mid november 1995 with the initiation of dome growth growth rates increased unevenly through early march 1996 with fluctuations on time scales from hours to months since march 1996 gravitational collapse of the unstable dome flank has affected an ever increasing area with pyroclastic flows surges and ashfalls major collapse of the eastern flank on 17 september 1996 resulted in a sub plinian explosive eruption later that day by february 1997 the dome had outgrown the confines of the crater and begun to spill into the surrounding valleys a large collapse on 25 june 1997 caused pyroclastic flows and surges on the northern flanks and resulted in the only deaths of the eruption in august september and october 1997 vulcanian explosions followed further collapses on the western and northern flanks the largest event of the eruption occurred on 26 december 1997 with failure of the southwestern flank of the volcano producing a debris avalanche and large dome collapse pyroclastic flows dome growth ceased in early march 1998 but residual volcanic activity has continued and consists of ash venting mild explosions and dome collapse pyroclastic flows

the nature and occurrence of discrete zoning in plagioclase from recently erupted andesitic volcanic rocks montserrat abstract recently erupted rocks from montserrat lesser antilles were found to be coarsely porphyritic including plagioclase magnesio hornblende orthopyroxene titano magnetite and rare quartz phenocrysts zonations within plagioclase show three distinct periods within the evolution of the andesitic magmas initial steady state oscillatory overgrowths an49 54 represent an early period of growth within a confined andesitic magma chamber interaction with an intruding more mafic hotter magma resulted in thick zones of dissolution features that cut through these early growth layers the dissolution zones and crystal fragments are overgrown by thin calcic an86 plagioclase rims a plagioclase growth rate of 10 7cm s suggests that the calcic plagioclase rims were formed in approximately 10 h consistent with the idea that mineral growth may increase magma viscosity inducing conduit plugging and shallow pressurization resulting in cycles of dome inflation eruption and explosive activity thermodynamic modeling of heating and mixing of the andesitic magma with a basaltic liquid produces plagioclase of composition an79 not the observed composition of the late plagioclase an86 we conclude that the final plagioclase grew under non equilibrium conditions as a result of the mixing event

zedong terrane revisited an intra oceanic arc within neo tethys or a part of the asian active continental margin abstract precise timing of the india asia collision is important to constrain the evolution history of both the himalayan orogen and the tibetan plateau it has been proposed that the indian plate first collided with an intra oceanic arc at 55 ma and then the composite terrane collided with the asian continent at 35 ma the zedong terrane has been suggested to represent the vestige of such an intra oceanic arc developed within the neo tethys ocean as some volcanic rocks with high k2o have been classified as shoshonites in this study we present detailed geochemical and geochronological data of various types of magmatic rocks including volcanic cumulate and granitic rocks widely exposed in the zedong terrane to constrain the formation age and tectonic setting of the zedong terrane we found that the zedong volcanic rocks belong to calc alkaline series rather than shoshonites and high k2o contents in some volcanic rocks resulted from alteration the basalts are highly enriched in lree and lile but strongly depleted in hfse indicating they were derived from a metasomatized mantle presence of hornblende phenocryst in both gabbros and hornblendites indicates that the cumulates were produced from hydrous basalts through crystallization the granitic rocks have adakite like compositional characteristics i e high sr y ratios but low y contents which were formed by melting of a thickened lower crust zircons from six samples including a volcanic rock an andesite three cumulates a hornblendites a hornblende bearing gabbro and a gabbro and two granitic a tonalite and a granodiorite rocks have been dated to yield identical ages of 155 160 ma this suggests that the volcanic eruption and plutonic emplacement were coevally developed in the zedong terrane zircons from both the andesite and the cumulates have similar positive ehf t values 11 6 to 16 7 indicating they were stemmed from similarly depleted mantle sources meanwhile zircons from the granitic rocks also have positive ehf t values of 12 6 to 15 2 implying their derivation from a juvenile lower crust therefore we proposed that the basalts in the zedong terrane were formed through partial melting of the mantle wedge metasomatized by slab released fluids melts a part of hydrous basalts were underplated in the thickened lower crust beneath the zedong terrane which gave rise to the cumulate and granitic rocks by comparison magmatic rocks in the zedong terrane show compositional similarities with the jurassic rocks exposed in the gangdese arc this suggests that the zedong terrane represents a slice of the active continental margin developed on the southern margin of the lhasa terrane as a result of the northward subduction of the neo tethys ocean during the late jurassic rather than the vestige of an intra oceanic arc

crystal size distribution in magmatic rocks proposition of a synthetic theoretical model the crystal size distribution csd corresponds to the number of crystals of a mineral per unit volume within a series of defined size intervals many crystal size distributions in igneous systems are straight lines when plotted on the classical diagram of ln population density versus size other common csds are concave down on such graphs for the low size values the effect of growth rate on csds seems to be small and nucleation apparently increases exponentially with time although magmatic systems are always multiphase only a few csd studies deal with several phases on the basis of the few available published examples it seems that the parallelism defined as y spacing constancy of the normalized csds i e where the crystal lengths are normalized to the greatest length for each phase is a common feature in igneous systems the aim of our modelling is to propose a mathematical framework to unify the csd typology to explain the common occurrence of straight lines on the classical csd diagram and the y spacing constancy of the normalized csds for multiphase rocks and also to provide a convenient tool to easily test petrogenetic scenarios through efficient computer simulations the bulk balance modelling is based on a square matrix having for coefficients the number of nuclei of one phase by the crystallized volume of another phase this interaction matrix is the mainspring of our csd modelling which requires straightforward assumptions about the uniformity of crystal shape and growth 1 at each time every crystal of a phase has the same shape and 2 the one dimensional growth rate of a crystal is independent of its size consequently the values of the normalized csd are linked together by means of an integral equation for the most part the behaviour of the csds theoretically obtained with a constant matrix may also be observed in computer simulations of more general cases this extended approach makes feasible the application of the model to natural cases concave down shapes for the lowest size are classically interpreted as a consequence of the ending of crystallization by roughly taking into account crystal interactions during the growth process through the concept of mean clutter we successfully simulate such concave down csds

geochemical confirmation of the kula farallon slab window beneath the pacific northwest in eocene time plate tectonic models indicate subduction of the kula farallon spreading ridge and thus imply formation of the kula farallon slab window beneath western north america from late cretaceous to middle eocene time seafloor magnetic anomalies however pro vide few constraints on the configuration of the subducted ridge and the location of the slab window we use geochemical data from eocene igneous rocks to provide an estimate of the slab window s position at 50 ma contouring of the trace element ratios k 2o sio2 rb zr and ta ce for lavas of the eocene challis kamloops volcanic belt demonstrates a southward trend toward enriched geochemical character this trend is consistent with a pacific northwest position for the slab window this is supported by the presence of adak ites slab melts near the canada united states border and by ca 51 ma forearc intru sions and volcanic rocks on vancouver island that are regarded as products of ridge subduction a new reconstruction of the slab window s geometry over time indicates that its chemically defined position is kinematically viable

the role of magma mixing in triggering the current eruption at the soufriere hills volcano montserrat west indies the andesite lava currently erupting at the soufriere hills volcano montserrat contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite the andesite phenocrysts have a range of textures and zonation patterns which suggest that non uniform reheating of the magma occurred shortly before the current eruption reheating resulted in remobilisation of the resident magma and may have induced eruption

remobilization of andesite magma by intrusion of mafic magma at the soufriere hills volcano montserrat west indies the 1995 1999 eruption of the soufriere hills volcano montintroduction serrat has produced a crystal rich andesite containing quenchthe soufriere hills volcano montserrat located in the textured mafic inclusions which show evidence of having been molten lesser antilles island arc in the west indies began to when incorporated into the host magma individual crystals in the erupt on 18 july 1995 after about 350 years of dormancy andesite record diverse histories amphibole phenocrysts vary from young et al 1998b the eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms which began in january 1992 anhydrous reaction products plagioclase phenocrysts are commonly previous earthquake crises have occurred on montserrat reverse zoned often with dusty sieve textures reverse zoned rims are in 1896 1897 1932 1935 and 1966 1967 powell 1938 also common on orthopyroxene phenocrysts pyroxene geothermometry shepherd et al 1971 initial phreatic and phregives an average temperature of 858 20 c for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores whereas reverse zoned rims record temperatures andesitic lava dome in november 1995 pyroclastic flows from about 880 to 1050 c the heterogeneity in mineral rim generated by gravitational collapse of the unstable dome compositions zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non uniform reheating and remobilization of the resident magma eruption but periods of explosive activity producing body by intrusion of hotter mafic magma convective remobilization pumice flows and falls have also occurred robertson et results in mixing together of phenocrysts that have experienced al 1998 young et al 1998b dome growth ceased different thermal histories depending on proximity to the intruding suddenly in march 1998 and further activity up to may mafic magma the low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool highly crystalline dome and periods of ash venting with some small scale magma body beneath the soufriere hills volcano the petrological vulcanian explosive events observations in combination with data on seismicity extrusion rate the soufriere hills volcano is the only active centre and so2 fluxes indicate that the current eruption was triggered by on montserrat the volcano is a composite of at least recent influx of hot mafic magma five andesitic lava domes flanked by pyroclastic deposits rea 1974 identified four other volcanic centres all

mantle flow through the northern cordilleran slab window revealed by volcanic geochemistry the northern cordilleran slab window formed beneath western canada concurrently with the opening of the californian slab window beneath the southwestern united states beginning in late oligocene miocene time a database of 3530 analyses from miocene holocene volcanoes along a 3500 km long transect from the northern cascade arc to the aleutian arc was used to investigate mantle conditions in the northern cordilleran slab window using geochemical ratios sensitive to tectonic affi nity such as nb zr we show that typical volcanic arc compositions in the cascade and aleutian systems derived from subduction hydrated mantle are separated by an extensive volcanic fi eld with intraplate compositions derived from relatively anhydrous mantle this chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the northern cordilleran slab window we suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge which had developed during extensive early cenozoic arc and backarc volcanism in western canada high heat fl ow throughout the western canadian cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window induced mantle upwelling

recent remobilisation of shallow level intrusions on montserrat revealed by hydrogen isotope composition of amphiboles abstract ion probe measurements of hydrogen isotopes were made on amphiboles representing different stages of the ongoing eruption of the soufriere hills volcano montserrat the majority 80 of the andesitic amphiboles show relative intra and inter crystal d homogeneity with a mean of 38 12 consistent with known primary magmatic values the remainder 20 of amphiboles show marked d heterogeneity with a mean of 6 30 the heterogeneous amphiboles have 100 m rims with d values similar to the homogeneous crystals but core values which are significantly heavier than primary magmatic values early in the eruption 50 of amphiboles are isotopically heterogeneous but post spring 1996 all amphiboles are homogeneous we interpret these ion probe data in terms of development of a shallow andesitic magma chamber by repeated emplacement of andesitic magma in the shallow crust over at least the last 100 yr we suggest such shallow intrusions emplaced during the volcano seismic crises preceding the ongoing eruption solidified some regions interacted with hydrothermal fluids and isotopic exchange took place generating the heavy hydrogen isotope signatures in the heterogeneous amphiboles on onset of the ongoing eruption such old igneous material was remobilised with intimate mixing between the new magma batch and the old intrusions indicated by the close proximity of homogeneous and heterogeneous crystals the isotopically primary magmatic rims on the heterogeneous crystals indicate hydrogen isotope exchange over a period of a few weeks consistent with timescales of magma ascent at the soufriere hills volcano once the old igneous material was flushed out by spring 1996 the eruption proceeded by extruding material dominated by the new batch of magma this resulted in a marked increase in extrusion rate

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

transition from oceanic to continental lithosphere subduction in southern tibet evidence from the late cretaceous early oligocene 91 30 ma intrusive rocks in the chanang zedong area southern gangdese little is known about the detailed processes associated with the transition from oceanic to continental lithosphere subduction in the gangdese belt of southern tibet gbst here we report zircon u pb age major and trace element and sr nd hf isotopic data for late cretaceous early oligocene similar to 91 30 ma intermediate acid intrusive rocks in the chanang zedong area immediately north of the yarlung tsangpo suture zone these rocks represent five magmatic episodes at similar to 91 similar to 77 similar to 62 similar to 48 and similar to 30 ma respectively the 91 48 ma rocks have slightly lower initial sr 87 sr 86 0 7037 to 0 7047 and higher epsilon nd t 1 8 to 4 3 and epsilon hf t 3 5 to 14 7 values in comparison with those 0 7057 to 0 7062 3 3 to 2 5 and 2 2 to 6 6 of the similar to 30 ma intrusive rocks the similar to 91 similar to 62 and similar to 30 ma rocks are geochemically similar to slab derived adakites the similar to 91 ma somka adakitic granodiorites were likely derived by partial melting of the subducting neo tethyan oceanic crust with minor oceanic sediments and the similar to 91 ma somka dioritic rocks with a geochemical affinity of adakitic magnesian andesites likely resulted from interactions between adakitic magmas and overlying mantle wedge peridotite the similar to 77 ma luomu diorites were probably generated by partial melting of juvenile basaltic lower crust the similar to 62 ma naika and zedong adaldtic diorites and granodiorites were likely generated mainly by partial melting of thickened juvenile mafic lower crust but the source region of the zedong adakitic rocks also contained enriched components corresponding to indian continental crust the similar to 48 ma lamda granites were possibly generated by melting of a juvenile basaltic crust the younger similar to 30 ma chongmuda adakitic quartz monzonites and minor granodiorites were most probably derived by partial melting of early oligocene north ward subducted indian lower crust beneath the southern lhasa block taking into account the regional tectonic and magmatic data we suggest that the gangdese belt of southern tibet gbst underwent a tectonodynamic transition from oceanic subduction to continental subduction between 100 and 30 ma it evolved through four stages 100 65 ma roll back of subducted neo tethyan oceanic lithosphere 65 60 ma initial collision between indian and asian continents 60 40 ma breakoff of subducted neo tethyan oceanic lithosphere and similar to 30 ma northward subduction of the indian continent c 2014 elsevier b v all rights reserved

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

mesozoic and cenozoic volcanic rocks from central and southern tibet 39ar 40ar dating petrological characteristics and geodynamical significance field relations radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the lhasa block tibet are studied volcanic activity started before the deposition of the albian aptian sedimentary units takena formation and xigaze group volcanism is volumetrically limited in the northern part of the lhasa block basalts andesites and dacites are present while ignimbrites are scarce radiometric ages of these rocks range between 110 and 80 ma conversely in the southern part of the lhasa block volcanism is largely developed gangdese belt there paroxysmal activity occurred later than in the north between 60 and 50 ma lingzizong formation this volcanism is mainly represented by andesites and ignimbrites a younger episode at about 15 10 ma took place in this southern region the mineralogical and geochemical characteristics indicate that as a whole the volcanic rocks from the lhasa block belong to a calc alkaline continental margin series enriched in k20 and other incompatible elements subordinate potassic alkaline lavas are present in the northern margin of the gangdese belt some basalts exhibit a tholeiitic affinity daquin area southern region the geodynamic significance of these cenozoic volcanic rocks lies in the relative motion of india and the lhasa block after accretion of the latter to the northern qantang block a geodynamic model based on changes of the type of subduction of the indian oceanic crust beneath the lhasa block for the period 120 40 ma is proposed to account for the abundance age and magmatic affinity of the volcanic products

the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya ultrahigh pressure uhp rocks in the nw himalaya are some of the youngest on earth and allow testing of critical questions of uhp formation and exhumation and the timing of the india asia collision initial collision of india with asia is widely cited as being at 55f1 ma based on a paleomagnetically determined slowdown of india s plate velocity and as being at ca 51 ma based on the termination of marine carbonate deposition even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid india asia convergence rate of 134 mm a at the time of collision new u pb shrimp dating of zircon shows that indian rocks of the tso morari complex reached uhp depths at 53 3f0 7 ma given the high rate of indian subduction this dating implies that indian continental crust arrived at the asian trench no later than 57f1 ma providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates india s collision with asia may be compared to modern processes in the timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition the indian uhp rocks must have traveled rapidly along a short hence steep path into the mantle early continental subduction was at a steep angle probably vertical comparable to modern continental subduction in the hindu kush despite evidence for modern day low angle subduction of india beneath tibet oceanic slab break off likely coincided with exhumation of uhp terranes in the western himalaya and led to the initiation of low angle subduction and leucogranite generation d 2005 elsevier b v all rights reserved

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

paleomagnetic results from the early cretaceous zenong group volcanic rocks cuoqin tibet and their paleogeographic implications article i nfo article history a paleomagnetic study has been carried out on the early cretaceous zenong group volcanic rocks ca 110 130 ma near cuoqin town in the central lhasa terrane stepwise thermal demagnetization up to 680 c isolated a stable high temperature component htc including antipodal dual polarities the htc directions passed both a fold test at 99 confidence and a reversal test at 95 confidence suggesting a primary origin the tilt corrected average direction for 18 sites is d 327 0 i 35 7 59 3 95 4 5 which corresponds to a paleopole at 58 2 n 341 9 e a95 4 6 yielding a paleolatitude of 19 8 4 6 n for the study area our results combined with previous paleomagnetic data suggest that southern tibet was at a mean paleolatitude of 20 n during the cretaceous and paleogene when compared with the apparent polar wander paths of asia the difference suggests that only ca 870 km north south crustal shortening has accumulated between the lhasa terrane and stable asia since ca 110 ma when comparing the paleolatitudes of the lhasa terrane and india the results indicate that the width of the neotethys was 6720 690 km during the early cretaceous

geochemical sr nd and zircon u pb hf isotopic studies of late carboniferous magmatism in the west junggar xinjiang implications for ridge subduction abstract voluminous granitic intrusions are distributed in the west junggar nw china and they can be classified as the dioritic rocks charnockite and alkali feldspar granite groups the dioritic rocks sio 2 50 4 63 8 wt are calc alkaline and mg enriched average mgo 4 54 wt mg 0 39 0 64 with high sr y ratios average 21 2 weak negative eu average eu eu 0 80 and pronounced negative nb ta anomalies their sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7035 0 7042 e nd t 4 5 7 9 e hf t 14 1 14 5 show a depleted mantle like signature these features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials the charnockites sio 2 60 0 65 3 wt show transitional geochemical characteristics from calc alkaline to alkaline with weak negative eu average eu eu 0 75 but pronounced negative nb ta anomalies sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7037 0 7039 e nd t 5 2 8 0 e hf t 13 9 14 7 also indicate a depleted source suggesting melts from a hot juvenile lower crust alkali feldspar granites sio 2 70 0 78 4 wt are alkali and fe enriched and have distinct negative eu and nb ta anomalies average eu eu 0 26 low sr y ratios average 2 11 and depleted sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7024 0 7045 e nd t 5 1 8 9 e hf t 13 7 14 2 these characteristics are also comparable with those of rocks derived from juvenile lower crust despite of the differences in petrology geochemistry and possibly different origins zircon ages indicate that these three groups of rocks were coevally emplaced at 305 ma a ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks as the slab window opened upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms partial melting of the subducting slab formed the dioritic rocks partial melting of the hot juvenile lower crust produced charnockite and alkali feldspar granite and partial melting in the mantle wedge generated mafic rocks in the region these results suggest that subduction was ongoing in the late carboniferous and thus support that the accretion and collision in the central asian orogenic belt took place in north xinjiang after 305 ma and possibly in the permian

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

late cretaceous volcanic rocks in the sangri area southern lhasa terrane tibet evidence for oceanic ridge subduction abstract late cretaceous magmatism in the southern lhasa subterrane tibet provides critical constraints on the subduction processes of the neo tethyan oceanic plate here we provide data on the whole rock geochemistry zircon u pb ages trace element contents and hf isotopes of early late cretaceous volcanic rocks from sangri county to sangye temple ss along the southern lhasa subterrane the ss volcanic rocks include basalts andesites and dacites sims and la icp ms zircon u pb data indicate that the ss dacites were erupted at ca 95 ma coeval with the early late cretaceous flare up event in the southern lhasa subterrane the ss volcanic rocks belong to the calc alkaline to high k calc alkaline series and have the geochemical features of subduction related volcanic rocks with negative nb ta and ti anomalies the basalts have similar rare earth element ree patterns to e morb and are characterized by high zr abundances 65 190 ppm and zr y ratios 3 3 6 5 they were possibly derived by high degree partial melting of mantle peridotite that had been metasomatized by subduction related fluids with an input of asthenospheric components positive whole rock end t 2 9 to 3 2 values for the andesites and their variable mg values 35 61 and mgo 2 29 7 32 wt cr 12 232 ppm and ni 10 127 ppm contents can be attributed to medium degree partial melting of mantle peridotite followed by fractional crystallization of clinopyroxene and olivine the calc alkaline ss dacites are similar to adakites with high sr 663 1188 ppm and low heavy rare earth element hree and y 10 7 11 5 ppm contents these adakitic dacites have positive whole rock end t 2 9 to 3 4 and zircon ehf t 8 4 to 14 9 values with relatively high values of mg 36 57 and high contents of compatible elements indicating that they were derived from the partial melting of subducted oceanic slab and with the ascending magma interacting with mantle wedge peridotite our new data indicate a distinct rock association of coeval asthenosphere component involved basalts and slab derived adakitic dacites such an association together with the coeval high temperature charnockites and high temperature granulite facies metamorphism of the charnockitic country rocks in the southern lhasa subterrane indicates an anomalous input of materials and heat during the early late cretaceous flare up event such an event can probably be attributed to the northwards subduction of a neo tethyan oceanic ridge that allowed for contributions from upwelling asthenosphere that differed from the typical results of normal angle subduction or low angle flat subduction and subsequent slab rollback

late cretaceous gangdese intrusions of adakitic geochemical characteristics se tibet petrogenesis and tectonic implications article i nfo the gangdese batholith emplaced from the cretaceous to eocene in southern tibet has been widely regarded as the major constituent of an andean type convergent margin resulting from northward subduction of the neo tethyan oceanic lithosphere under asia while the gangdese batholith consists predominantly of calc alkaline rocks we identify from the eastern part of the batholith a suite of epidote bearing granodiorites that shows adakitic geochemical characteristics marked with apparently higher la yb and sr y and lower y and hree than other gangdese rocks and common arc magmas shrimp zircon u pb analyses of two of the samples yielded 206 pb 238 u dates of 80 4 1 1 and 82 7 1 6 2 ma which constrain the emplacement ages of the adakitic rocks trace element modeling suggests that these rocks originated from partial melting of a garnet amphibolite source that on the basis of the sr and nd isotope data isr 0 7044 0 7048 end t 3 2 to 0 9 we interpret to be a newly underplated mafic lower crust rather than the subducted neo tethyan oceanic crust this juvenile crust was produced by cretaceous basaltic underplating above the mantle wedge and then thickened by the tectonic contraction owing to flattening of the neo tethyan subduction a process that also led to the adakitic magmatism our interpretation involving a late cretaceous flat slab subduction and related orogenesis in southern tibet is consistent with petrographic data such as the occurrence of magmatic epidote and muscovite rimmed with resorption texture in the granodiorites which indicate deep seated emplacement followed by rapid tectonic exhumation

zircon u pb and hf isotope constraints on the mesozoic tectonics and crustal evolution of southern tibet the first in situ hf and u pb isotope analyses of zircon separates from mesozoic granites in southern tibet identify a significant previously unknown stage of magmatism igneous zircons n 34 from a granite within the gangdese batholith show a weighted mean 206 pb 238 u age of 188 1 1 4 ma and e hf t the parts in 10 4 deviation of initial hf isotope ratios between the zircon sample and the chondritic reservoir values between 10 4 and 16 8 suggesting predominantly early jurassic intrusive activity with a juvenile mantle contribution of 40 inherited zircons from two cretaceous s type granites in the northern magmatic belt 23 delineate a slightly older 206 pb 238 u age cluster between 188 and 210 ma these zircons have e hf t values from 3 9 to 13 7 yielding crustal hf model ages from ca 1 4 to 2 1 ga suggesting a major episode of crustal growth in proterozoic time and remelting of this crust in early jurassic time combining these with literature data we interpret the jurassic gangdese magmatism as an early product of the neo tethyan subduction that played a long lasting role in the tectonic evolution of southern tibet prior to the india asia collision

late cretaceous 81 ma high temperature metamorphism in the southeastern lhasa terrane implication for the neo tethys ocean ridge subduction abstract an integrated study of u pb zircon dating geochemical and sr nd hf isotopic compositions of garnet bearing granulite and marble from the granulite facies domain of the nyingchi complex eastern himalayan syntaxis has provided insights into the tectonic evolution of the southern lhasa terrane the peak metamorphism of the garnet bearing granulite is marked by a mineral assemblage of garnet orthopyroxene high ti amphibole plagioclase quartz rutile abundant exsolved rutile needles are observed within amphibole garnet and quartz the peak metamorphic temperatures are estimated at 803 924 c geochemical data from the garnet bearing granulites provide evidence for a basaltic protolith that formed in a continental margin arc setting sr nd hf isotopic compositions indicate that this protolith was sourced from partial melting of a depleted mantle la icp ms u pb zircon dating shows that the protolith age and metamorphic age of the garnet bearing granulites are 89 3 0 6 ma and 81 1 0 8 ma respectively the detrital magmatic zircons from the marble yield ages from 86 3 to 167 ma the age distribution and hf isotopic composition e hf t 5 9 to 17 5 of the detrital magmatic zircon in the marble are consistent with the isotopic data of zircons from the jurassic cretaceous gangdese batholiths suggesting that the clastic sediments were partially derived from these intrusives or associated volcanic rocks and deposited in the fore arc basin of the gangdese arc the metamorphic zircons in the marble yield a metamorphic age of 81 4 0 5 ma these results show that both the arc magmatic rocks and forearc sedimentary rocks underwent high temperature ht granulite facies metamorphism at 81 ma indicating anomalously high heat input in the forearc region a range of tectonic observations including a coeval hiatus in arc magmatism and a period of regional uplift indicate that ht metamorphism resulted from the subduction of the neo tethys ocean ridge beneath the southern lhasa terrane during the late cretaceous

effects of magma storage and ascent on the kinetics of crystal growth the size distributions of crystals of olivine plagioclase and oxides of the 1991 93 eruption at mt etna italy are analyzed the simultaneous collection of this information for different minerals gives precious insight into the cooling history of lavas three distinct episodes are detectable a storage of the magma in a deep reservoir characterized by nearly constant and low nucleation and growth rates near to equilibrium an ascent phase with an ever increasing nucleation rate related to volatile exsolution and finally a quenching phase in addition to geochemical and geophysical evidence the similarity of the crystal size distributions of the present eruption with those of previous ones of this century makes it possible to exclude that crystal size distributions of etnean lavas are due to mixing of different populations this strongly suggests that the main features of the volcano feeding system have not changed despite observed variations in the magma output rates

formation of the cretaceous skarn cu au deposits of the southern gangdese belt tibet case studies of the kelu and sangbujiala deposits abstract mesozoic subduction of neo tethyan oceanic crust caused magmatism that formed numerous jurassic and cretaceous intrusions within the southern gangdese belt of tibet these intrusions are associated with the formation of giant jurassic xiongcun porphyry cu deposits but the prospectivity of other cretaceous intrusions in this area remains unclear this study presents new molybdenite re os ages for the kelu and sangbujiala cu au skarn deposits within the southern gangdese belt as well as new geochemical zircon u pb geochronological and sr nd hf isotopic data for the intrusions associated with these deposits these new data indicate that 1 the kelu deposit formed at ca 92 ma coeval with emplacement of the diorite and biotite granodiorite intrusions ca 92 90 ma in this area and 2 the sangbujiala deposit formed at ca 95 ma contemporaneous with emplacement of the mineralized biotite granodiorite ca 95 92 ma in this area these data suggest that the skarn cu au mineralization in this area formed as a result of cretaceous magmatism in the southern gangdese belt the intrusions associated with the kelu and sangbujiala deposits have adakitic affinities with low contents of y and heavy rare earth elements and high sr y ratios 22 4 123 these intrusions also have more primitive sr nd isotopic compositions end t 3 99 4 81 87sr 86sr i 0 7040 0 7044 lower ehf t values 8 0 13 18 and higher zircon eun eu values mean of 0 50 than those of contemporaneous but barren adakitic intrusions in this region this suggests that the mineralization related intrusions were oxidized and formed from magmas that were generated predominantly by partial melting of subducted neo tethyan slab material without any significant crustal contamination or assimilation of sedimentary material however these intrusions contain elevated th ni and cr contents and have high mg ratios 43 4 56 9 suggesting that they formed from magmas that interacted with metasomatized mantle wedge peridotite material combining our new data with the results of previous research suggests that the southern gangdese belt records two stages of cu au mineralization related to neo tethyan subduction the first stage generated the large xiongcun porphyry cu au deposit and was associated with normal subduction during the jurassic whereas the second stage was associated with the formation of the kelu and sangbujiala skarn deposits during cretaceous slab rollback

neogene kinematic history of nazca antarctic phoenix slab windows beneath patagonia and the antarctic peninsula article i nfo the patagonian slab window is a subsurface tectonic feature resulting from subduction of the nazca antarctic spreading ridge system chile rise beneath southern south america the geometry of the slab window had not been rigorously defined in part because of the complex nature of the history of ridge subduction in the southeast pacific region which includes four interrelated spreading ridge systems since 20 ma first the nazca phoenix ridge beneath south america then simultaneous subduction of the nazca antarctic and the northern phoenix antarctic spreading ridge systems beneath south america and the southern phoenix antarctic spreading ridge system beneath antarctica spreading ridge paleo geographies and rotation poles for all relevant plate pairs nazca phoenix antarctic south america are available from 20 ma onward and form the mathematical basis of our kinematic reconstruction of the geometry of the patagonia and antarctic slab windows through neogene time at approximately 18 ma the nazca phoenix antarctic oceanic ridge ridge ridge triple junction enters the south american trench we recognize this condition as an unstable quadruple junction heat flow at this junction and for some distance beneath the forearc would be considerably higher than is generally recognized in cases of ridge subduction from 16 ma onward the geometry of the patagonia slab window developed from the subduction of the trailing arms of the former oceanic triple junction the majority of the slab window s areal extent and geometry is controlled by the highly oblique near parallel subduction angle of the nazca antarctic ridge system and by the high contrast in relative convergence rates between these two plates relative to south america the very slow convergence rate of the antarctic slab is manifested by the shallow levels achieved by the slab edge b45 km thus no point on the antarctic slab is sufficiently deep to generate normal mantle derived arc type magmas upwelling beneath the region may have contributed to uplift and eastward transfer of extension in the scotia sea

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

origin of anorthosite by textural coarsening quantitative measurements of a natural sequence of textural development the textures of plagioclase crystals within large olivine oikocrysts preserve the sequence of the formation of anorthosite such textures have been quantified in a troctolite anorthosite from the proterozoic lac st jean anorthosite complex canada crystal size distributions csds indicate that initially plagioclase nucleated and grew in an introduction environment of linearly increasing undercooling producing a straight the textures of plutonic rocks are one of their most line csd during this phase latent heat of crystallization was striking aspects but they have been little used to elucidate largely removed by circulation of magma through the porous crystal petrological processes as compared with chemical and mush by about 25 solidification the crystallinity was such that isotopic methods where used discussion has tended it reduced but did not eliminate the circulation of magma resulting to be qualitative in contrast to quantitative chemical in the retention of more latent heat within the crystal pile the methods hence reducing the weight of this approach temperature rose until it was buvered by the solution of plagioclase indeed it is not possible to verify physical models if close to the liquidus temperature of plagioclase nucleation of quantitative data are not available quantitative textural plagioclase was inhibited and conditions were suitable for textural studies of volcanic rocks are becoming more common coarsening of both plagioclase and olivine to occur ostwald and these methods are applied here to the development ripening in this process small crystals were resorbed whereas of anorthosite larger crystals grew from both material recycled by the resorption of quantitative textural studies of plutonic rocks really crystals smaller than the critical size and new material brought in started with jackson s studies of the stillwater complex by the circulating magmatic fluid the results of this process resemble in 1961 jackson 1961 he measured crystal size disthose of high temperature metasomatism but there is no necessity for tributions but did not record the area that he measured a magmatic fluid with a diverent composition the shapes of the hence his data cannot be completely compared with plagioclase csds fit better the communicating neighbours equation of modern data his pioneering work was not really followed textural coarsening rather than the classic lifshitz slyozov wagner up and since then most studies have looked only at equation as do other examples drawn from the literature if olivine average or typical crystal sizes these data are much started to nucleate at a higher temperature than plagioclase then less revealing of petrological processes than crystal size during the textural coarsening phase olivine would have been more distributions partly because they only give one parameter undercooled than plagioclase and would have had a higher maximum for each sample the few existing studies of crystal size growth rate in these conditions olivine would coarsen more rapidly distributions in plutonic rocks have not treated than plagioclase and engulf it hence the order of crystallization plagioclase determined from the textures would be the reverse of the order of one problem especially pertinent to quantitative texfirst nucleation of the two phases from equilibrium phase diagrams tural studies is the way that many igneous petrologists maintenance of the temperature near the plagioclase liquidus may look at rocks they commonly try to reconstruct how a rock has formed by examining the final fully crystallized also inhibit the nucleation and growth of other phases

geochemistry of magnesites and dolomites at the oberdorf laming austria deposit and implications for their origin the major and trace element geochemistry of magnesite and dolomite samples from alternating rock masses at the oberdorf laming styria austria mineralization has been studied the rare earth elements ree are considered to be of importance for the discussion of the origin of sparry magnesite deposits the al and ree contents are positively correlated indicating a clay component as the main source for ree in the magnesites and their precursor rocks our data support the view that the oberdorf and probably other magnesite mineralizations in the eastern alps have been formed by metasomatism through this process calcite which was deposited by sedimentation in a marine environment underwent dolomitization subsequently mg rich solutions circulating through the rock masses formed the magnesite bodies

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

eocene volcanism above a depleted mantle slab window in southern alaska the caribou creek volcanic fi eld lies along the continent side edge of forearc basin rocks in south central alaska and consists of over 1000 m of shallow dipping basalt and andesite lavas with minor mafi c pyroclastic deposits dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas the basalts are tholeiitic and strongly depleted in the light rare earth elements la yb 0 18 1 5 with concentrations of high fi eld strength elements e g zr hf ti y similar to mid ocean ridge basalt and with variable enrichment in fl uid mobile elements e g cs ba and pb intermediate and felsic rocks show enrichment in the rare earth elements and fl uid mobile elements plus rb and k but retain low la yb ratios 0 48 3 6 a few andesite and dacite samples are strongly depleted in the heavy rare earth elements and are geochemically similar to adakites e g sr y up to 52 ten 40 ar 39 ar ages for the caribou creek volcanic rocks range from 49 4 2 2 to 35 6 0 2 ma an adakite like tuff beneath the other volcanic rocks yields an age of 59 0 0 4 ma caribou creek basalts were derived from mid ocean ridge like depleted mantle that was emplaced beneath the southern margin of alaska through a slab window following spreading ridge subduction caribou creek volcanism was coeval with oblique subduction oroclinal bending and right lateral strike slip faulting in south central alaska all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas the basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of jurassic arc basement rocks to form the intermediate and felsic magmas enrichment of the basaltic parent magmas in fl uid mobile elements occurred by contamination from the jurassic arc rocks and or by contamination with metasomatic mantle remnant from preceding subduction high heat fl ow through the slab window induced partial melting of garnet bearing mafi c parts of the jurassic arc basement to form the adakite like rocks the caribou creek volcanic rocks demonstrate that slab windows can directly infl uence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime e g crustal extension and that the infl uence of a slab window on continental margin magmatism can be long lived 20 m y

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

metagabbros of the gangdese arc root south tibet implications for the growth of continental crust abstract the gangdese batholith of the southern tibet is regarded as a principal component of the pre himalayan collision andean type continental margin however the gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc in this paper we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the late cretaceous lilong batholith in the eastern gangdese arc the gabbros with protolith ages of 82 95 ma underwent slightly late 68 77 ma granulite facies metamorphism and nearly coeval anatexis typical of lower arc crusts the metagabbros and diorites of the lilong batholith show geochemical features of juvenile crust in continental magmatic arcs and an original magmatic differentiation trend with gabbros formed as the cumulates whereas diorites as the crystalline products of evolved magmas the lilong batholith forms an arc crustal section with a possible thickness of 20 30 km and may have an overall andesitic composition providing a support for the andesite model of continental crust growth

overview of the eruption of soufriere hills volcano montserrat 18 july 1995 to december 1997 the onset of phreatic volcanic activity at the soufriere hills volcano montserrat on 18 july 1995 followed a three year period of heightened volcano seismic activity beneath the island phreatic explosions gave way to continuous eruption of juvenile andesitic magma in the form of a lava dome on or around 15 november 1995 magma production rate has varied leading to changes in eruptive style an explosive eruption on 17 september 1996 followed a period of enhanced dome growth and large scale gravitational collapses from its eastern flank increasing dome volume led to stressing and overtopping of the confining crater walls to the southwest north and west during early 1997 sustained high magma production rate since june 1997 has led to three periods of major gravitational dome collapses followed by vulcanian explosive eruptions dome growth re started immediately after the cessation of the latest of these explosive phases in october 1997 and continues as of december 1997

high density carbonic fluids in a slab window evidence from the gangdese charnockite lhasa terrane southern tibet abstract charnockites occurring in the well studied gangdese batholith in the southern lhasa terrane eastern himalayan orogen consist of orthopyroxene clinopyroxene plagioclase and minor quartz and k feldspar and show adakitic affinity here we present a systematic study on the fluid inclusions in gangdese charnockites which reveals that the primary fluid inclusions occur isolated or randomly distributed and as trails along intragranular fractures in quartz and that the secondary ones occur along healed mircofractures and coexist commonly with mineral inclusions of calcite magnetite and hematite within the host plagioclase and quartz the fluid inclusions contain dominantly near pure co 2 with traces of n 2 most of the primary fluid inclusions have low co 2 homogenization temperatures and with densities of 1 138 1 013 g cm 3 suggesting trapping pressures of 0 7 1 0 gpa at temperatures of 850 950 c we propose a model in which the southward subduction of neo tethyan mid oceanic ridge beneath the lhasa terrane resulted in the release of heat and co 2 from the upwelling asthenosphere through a slab window providing high temperatures ht and dry co 2 rich fluids for the formation and stabilization of the adakitic charnockite as well as the associated ht granulite facies metamorphic rocks in the late cretaceous prior to the final collision of the indian plate with the asian continent our study provides new insights into the geodynamics of andean type orogeny in the southern tibetan plateau during the late mesozoic

the role of magma chamber fault interaction in caldera forming eruptions this paper examines the role of the position and orientation of a regional fault in the roof of a magma chamber on stress distribution mechanical failure and dyking using 2d finite element numerical simulations the study pertains to the magma chamber behavior in the relatively short time intervals of several hundreds to thousand of years the magma chamber is represented as an elliptical inclusion eccentricity a b 0 12 at a relative depth h a of 0 9 the fault has a 45 dip and is represented by a frictionless fracture the temperature field in the host rock is calculated assuming a quasisteady state thermal regime that develops through periodic episodes of magma supply the rheology of the surrounding rocks is treated using viscoelasticity with temperature activated strain rate dependent viscosity strain weakening of the rocks in the ductile zone is described within the frame of the dynamic power law model the magma pressure is coupled with the deformation of the rock mass hosting the chamber including the fault the variation of magma pressure in response to magma supply and chamber deformation is calculated in the elastic and viscoelastic regimes the latter corresponds to slow filling while the former represents a filling time much less than the viscous relaxation time scale the resulting equation of state for the magma chamber couples the magma pressure with the chamber volume in the elastic regime and with the filling rate for the viscoelastic regime analysis of stresses is used to predict dyke propagation conditions and the mechanical failure of the chamber roof for different fault positions and magma overpressures results show that an outward dipping fault located on the periphery of the chamber roof hinders the propagation of dykes to the surface causing magma to accumulate under the footwall of the fault at high to moderate overpressures 30 40 mpa the fault causes localized shear failure and chamber roof collapse that might lead to the first stage of a caldera forming eruption

origin of mesozoic adakitic intrusive rocks in the ningzhen area of east china partial melting of delaminated lower continental crust to the best of our knowledge modern adakites have not been documented in a nonarc environment we report geochemical and isotopic data for early cretaceous anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction the anjishan adakitic intrusive rocks which are exposed in the ningzhen area of east china have high sr y and la yb ratios coupled with low yb and y as well as relatively high mgo contents and mg numbers mg 0 4 0 6 similar to products from slab melting however low n d t values 6 8 to 9 7 and high 8 7 sr 8 6 sr i 0 7053 0 7066 are inconsistent with an origin by slab melting the tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust high sr y and la yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting implying that the crustal thickness exceeded 40 km in the early cretaceous the present thickness of the crust in the ningzhen area is only 30 km and therefore the crust appears to have been thinned by at least 10 km since the early cretaceous the relatively high mgo contents and mg of the anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks possibly coinciding with lower crustal delamination which would also account for the observed thinning

magma dynamics beneath kameni volcano thera greece as revealed by crystal size and shape measurements size distributions of plagioclase crystals in series of recent porphyritic dacite lavas from kameni volcano greece can be modelled by mixing two populations of crystals each with overlapping linear crystal size distributions csd termed microlites and megacrysts the magmas bearing the microlites and megacrysts started to crystallise 6 13 and 24 96 years respectively before each eruption the dates of initiation of crystallisation of the megacrysts indicate that they are left overs of earlier injections of new magma into a shallow chamber some magma remains after each eruption and continues to crystallise new magma with few or no crystals is then introduced and the microlites crystallise from the mixed magma eruption followed 6 13 years after mixing such a model would suggest that some porphyritic magmas are products of a shallow magma chamber that is never completely emptied just topped up from time to time

high precision pb sr nd hf isotopic characterization of usgs bhvo 1 and bhvo 2 reference materials the recent development of multiple collector inductively coupled plasma mass spectrometry mc icp ms and increasing use of the technique have created the need for well characterized rock standards especially for isotopic systems where no internal fractionation correction can be applied this paper presents a careful leaching experiment on the u s geological survey usgs reference materials bhvo 1 and bhvo 2 hawaiian basalts and documents the evidence for contamination of the rock powders during processing this contamination accounts for the difference in pb isotopic ratios of bhvo 1 and bhvo 2 as well as for their lack of homogeneity both in pb isotopic compositions and in some trace element contents copyright 2005 by the american geophysical union

possible manifestations of slab window magmatisms in cretaceous southwest japan abstract the slab window system related to ridge subduction is studied to explain the conspicuous activity and the along arc migration of the magmatisms in the eastern margin of eurasian continent in the late mesozoic the trend of the granite ages is scrutinized with systems approach and analyzed by the statistical method in cretaceous southwest japan a reference field of the eurasian eastern margin the magmatic distribution of simultaneous activity was determined to be the v shape of the slab window excluding dating errors from the located age data the slab window magmatism has the most dominant zone of the acidic rocks at or near the centerline of the window the spreading center of two plates and also has the subdominant zone of the acidic to intermediate rocks on either side of the window as the slab window opens due to the plate motions the upwelling current is adiabatically induced in the asthenosphere to fill the window gap the partial melt is generated in the upwelling flux and transports heat to the lower crust to make greater granitic magma chambers by heat conduction to and assimilation with the crustal matter the younger and hotter slabs outside the window also play a role in partial slab melting and or dehydration to the asthenosphere and generate the subdominant granodioritic and adakitic and or high magnesian andesitic magmatisms as the pre and post activities of the main granite genesis the slab window formed by the subducted ridge between the kula and pacific plates was the strong heat source of the active magmatisms and migrated at the rate of 2 8 cm year from southwest to northeast about 8000 km along the continental margin the growth of the continent associated with the active acidic to intermediate magmatisms including the adakitic and or high magnesian andesitic activities is modeled by the slab window system

early late cretaceous ca 93 ma norites and hornblendites in the milin area eastern gangdese lithosphere asthenosphere interaction during slab roll back and an insight into early late cretaceous ca 100 80 ma magmatic flare up in southern lhasa tibet abstract at more than 500 km in length the mainly jurassic early eocene gangdese batholith is one of the most important constituents of the southern lhasa sub block and provides an ideal site for study of tibetan orogenesis recent studies on gangdese intermediate felsic intrusive rocks mainly granites demonstrate that remarkable crustal growth as well as an early late cretaceous ca 100 80 ma magmatic flare up event occurred in southern tibet however the mechanism that drove this magmatic event and its relationship to crustal growth event are not yet clear here we report detailed petrological geochronological geochemical and sr nd hf isotopic data for recently identified norites and hypersthene bearing hornblendites in the milin area southern lhasa sub block these mafic rocks are dated at early late cretaceous ca 93 ma and are characterized by relatively uniform sr nd hf isotopic compositions 87sr 86sr i 0 7042 to 0 7047 end t 2 9 to 3 6 and ehf t zircon 10 9 to 17 0 suggesting that they evolved from similar parental magmas the milin norites and hornblendites are likely to be the products of mineral fractionation and accumulation from a common parental magma during the early and late stages of the magma evolution respectively thermometric calculations indicate that pyroxenes from the milin norites have high crystallization temperatures 1240 1349 c the parental magmatic compositions calculated from pyroxene trace element compositions in the milin norites show slightly flat to enriched light rare earth element lree patterns la yb n 2 9 3 4 la gd n 2 1 2 9 with variable negative nb anomalies nb la n 0 18 0 81 indicating their dual or hybrid geochemical characteristics we suggest that their parental magmas may have been generated by the interaction of upwelling asthenospheric and metasomatized lithospheric mantle taking into account the spatial and temporal distribution of the mesozoic magmatic rocks and regional paleomagnetic data we further suggest that the early late cretaceous magmatic flare up in the southern lhasa sub block was also triggered by the asthenospheric upwelling which resulted from roll back of subducted tethyan oceanic slab

early cretaceous subduction related adakite like rocks of the gangdese belt southern tibet products of slab melting and subsequent melt peridotite interaction abstract the limited geochronology and geochemistry data available for the early cretaceous igneous rocks of the southern gangdese belt southern tibet has resulted in the proposal of conflicting geodynamic models for the generation of the widespread cretaceous igneous rocks in the middle and northern parts of the belt to explore this issue we present shrimp u pb zircon data and geochemical and sr nd pb hf isotopic data for the mamen andesites from the southern margin of the gangdese belt the mamen andesites emplaced at 136 5 ma are sodic na2o k2o 1 2 2 3 and have geochemical characteristics typical of adakites i e high al2o3 high la yb ratios and sr contents low y and hree contents and positive eu anomalies except for high cr ni and mgo contents the andesites have initial 87sr 86sr t ratios of 0 70413 0 70513 positive end t values of 3 7 5 8 and 206pb 204pb t ratios of 18 37 18 51 207pb 204pb t ratios of 15 59 15 65 and 208pb 204pb t ratios of 38 43 38 72 in situ hf isotopic analyses of zircons that had previously been dated by shrimp yielded positive initial ehf t values ranging from 11 0 to 15 5 a model calculation using trace element and sr nd pb isotopic data indicates that several percent of subducted sediment is required to generate the mamen andesites which were derived via the partial melting of subducted neo tethyan slab morb sediment fluid and subsequently hybridized by peridotite in the mantle wedge our data indicate that the neo tethyan oceanic crust was subducted northward beneath the gangdese belt during the early cretaceous at a high angle our results are inconsistent with a tectonic model that advocates the low angle or flat slab subduction of neo tethyan oceanic crust in generating the widespread cretaceous magmatism recorded in the gangdese belt

rapid forearc spreading between 130 and 120 ma evidence from geochronology and geochemistry of the xigaze ophiolite southern tibet article i nfo the cretaceous xigaze ophiolite is best exposed at the central yarlung zangbo suture zone yzsz tibet which also includes the gangdese arc and the xigaze forearc basin this study reports new geochronological and geochemical data for this ophiolite to revisit its geodynamic and petrogenetic evolution the xigaze peridotites have low cao and al2o3 contents and u shaped rare earth element ree patterns suggesting that they are residues after moderate to high degrees of partial melting and were modified by infiltration of light rare earth element lree enriched boninitic melts the xigaze crustal rocks belong to two groups mid ocean ridge basalt morb like rocks and boninitic rocks showing a uniform lree depletion and flat to lree enrichment on chondrite normalized patterns respectively geochemically both groups show the influence of subducting oceanic slab derived fluids la icpms zircon u pb and lu hf analyses from dolerite and quartz diorite dikes which intruded into the mantle peridotite and dolerite sheeted sills show that they were generated between 127 and 124 ma the zircons possess positive ehf t values ranging from 7 5 to 17 3 taking into account the geological and geochronological characteristics of the central western yzsz we propose that ophiolites in this region formed in a forearc spreading setting through rapid slab roll back during subduction initiation between 130 and 120 ma following this stage of spreading the forearc was stabilized and the zone of melting migrated beneath the gangdese arc producing the voluminous late cretaceous granitoids displaying depleted mantle type hf isotopic compositions our model provides a new explanation for the generation and evolution of forearc type ophiolites

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

crystal size distribution of plagioclase and amphibole from soufriere hills volcano montserrat evidence for dynamic crystallization textural coarsening cycles the crystal size distributions csds of plagioclase and amphibole were determined from andesites of the soufriere hills volcano montserrat plagioclase occurs as separate crystals and as chadocrysts in large amphibole oikocrysts the chadocrysts represent an earlier stage of textural development preserved by growth of the oikocryst seventeen rock and eight chadocryst plagioclase csds are considered together as a series of samples of textural development all are curved concave up and coincident differing only in their maximum crystal size three amphibole csds have a similar shape and behaviour but at a different position from the plagioclase csds a dynamic model is proposed for the origin of textures in these rocks crystallization of plagioclase started following emplacement of andesite magma at a depth of at least 5km a steep straight csd developed by nucleation and growth this process was interrupted by the injection of mafic magma into the chamber or convective overturn of hotter magma the magma temperature rose until it was buffered initially by plagioclase solution and later by crystallization during this period textural coarsening ostwald ripening of plagioclase and amphibole occurred small crystals dissolved simultaneously with the growth of large crystals the csd became less steep and extended to larger crystal sizes early stages of this process are preserved in coarsened amphibole oikocrysts repetitions of this cycle generated the observed family of csds textural coarsening followed the communicating neighbours model hence each crystal has its own unique growth solution history without appealing to mixing of magmas that crystallized in different environments

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

partial melting of slab window margins genesis of adakitic and non adakitic magmas abstract when a mid ocean spreading ridge subducts it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window we examine the fate of the slab edges using simplified slab window geometries specific thermal parameters and assumptions regarding shear stress and slab hydration six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions non adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5 65 km whereas adakitic melts form proximal to the edges at depths of 25 90 km as anatexis proceeds the subducted crust is converted to a migmatite of slab melt and eclogitic restite much of the migmatite may shear away from the slab and become incorporated into the mantle the melts will rise and leave behind fragments of restite within mantle peridotite if the peridotite is part of the overriding plate then the restite fragments may become long term residents of the continental lithospheric mantle however if the restite becomes entrained in the asthenosphere it may undergo upwelling and melting or flow away as ductile streaks to become long term mantle heterogeneities slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle