concentration mechanisms of rare earth element nb zr be mineralization in the baerzhe deposit northeast china insights from textural and chemical features of amphibole and rare metal minerals

an interpretation of the active deformation of southern taiwan based on numerical simulation and gps studies abstract the fold and thrust belt of taiwan results from oblique convergence between the eurasia and the philippine sea plate and its front structures are strongly influenced by the presence of large horsts and basins in the foreland using a 2 d numerical modelling finite element and distinct element methods we evaluate the influences on the active deformation of southern taiwan of 1 the foreland structural highs 2 the major fault zones in the belt and 3 the presence of a subduction zone to the south to constrain our models we take into account for the first time the present day velocity field of southern taiwan estimated based on new geodetic data obtained through the global positioning system gps particular attention is paid to the role of geological discontinuities through an evaluation of the presence and role of mechanical decoupling along major faults which plays an important role in the distribution of the regional and local velocity and stress patterns this particular analysis of the behaviour and influence of weak shear zones in taiwan is carried out by using for the first time the distinct element method additional 3 d distinct element modelling allows better consideration of oblique shearing such as for the longitudinal valley fault of eastern taiwan we conclude that the active velocity field and tectonic stress pattern in southwestern taiwan strongly depend on 1 the presence and shape of the peikang high 2 the presence of the major active regional discontinuities the longitudinal valley fault and the major thrusts of western taiwan and 3 the neighbouring weakness zone of the accretionary prism of the northern manila subduction zone and cannot be explained by any of these factors taken solely

fluvio lacustrine sedimentation and volcanism in a late carboniferous tensional intra arc basin northern chile abstract extensive outcrops of late carboniferous to triassic volcanoplutonic magnetic arc complexes occur in the andes of northern chile in the salar de atacama area terrestrial volcanosedimentary successions include a 200 600 m thick fluvio lacustrine sequence miembro medio the terrestrial basin which accommodated this sequence formed during the latest carboniferous on top of the deposits of the pre existing carboniferous composite volcanoes the lower part of the miembro medio consists of green limnic and multicoloured alluvial fan deposits the upper part is made up of red fluvial sedimentary rocks basic volcanic rocks occur locally throughout the sequence the climate is inferred to have been warm and humid the limnic freshwater environment had a minimum extension of 300 100 km limnic facies is inferred from the existence of fine parallel bedded sediments ooliths and a limnic benthic fauna lake shore deposits prevail in the outcrops in some sections the green limnic sequence is followed by red fine grained floodplain or alluvial plain and fluvial channel deposits the framework composition of the miembro medio is dominated by volcanogenic detritus presumably eroded from the successions of the previous late carboniferous volcanic arc another volcanogenic clast type was provided by syndepositional basalto andesitic phreatomagmatic activity which also produced peperitic sills dykes and lavas the limnic alluvial fan facies association the predominance of volcanogenic detritus and the accompanying intrabasinal basic calc alkaline volcanism together with the considerable size of the basin point to a nw se trending half graben setting controlled by trans tensional arc tectonics with regard to the late carboniferous triassic volcanosedimentary successions of northern chile this latest carboniferous tensional setting is the only feature in this area supporting those models which inferred an extensional magmatic arc for the entire central and southern andes during the permo triassic

tectonics of short lived intra arc basins in the arc continent collision terrane of the coastal range eastern taiwan the coastal range in eastern taiwan was originated from an oblique collision between the luzon volcanic arc and asian continent since the late neogene in this collision terrane two intra arc basins the pliocene chingpu and pleistocene chengkung basins were developed on the eastern part of the neogene chimei and chengkuangao volcanic islands respectively prior to their accretion to eastern taiwan the tectonic evolution of these neogene volcanic islands and associated intra arc basins is reconstructed by stratigraphic and sedimentological analysis igneous rock geochemistry and comparison with observations in modern collision zone in the regions off southeastern taiwan in the coastal range the intra arc basin sequences are 1 5 10 km wide and 40 km long comparable in size to their modern analogues in the active collision zone the basin axis trends subparallel to the volcanic ridge in both basins deepwater flysch overlies shallow marine reef carbonates which in turn rest on volcanic basement indicating rapid arc collapse minimum rate of 1 km m y soon after the arc continent collision the arc collapse occurred earlier in the north chimei between 5 1 and 3 5 ma and later in the south chengkuangao between 2 9 and 1 8 ma in concert with a southward propagation of the oblique collision sedimentation ended about 2 ma and 1 ma in the chingpu and chengkung basins respectively coeval with rotation of the neogene volcanic islands this suggests that the rotation inverted the intra arc basin into thrusting uplifting and final emergence thus the duration of sedimentation for the intra arc basins spanned only about 0 8 3 1 m y on the basis of land geology offshore observations and a clay model experiment simulating oblique arc continent collision a model for the intra arc basin evolution in eastern taiwan is proposed during the collision strike slip faults would have been developed in the eastern part of volcanic islands to induce transtension movements thus forming pull apart intra arc basins on the collapsed volcanic island this mechanism is believed to be responsible for the formation of the pliocene chingpu and pleistocene chengkung basins as well as the present day offshore intra arc basins found on the lutao and lanhsu volcanic islands the two intra arc basins on lutao and lanhsu are predicted to be short lived as collision continues these two basins together with their underlying northern part of the luzon arc will be rotated thrust and uplifted in the next 1 m y and finally will become part of the southern extension of the coastal range

comprehensive chemical analyses of natural cordierites implications for exchange mechanisms abstract cordierite samples from pegmatites and metamorphic rocks have been analysed for major electron microprobe analysis empa and trace elements inductively coupled plasma mass spectrometry icp ms secondary ion mass spectrometry analyses sims as well as for h 2 o and co 2 coulometric titration and the results evaluated in conjunction with published data in order to determine which exchange mechanisms are significant apart from the homovalent substitutions femg 1 and mnmg 1 on the octahedral site some minor kna 1 on the ch0 channel site and fe 3 al 1 on the t 1 1 tetrahedral site the three most important substitution mechanisms are those for the incorporation of li on the octahedral sites nali 1 mg 1 and of be and other divalent cations on the tetrahedral t 1 1 site nabe 1 al 1 and na mg fe 2 1 al 1 the dominant role of the last vector is clearly demonstrated we propose a new generalized formula for cordierite ch na k 0 1 vi mg fe 2 mn li 2 iv si 5 iv al 3 iv al be mg fe 2 fe 3 o 18 x ch h 2 o co 2 our results show that the population of mg fe 2 on the t 1 1 site is limited to about 0 08 a p f u other exchange mechanisms that were encountered in experiments operate only under p t conditions or in bulk compositions that are rarely realized in nature routine analyses by electron microprobe in which li and be are not determined can be plotted as mg fe mn versus si al to assess whether significant amounts of li and be could be present these amounts can be calculated as li a p f u al na 4 and be a p f u 10 2al m 2 na

facies analysis of an ancient dismembered large caldera complex and implications for intra arc subsidence middle jurassic strata of cobre ridge southern arizona usa abstract the middle jurassic 170 ma cobre ridge caldera is an elongate caldera complex that formed during the eruption of the tuff of pajarito a crystal rich rhyodacite ignimbrite with an estimated volume 1000 km 3 the caldera subsided in two subequal blocks to the southeast caldera forming ignimbrite is as much as 3000 m thick whereas the northwest half of the caldera subsided to lesser depths during the initial eruption but was the locus of subsidence during subsequent eruptions providing a depocenter for a 1 5 km thick section of secondary and post collapse volcanic and sedimentary strata these strata record moat formation and filling and waning volcanism including 1 areally restricted ignimbrites up to several tens of meters thick that may have been ponded between caldera margins and or fault scarps 2 a 600 m thick ignimbrite whose eruption probably caused further collapse of the caldera or enlargement of the caldera margins 3 a localized debris apron deposit up to 500 m thick that represents material reworked from intra caldera pyroclastic deposits and possibly caldera walls and 4 eolian and fluvial sandstones and water lain tuffs megabreccia blocks as large as 0 5 km along strike probably demarcate approximate caldera margins suggesting that the composite caldera was approximately 50 km long by 25 km wide the preserved thickness of the strata of cobre ridge 4500 m is greater than many ancient continental volcanic sequences suggesting external i e tectonic controls on subsidence but preservation of this great thickness of strata is apparently due entirely to volcanic subsidence

the marinduque intra arc basin philippines basin genesis and in situ ophiolite development in a strike slip setting the marinduque basin is a marine intra arc basin in the north central philippine volcanic arc system a suite of marine geophysical observations show the basin as a whole to be rhombic in shape with its long axis trending north northwest a conspicuous volcanic ridge trends east northeast across the basin dividing it into two smaller active depocenters symmetric linear magnetic anomalies striking parallel to and centered over the central volcanic ridge indicate that it formed by extension in a north south direction by a process analogous to sea floor spreading geometric and temporal relations between the central volcanic ridge and several en echelon sets of north trending left stepping faults indicate that sea floor spreading occurred in the extensional stepover zone of a left lateral strike slip fault system and that the marinduque basin as a whole is a composite pull apart basin whose floor is in part composed of oceanic type crust the evolution of the central volcanic ridge presents an actualistic model for in situ development and emplacement of ophiolites in an island arc setting the presently active left lateral philippine fault zone truncated the eastern flank of the basin as well as the central volcanic ridge following their initial formation shortening associated with a probable restraining bend in the philippine fault has resulted in uplift of basin flank sediments which have been thrust back toward the basin interior the philippine fault has also displaced fragments of the ridge and basin floor toward the north the overall history of the marinduque basin further suggests that strike slip processes may play an important role in the origin and growth of intra arc basins such basins may ultimately become isolated in a forearc or backarc setting

collapse of the luzon volcanic arc during onset of arc continent collision evidence from a miocene pliocene unconformity eastern taiwan a regional miocene pliocene unconformity in the coastal range of eastern taiwan provides information about vertical motions that affected the colliding volcanic arc during initiation of arc continent collision the unconformity separates precollisional volcanic and volcaniclastic rocks of the miocene luzon arc and forearc basin from an overlying thick sequence of syncollisional pliocene pleistocene sedimentary rocks derived from the metasedimentary accretionary wedge of taiwan the duration of time represented by the unconformity averages 1 2 m y and locally ranges up to 4 m y the basal unit of the younger syncollisional sequence consists of bathyal mega slump folded mudstone pebbly mudstone and thin bedded turbidites sharp stratigraphic juxtaposition of these deep marine deposits directly above shallow marine limestone and epiclastic rocks upper part of the older arc sequence requires extremely rapid subsidence during initiation of the collisional basin the coastal range unconformity is interpreted to represent a period of latest miocene nondeposition and or erosion in the luzon arc that was related to initial subduction of the eurasian continental margin along the manila trench this period was followed in pliocene time by very rapid subsidence or collapse of the arc platform to 1 2 km water depth there are two dissimilar mechanisms of basin subsidence that may have controlled initiation of the collisional basin 1 extension rifting and crustal thinning in the luzon arc massif to form a series of small rift or pull apart subbasins or 2 flexural subsidence of oceanic lithosphere in a foreland style basin due to crustal loading in the expanding accretionary wedge of proto taiwan although data are insufficient to provide a unique solution the flexural subsidence model is supported by systematic younging of the initial subsidence event from west to east which may record eastward migration of an outer flexural bulge notable differences between the unconformity and paleogeography of the early collisional basin and analogous present day basins in the offshore region to the south suggest that arc continent collision and basin development in eastern taiwan have experienced secular change since late pliocene time

a geophysical study of the manila trench luzon philippines 2 fore arc basin structural and stratigraphic evolution the pattern of sediment distribution within the manila trench subduction zone west of luzon exerts a strong influence on the tectonic evolution of the accretionary prism and the fore arc basins of this convergent plate margin multichannel common depth point seismic reflection data reveal up to 2600 m of turbidite sediments overlying a thin pelagic hemipelagic sediment layer and oceanic basement in the northern part of the manila trench these turbidites were transported from the uplifted collision zone of taiwan southward along the manila trench by gravity controlled processes similarly thinner turbidite deposits in the southern end of the trench were transported northward from the collision zone involving the north palawan block and the central philippines sedimentation within the manila trench is dominated by sources at the ends of the trench with longitudinal transport along the trench axis in contrast the fore arc basin system of the manila trench receives sediment through submarine canyons incised into the west luzon continental slope the rate of sediment supply to the fore arc basin system is not directly related to the rate of sediment supply to the trench the size of the accretionary prism varies significantly along strike of the trench and it strongly correlated with the local thickness of turbidite sediments within the trench a thickness of more than 200 m of trench turbidites is probably required before accretion and uplift become the dominant process affecting the fore arc region this generalization may apply to other convergent plate margins as well the uplift rate of the accretionary prism probably varies dramatically along the trench and is largely independent of variations in the local sediment accumulation rates in the fore arc basins this variability results in the formation of several different types of fore arc morphology along the manila trench the overall seismic stratigraphy of the west luzon trough fore arc basin is remarkably uniform with seismic sequences continuous for up to 110 km along strike the unconformities that bound these seismic sequences are probably the result of a complex interplay of relatively long term approximately 10 ma uplift of the accretionary prism and relatively short term approximately 1 2 ma variations in sediment supply to the fore arc basin modulated by eustatic sea level changes

rare gases in cyclosilicates and cogenetic minerals the component and isotopic composition of the rare gases in beryls cogenetic pegmatite minerals and metamorphic cordierites has been measured beryls contain large amounts of excess radiogenic gases and significant amounts of nonradiogenic gases beryls contain the most pristine samples of the rare gases in pegmatite volatile phases but have imposed significant elemental fractionations during crystallization both the excess radiogenic and nonradiogenic components are progressively depleted in the heavy gases relative to the light gases other pegmetite minerals contain rare gases with much smaller excess radiogenic components and apparently did not sample the same source as did the beryls the other minerals exhibit varying but mineral specific fractionation patterns for the nonradiogenic rare gases cordierites also contain large amounts of excess radiogenic gases and significant amounts of nonradiogenic gases in addition cordierites show a strong consistent fractionation of both gas components the fractionation pattern is significantly different from that found in beryls and is characterized by an enrichment of xe relative to the ar though both gases are depleted relative to he the cyclosilicates can serve as probes of the rare gases present in the volatile phases when the cyclosilicates crystallized the cyclosilicates contain a record of the rare gases present in geologically interesting environments over a wide range of time and physical conditions

khmaralite a new beryllium bearing mineral related to sapphirine a superstructure resulting from partial ordering of be al and si on tetrahedral sites khmaralite ca sub 0 04 mg sub 5 46 fe super 3 sub 0 12 fe super 2 sub 1 87 al sub 14 26 be sub 1 43 b sub 0 02 si sub 4 80 o 40 is a new mineral closely related to sapphirine from khmara bay enderby land antarctica it occurs in a pegmatite metamorphosed at t or 820 degrees c p or 10 kbar the minerals surinamite musgravite and sillimanite associated with khmaralite at casey bay saturate it in beo and thus its beo content could be close to the maximum possible optically khmaralite is biaxial at lambda 589 nm alpha 1 725 2 beta 1 740 2 gamma 1 741 2 2v meas 34 4 1 8 degrees v r strong and beta

rapid intra arc rifting in miocene northeast japan although life times of major continental rifts are tens of millions of years intra arc rifting in miocene ne japan lasted for only about 3 m y before back arc spreading began the rate of initial subsidence was over 1 km m y much faster than that in major continental rifts thus this intra arc rift evolved 10 times as fast as major intracontinental rifts the large temperature dependence of lithosphere rheology and the hot thermal regime of island arc lithosphere account for the rapid rifting less than 1 m y before the rifting spreading transition the inner ne japan arc subsided from shallow marine depths to middle or lower bathyal depths 1 0 2 5 km the average total basement subsidence was 2 3 km during the rifting the rapid submergence occurred during one third of the duration of rifting but accounts for two thirds of the total subsidence during rifting namely the initial subsidence rate accelerated toward the time of breakup of the arc crust at the same time the region along the present pacific coast remained only shallow marine or terrestrial throughout presumably influenced by global eustacy

solubility and stability of beryl in granitic melts beryllium contents of granitic melts at saturation in beryl or chrysoberyl phenakite or quartz have been determined by forward and reverse direction experiments at 650 850 degrees c and 200 mpa using natural beryl or the compositionally equivalent mixture of phenakite alumina quartz added to metaluminous haplogranite peraluminous haplogranite and macusanite a peraluminous li b f and p enriched rhyolite obsidian from macusani peru chrysoberyl coexists with beryl or quartz in moderately to strongly peraluminous systems asi approximately or 1 05 the assemblage phenakite beryl chrysoberyl is stable in silica undersaturated bulk composition the beo content of the melts which ranged between 0 03 0 40 wt varies principally with temperature the solubility of beryl falls with decreasing t and with the activity product of beryl alpha beo 3 alpha al2o3 alpha sio2 6 beryllia contents are lowest in strongly peraluminous and quartz saturated haplogranite melts however the highest beo content measured in glass products occurs for the strongly peraluminous macusanite at 850 degrees c 4016 ppm we infer that the greater solubility of beryl in macusanite melt results from melt speciation reactions involving li f b and p for all bulk compositions studied the beo content of melt projects to a narrow range of similar values at low t near the solidus of haplogranite thus metaluminous to peraluminous granitic magmas with and without common volatile and fluxing components will if cooled to subsolidus temperatures have similarly low beo requirements for beryl saturation the small beo content of melt near the solidus of haplogranite results in a minor 10 degrees c depression of the freezing point compared to be free haplogranite melt the mean melt composition at the beryl saturated granite minimum shifts slightly toward quartz beryl is common in peraluminous granitic rocks in part because lower beo contents are required to saturate these melts in beryl and possibly because these melts acquire higher beo contents by mica melting reactions at their sources beryl is also a common constituent of border facies assemblages in granitic pegmatite dikes regardless of their asi values this early crystallization of beryl results from the low beo content required to saturate any granitic melt in beryl at the low t environments in which pegmatite dikes solidify

strike slip faults offshore southern taiwan implications for the oblique arc continent collision processes taiwan is the site of present day oblique arc continent collision between the luzon arc of the philippine sea plate and the chinese continental margin the major structural pattern revealed from marine geophysical studies in the area offshore southern taiwan is that of a doubly vergent orogenic belt bounded by significant zones of thrusting on the west and east of the submarine accretionary wedge due to the oblique collision process strike slip faults could play an important role in this convergent domain topographic lineaments revealed from new digital bathymetry data and seismic reflection profiles confirm the existence of three sets of strike slip faults in the collision subduction zone offshore southern taiwan the n s trending left lateral strike slip faults within the luzon volcanic arc the ne sw trending right lateral strike slip faults across the accretionary wedge and the nne ssw trending left lateral strike slip faults lie in the frontal portion of the accretionary wedge these strike slip faults overprint pre existing folds and thrusts and may convert into oblique thrusts or thrusts as the forearc blocks accrete to the mountain belt a bookshelf rotation model is used to explain the observed geometrical relationships of these strike slip fault systems based on this model the counter clockwise rotation of the forearc blocks in the area offshore southern taiwan could have caused extrusion of the accretionary wedge material into the forearc basin the originally continuous forearc basin is thus deformed into several closed and separate proto collisional basins such as the southern longitudinal trough and taitung trough a tectonic evolution model which emphasizes on the development of various structures at different stages of the oblique arc continent collision for the taiwan mountain belt is proposed

geochemical and petrogenetic features of mineralization in rare element granitic pegmatites in the light of current research abstract tremendous expansion of high technology applications of rare metals and specialty minerals requires periodic reassessment of their geological sources keeping in mind the long range technological potential of all hi tech materials available from a given type of deposit granitic pegmatites are a classic example of diversified sources li rb cs be ga sc y ree sn nb ta u th zr hf optical quartz and fluorite high purity feldpar petatite and refractory spodumene ceramic amblygonite e tc such rare element pegmatites can be subdivided into three principal families lct pegmatites with li rb cs be ga sn ta nb b p f nyf pegmatites with nb ta ti y ree zr th and u f pegmatites with a mixed geochemical signature the less evolved members of the lct family produce mainly only be nb ta and sn but exotic types with be ta nb f b p with li rb and cs and are also known so far no differences ini mineralization styles have been observed between the spodumene petalite and amblygonite subtypes of the complex type except the mineralogy of their substantial li contents however extensive divergence in k rb cs fractionation is observed violating the classic criteria for for example pollucite potential and economic ga concentration is recognized the lepidolite subtype is commonly enriched in y and h ree albitespodumene pegmatites have largely simple li sn nb ta be mineralization but a suprisingly broad range of alkali fractionation levels the not so common albite type seems to be typically enriched in sn nb ta ti the nyf family is much less abundant a general subdivision of these pegmatites is not yet available except for the recognition of sporadic lree enriched and much more widespread y hree enriched subtypes scandium is an element recognized recently as much more abundant than previously thought internal evolution of individual pegmatites is better understood today in terms of london s model of crystallization from highly hydrous but homogeneous melts experimental evidence does not support the jahns and burnham proposal of early separation of a fluid phase peraluminous fertile s granites generating lct pegmatites are derived by partial melting of upper middle crustal rocks undergoing their first anatexis and the pegmatites commonly show regional zoning metaluminous granites yeilding poorly zoned groups of nyf pegmatites are largely of the a type generated by second melting of short lived depleted lower crustal protoliths however a juvenile igneous or fluid component may be involved mixed nyf lct granite pegmatite suites may originate in several ways peraluminous lct sequences were traditionally regarded as orogenic in contrast to anorogenic nyf systems tectonic correlation with geochemistry is now considered secondary to the control by source lithologies it is the understanding of the association of pegmatite types with their geological setting and petrogenetic affiliation which assists in the search for selected hi tech materials

plate convergence and deformation north luzon ridge philippines abstract marine geophysical and earthquake seismology data indicate that the north luzon ridge a volcano capped bathymetrie ridge system that extends between luzon and taiwan is presently undergoing deformation in response to the relative motion between the asian and philippine sea plates plate motion models predict convergence along the western side of the philippine sea plate from japan in the north to indonesia in the south and most of this plate margin is defined by active subduction zones however the western boundary of the philippine sea plate adjacent to the north luzon ridge shows no evidence of an active wnw dipping subduction zone this is in marked contrast to the presence of both the philippine trench east luzon trough subduction zones to the south and the ryukyu trench subduction zone to the north crustal shortening in response to ongoing plate convergence in the north luzon ridge region apparently takes place through a complex pattern of strike slip and thrust faulting rather than by the typical subduction of oceanic lithosphere along a discreet zone the curvilinear bathymetrie trends within the north luzon ridge represent the traces of active faults the distribution of these faults mapped by both multichannel and single channel seismic reflection methods and earthquake seismicity patterns and focal mechanism solutions suggest that right lateral oblique slip faulting occurs along ne trending faults and left lateral oblique slip faulting takes place on n and nnw trending faults the relative plate convergence accommodated by the deformation of the north luzon ridge will probably be taken up in the future by the northward propagating east luzon trough subduction zone

marine geology in the arc continent collision zone off southeastern taiwan implications for late neogene evolution of the coastal range abstract marine geology in the region between taiwan and luzon is characterized by a convergent tectonics in the south and an oblique arc continent collision in the north in the collision zone six geologic provinces are recognized they are the manila trench the accretionary prism of the hengchun ridge the sutural basin of the southern longitudinal trough the subduction melange of the huatung ridge the forearc basins of the taitung and the north luzon troughs and the volcanic arc of the north luzon ridge the oceanic crust of the south china sea is subducting eastward beneath the philippine sea plate along the manila trench south of 20 n further to the north the trench has changed progressively its axis toward the west from the subduction melanges off southeastern taiwan at 3 ma to the hengchun peninsula of southern taiwan at 2 1 ma and then to the present day location off southwestern taiwan because of the continuing collision and the westward shifting of the manila trench sediments deposited in the region are passively incorporated into the accretionary prism and deformed to the hengchun ridge after the collision the southern longitudinal trough consisting of a 2 km thick of orogenic sediments represents a sutural basin superposed on the plate boundary between the accretionary prism of the hengchun ridge asian continent and the subduction melange of the huatung ridge philippine sea plate the huatung ridge is a bathymetric high connected to the southern coastal range and is characteristic of low negative free air gravity and magnetic anomalies the ridge contains deeply weathered sedimentary blocks with abraded deep water late pliocene foraminifera and identical clay mineral compositions as those in the onshore lichi melange this implies that at least a portion of the huatung ridge is composed of melange materials and represents a former subduction zone extending from the lichi melange of the southern coastal range the north luzon trough and the taitung trough are the constructed foreac basins with 2 km of orogenic sediments unconformably overlying the volcanic islands and the subduction melange the taitung trough is southward connected to the north luzon trough through a bathymetric passage which was formed by narrowing the north luzon trough as a consequence of clockwise rotation of the lutao volcanic island during the late pleistocene a detailed study of the four neogene tectonostratigraphic units in the coastal range shows that the overall history of the formation of the range can be related to the present day geologic provinces and the on going collision processes in the region off southeastern taiwan the range had evolved from a southward accretion of the shuilien loho remnant forearc basin and the chimei volcanic pair at 2 ma followed by the chengkung taiyuan remnant forearc basin and the chengkuangao volcanic island pair at 1 ma and the taitung trough and the lutao volcanic island pair presently in process by analogy the northern part of the north luzon trough and the lanhsu volcanic island will be accreted to the coastal range in the next million years the paired forearc basin volcanic island accretion model proposed in this study may thus have implications for mountain ranges that evolved from similar tectonics of oblique arc continent collision

onland signatures of the palawan microcontinental block and philippine mobile belt collision and crustal growth process a review abstract the collision of the palawan microcontinental block with the philippine mobile belt had significantly influenced the geological evolution of the philippines multiple collisions involving several fragments through space and time resulted into the collage of terranes of varying origin exposed in this part of central philippines cusping of the overriding plate volcanic arc gap ophiolite emplacement incipient back arc rifting island rotation and tilting raised coastal terraces metamorphism intrusion of igneous rocks and steepened subducted slab as seen in focal mechanism solutions are some of the manifestations of this collision a late early miocene to early middle miocene age 20 16 ma is proposed for the major collision between the palawan indenter and the philippine mobile belt the collision boundary is located from the northern part of mindoro through the central mountain range swinging east of sibuyan island in the romblon island group and finally threading along the buruanga peninsula and eastern side of the antique ophiolite complex before exiting and connecting with the negros trench the collision through accretion and crustal thickening has contributed to the crustal growth of the philippine archipelago

be daughter minerals in fluid and melt inclusions implications for the enrichment of be in granite pegmatite systems the study of re homogenized melt inclusions in the same growth planes of quartz of pegmatites genetically linked to the variscan granite of the ehrenfriedersdorf complex erzgebirge germany by ion microprobe analyses has determined high concentrations of be up to 10 000 ppm in one type of melt inclusion as well as moderate concentrations in the 100 ppm range in a second type of melt inclusion generally the high be concentrations are associated with the h2o and other volatile rich type b melt inclusions and the lower be concentration levels are connected to h2o poor type a melt inclusions both inclusion types representing conjugate melt pairs are formed by a liquid liquid immiscibility separation process this extremely strong and very systematic scattering in be provides insights into the origin of be concentration and transport mechanisms in pegmatite forming melts in this contribution we present more than 250 new analytical data and show with ion microprobe and fs la icpms studies on quenched glasses as well as with confocal raman spectroscopy of daughter minerals in unheated melt inclusions that the concentrations of be may achieve such extreme levels during melt melt immiscibility of h2o b f p li enriched pegmatite forming magmas starting from host granite with about 10 ppm be melt inclusions with 10 000 ppm be correspond to enrichment by a factor of over 1 000 this strong enrichment of be is the result of processes of fractional crystallization and further enrichment in melt patches of pegmatite bodies due to melt melt immiscibility at fluid saturation we also draw additional conclusions regarding the speciation of be in pegmatite forming melt systems from investigation of the be bearing daughter mineral phases in the most h2o rich melt inclusions in the case of evolved volatile and h2o rich pegmatite systems b p and carbonates are important for the enrichment and formation of stable be complexes

mountain building in taiwan insights from 3 d geodynamic models taiwan is widely considered to be a typical example of an arc continent collision surrounded by two opposite dipping subduction zones the manner by which the interaction of the two neighboring slabs caused plate collision and mountain building is insufficiently understood various hypotheses have been proposed but the geodynamic feasibility of those remains to be tested here we present 3 d thermomechanical models to study the geodynamic evolution process of a taiwan like setting after an initial transform fault was consumed in our model setup the boundary between the eurasian plate and the south china sea is northeast trending the results show that all simulations result in toroidal mantle flow around the slab edges and that slab breakoff as well as a small scale mountain belt with high topography and crustal exhumation occurs in most cases the eurasian continental crust is exhumed in a dome like manner exposing higher grade metamorphic rocks facilitated by high erosion rates and a weak continental lower crust rheology but inhibited by the presence of a weak arc a high topography within the orogen as well as continental slab detachment can develop for the convergence direction of n307 degrees and large convergence rates our modeling results are thus generally consistent with the eurasian slab tearing model proposed for taiwan based on seismic tomographic studies and we suggest that the main characteristic features in taiwan can be explained by the combined effects of fast erosion a weak lower crust fast convergence and a small convergence azimuth

holocene mean uplift rates across an active plate collision boundary in taiwan samples of holocene fossil coral from uplifted reefs of three tectonically distinct yet geographically proximal regions of taiwan have been dated by uranium series and 14c isotopes applying corrections for altitude change caused by sea level fluctuations enables evaluation of long term average holocene uplift rates for three areas across an active convergent margin i the hengchun peninsula of the eurasian tectonic plate ii the eastern coastal range of taiwan a plate boundary and iii two offshore islands lanyu and lutao both situated on the leading edge of the adjoining philippine plate the data indicate that while all three areas have experienced uplift through the holocene plate collision has caused significantly higher uplift rates in the region directly along the plate boundary

transverse intra arc rifting palaeogene wagwater belt jamaica abstract the wagwater belt in eastern jamaica is an exceptionally well exposed example of a transverse intra arc rift formed during an abrupt transition from convergent to strike slip tectonics the belt consists of the uplifted wagwater half graben containing at least 5 6 km of coarse clastic terrestrial sedimentary rocks which is overlain and flanked by a much wider basin filled with about 1 2 km of upward fining marine sedimentary rocks neogene strike slip movements structurally inverted the half graben and exposed thick sections of both the half graben and overlying basin early palaeocene early eocene rifting preceded the middle eocene formation of oceanic crust in the cayman trough pull apart basin and affected a 50000 km 2 area near the eastern distal end of the pull apart fifteen half grabens identified in jamaica and in offshore areas using seismic profiling range in strike from north east to north west rifts in jamaica are presently oblique to the generally east west strike of plutons and sedimentary facies belts within a maestrichtian palaeocene island arc based on their apparent relationship to the strike of the enclosing palaeo arc we interpret the jamaican rifts as transverse intra arc rifts following the deposition of palaeocene redbeds and evaporites wagwater formation in the half graben subsidence was widespread in a marine sedimentary basin this marine basin was filled in early eocene time by marine conglomerates port maria and albany members of the richmond formation sandstone and siltstones roadside member and finally in the latest early eocene to earliest middle eocene time by a calcareous mudstone langley member a bimodal suite of volcanic rocks newcastle and nutfield volcanics was erupted during the deposition of the wagwater formation and the roadside member the structure and tectonic setting of transverse intra arc basins in jamaica is somewhat similar to rifts of the neogene pannonian basin in hungary in both areas an abrupt transition between convergent and strike slip tectonics resulted in transverse intra arc rifts striking almost perpendicular to a dying volcanic arc transverse intra arc rifts may be common features of small arc fringed plates such as the caribbean and pannonian plates that respond quickly to collision by strike slip motion towards an unopposed oceanic free face

seismogenic strain across the transition from fore arc slivering to collision in southern taiwan tectonic slivers have profound implications on the mechanical behavior of fore arcs and subduction zones southern taiwan is characterized by precollision fore arc slivering that likely exerts important controls on the evolution of collision inversion of focal mechanism solutions for seismogenic strain reveals spatially partitioned plate motion south of taiwan with dip slip faulting nearer the manila trench and strike slip faulting nearer the luzon arc to the north these kinematics are less clearly segregated and both types of faults appear to occur in the same volume of crust further north deformation is dominated by oblique slip on potentially reactivated steep reverse faults in the collision zone existing work on the active fault zones in taiwan suggests that the subduction to collision transition is marked by a releasing step between the strike slip fault that bounds the fore arc sliver and the oblique thrusts of the collision zone this configuration may help to explain the lack of fore arc basement and cover exposed in the suture zone marked by the longitudinal valley

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

chemical and paragenetic data on gadolinite group minerals from baveno and cuasso al monte southern alps italy a chemical and paragenetic study was performed on gadolinite group minerals occurring in miarolitic pink granite and granophyric leucogranite of the subvolcanic hercynian plutons at baveno and cuasso al monte southern alps italy in the localities investigated gadolinite group minerals are hosted in massive pegmatite in aplite and in miarolitic cavities having different degrees of evolution the petrological relations indicate that progressive crystallization has occurred from magmatic through to hydrothermal conditions at baveno ce rich gadolinite y with ree y formed during the primitive stages of pegmatite crystallization gadolinite y with ree y gadolinite y with ree y formed during intermediate stages of evolution and hingganite y is dominant in highly evolved miarolitic cavities together with several rare element phases the chemical differences observed in the gadolinites from the two localities may indicate a different parental magma composition and reflect a difference in the crystallization processes in contrast to baveno the crystallization at cuasso al monte occurred under open system conditions which prevented the formation of a zeolite datolite bearing stage and generated a typical medium to low temperature hydrothermal mineral assemblage consisting of quartz fluorite barite sulfides and carbonates the large variations in the y dy ratio observed in the studied samples may be due to a change in the fluorine abundance in hydrothermal fluids related to paragenetic effects and mixing processes this work reports paragenetic and chemical data obtained on gadolinite group minerals found in the miarolitic nyfpegmatites the granite and the granophyre of both the baveno and cuasso al monte localities these minerals are recognized in pegmatitic systems as crystallizing from primitive magmatic conditions through to highly evolved hydrothermal conditions which show extensive changes in their compositions particularly during the latest stages of evolution of the miarolitic cavities our results show that the gadolinite group minerals may be of petrological interest in the characterization of the pegmatitic pneumatolitic and hydrothermal stages of granite plutons geologic setting

taiwan mountain building insights from 2 d thermomechanical modelling of a rheologically stratified lithosphere summary the taiwan orogen has long been regarded as a case example for studying mountain building in association with subduction processes in this paper we present a fully coupled thermomechanical modelling of the taiwan collision based on a realistic viscous elastic plastic rheology it satisfactorily reproduces available thermochronometric data long short term deformation patterns heat flux and erosion sedimentation distribution across the taiwan orogeny we found that a deep seated flux of asian crustal material into the orogenic wedge should be invoked to counter balance observed exhumation and erosion in the central range however in contrast with recent thermokinematic models of exhumation and deformation suggesting that underplating plays a significant role we show that most constraints on exhumation and deformation can be more straightforwardly interpreted by the frontal accretion of the rheologically layered asian crust we finally infer that such a model is in better agreement with the basic expectation that the hot young and buoyant chinese continental margin should hardly be subducted beneath the cold old and dense oceanic plate of the philippines sea

formation and evolution of the hogtuva beryllium deposit norway metagranite in the hogtuva tectonic window nordland norway hosts several spatially restricted be mineralisations with phenakite as the main ore mineral geochronology of magmatic zircon zircon i indicates crystallisation of phenakite bearing and beryl free metaluminous granite at 1787 57 ma and of phenakite and beryl beryl i bearing peraluminous aplites and pegmatites at 1710 59 ma crystallisation of texturally distinct zircon zircon ii cogenetic with fluorite in peraluminous aplites and pegmatites occurred at 434 14 ma during caledonian metamorphism in be mineralised and barren metagranite breakdown of phenakite post dates growth of zircon ii and resulted in the crystallisation of metamorphic hogtuvaite ca2 text fe 3 2 text fe 3 3 si4bealo20 in metaluminous metagranite and metamorphic beryl beryl ii in peraluminous metamorphosed aplites and pegmatites qualitative and quantitative assessment of mineral reactions implies that be has not been mobilised to significant degrees during metamorphism in spite of the presence of a fluorine rich fluid and that phenakite underwent in situ metamorphic reactions hogtuvaite crystallised as a metamorphic phase at the expense of phenakite the anorthite component in albitic plagioclase magnetite and possibly clinopyroxene in peraluminous environments the breakdown of phenakite to metamorphic beryl ii is accompanied by the formation of albite a decreasing activity ratio of a k h in the metamorphic fluid results in the reaction of k feldspar and phenakite to beryl ii further decrease in a k h finally results in complete transformation of the remaining k feldspar into albite the metamorphic breakdown of phenakite in the distinct be mineralisation types at hogtuva is probably controlled by changes in the activities of na k and h in the coexisting metamorphic fluid

differential preservation in the geologic record of intraoceanic arc sedimentary and tectonic processes abstract records of ancient intraoceanic arc activity now preserved in continental suture zones are commonly used to reconstruct paleogeography and plate motion and to understand how continental crust is formed recycled and maintained through time however interpreting tectonic and sedimentary records from ancient terranes after arc continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others in this synthesis we examine what is lost and what is preserved in the translation from modern processes to the ancient record of intraoceanic arcs composition of accreted arc terranes differs as a function of arc continent collision geometry forward facing collision can accrete an oceanic arc on to either a passive or an active continental margin with the arc facing the continent and colliding trench and forearc side first in a backward facing collision involving two subduction zones with similar polarity the arc collides backarc first with an active continental margin the preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case preservation of arc terranes likely is biased towards those that were in a state of tectonic accretion for tens of millions of years before collision rather than tectonic erosion the prevalence of tectonic erosion in modern intraoceanic arcs implies that valuable records of arc processes are commonly destroyed even before the arc collides with a continent arc systems are most likely to undergo tectonic accretion shortly before forward facing collision with a continent and thus most forearc and accretionary prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity when sediment flux to the trench was greatest and tectonic accretion prevailed collision geometry and tectonic erosion vs accretion are important controls on the ultimate survival of material from the trench forearc arc massif intra arc basins and backarc basins and thus on how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of aseismic ridges and seamounts oblique plate convergence and arc rifting forward facing collision involves substantial recycling melting and fractionation of continent derived material during and after collision and so produces melts rich in silica and incompatible trace elements as a result forward facing collision can drive the composition of accreted arc crust toward that of average continental crust

age geochemical and isotopic variations in volcanic rocks from the coastal range of taiwan implications for magma generation in the northern luzon arc abstract this paper reports the first systematic analysis of age and geochemical variations in volcanic rocks from the coastal range of taiwan the northern luzon arc the rocks recovered from four main volcanoes vary from low k tholeiitic to medium k calc alkaline basalts to dacites the rocks are typical of arc magmatic products exhibiting enrichment in the large ion lithophile elements and depletion in the high field strength elements our new 40ar 39ar age data constrain the youngest eruption time in each of the four volcanoes i e from north to south at 7 2 ma yuemei 4 2 ma chimei 6 2 ma chengkuang ao and 8 5 ma tuluanshan respectively these data indicate that volcanism in the northern luzon arc did not cease progressively from north to south as previously alleged the high and broadly uniform nd isotope ratios end 10 1 to 8 8 and trace element characteristics of the rocks suggest a principal magma source from the depleted mantle wedge their overall geochemical variations are ascribed to magma chamber processes the effects of magmatic differentiation and crustal contamination differ among each volcano most likely owing to the discrepancy of residence time in individual magma chambers consequently we propose a binary mixing model for the magma generation that involves arc magmas sourced from the depleted mantle wedge and up to 5 crustal contamination with a continental fragment split off from the eurasian margin

evolution and petroleum geology of amlia and amukta intra arc summit basins aleutian ridge abstract amlia and amukta basins are the largest of many intra arc basins formed in late cenozoic time along the crest of the aleutian arc both basins are grabens filled with 2 5 km of arc derived sediment a complex system of normal faults deformed the basinal strata although initial deposits of late micocene age may be non marine in origin by early pliocene time most of the basinfill consisted of pelagic and hemipelagic debris and terrigenous turbidite deposits derived from wavebase and subaerial erosion of the arc s crestal areas late cenozoic volcanism along the arc commenced during or shortly after initial subsidence and greatly contributed to active deposition in amlia and amukta basins two groups of normal faults occur major boundary faults common to both basins and intra basin faults that arise primarily from arc parallel extension of the arc the most significant boundary fault amlia amukta fault is a south dipping growth fault striking parallel to the trend of the arc displacement across this fault forms a large half graben that is separated into the two depocentres of amlia and amukta basins by the formation of a late cenozoic volcanic centre seguam island faults of the second group reflect regional deformation of the arc and offset the basement floor as well as the overlying basinal section intra basin faults in amlia basin are predominantly aligned normal to the trend of the arc thereby indicating arc parallel extension those in amukta basin are aligned in multiple orientations and probably indicate a more complex mechanism of faulting displacement across intra basin faults is attributed to tectonic subsidence of the massif aided by depositional loading within the basins in addition most intra basin faults are listric and are associated with high growth rates although the hydrocarbon potential of amlia and amukta basins is difficult to assess based on existing data regional considerations imply that an adequate thermal history conducive to hydrocarbon generation has prevailed during the past 6 5 my the possibility for source rocks existing in the lower sections of the basins is suggested by exposures of middle and upper miocene carbonaceous mudstone on nearby atka island and the implication that euxinic conditions may have prevailed during the initial formation of the basins large structures have evolved to trap migrating hydrocarbons but questions remain concerning the preservation of primary porosity in a sedimentary section rich in reactive volcaniclastic debris

an assessment of 40ar39ar dating for the whole rock volcanic samples from the luzon arc near taiwan from electron microprobe petrographic and image analyses of samples from the northern tip of the luzon arc the effects of various alteration reactions phenocryst and groundmass assemblages on the whole rock 40ar39ar step heating spectra have been delineated the alteration of primary hornblende to an assemblage of zoisite chlorite sphene the palagonitization of glass and the crystallization of phillipsite tends to cause discordant 40ar39ar dates in the low temperature steps of the age spectra discordant 40ar39ar dates in the high temperature steps appear to result from degassing of inherited or trapped 40ar from coarse grained phenocrysts samples in which most of the k resides in groundmass plagioclase tend to yield meaningful plateau dates whereas samples in which most of the k resides primarily in palagonitized glass tend to yield meaningless plateau and isochron dates substantial loss of 39ar 13 40 occurred from irradiated samples prior to 40ar39ar step heating analysis which appears to be correlated with the abundance of groundmass and the degree of alteration this argon loss is attributed to thermally induced argon diffusion from the fine grained groundmass plagioclase glass and alteration phases during the pre step heating bakeout

a tear fault boundary between the taiwan orogen and the ryukyu subduction zone more than twenty seismic lines were analysed across the intersection of the taiwan orogen and the ryukyu arc forearc system the main results of this study concern the recognition of two superposed sedimentary basins that were previously unknown we have named the recent one the hoping basin and the older tectonized one the suao basin the suao basin containing sediments more than 3 km thick records subsidence of about 3 km on its southwestern portion after being shortened in the direction of plate convergence on the basis of seismicity gravity present day plate kinematics as well as seismic reflection data we relate the dramatic subsidence of the suao basin to the initiation of a wnw trending tear fault within the philippine sea plate this tear fault decouples the subducting philippine sea plate to the northeast and the apparent overthrusting philippine sea plate in the coastal range to the southwest back arc rifting may thus occur in the westernmost section of the okinawa trough above the sinking philippine sea slab while lithospheric shortening prevails south of hualien as a consequence the triangular area located southwest of the tear fault between the taiwan coastline and the toe of the yaeyama ridge no longer belongs to the ryukyu forearc it is now passively transported with the non subducting portion of the philippine sea plate we propose an evolutionary model for the relative chronology of tectono sedimentary events in this region over the last 5 m y

depleted deep south china sea δ13c paleoceanographic events in response to tectonic evolution in taiwan luzon strait since middle miocene abstract the most distinctive feature of the deep south china sea scs paleoceanography is the occurrence of long term depleted deep sea benthic foraminiferal 13c values they are lower than the global and the pacific composite records in the last 16 ma especially at 13 2 10 5 6 5 3 0 and 1 2 0 4 ma this distinct deep scs paleoceanograhic history coincides with the subduction collision history in the taiwan region where waters of the west pacific wp and the scs exchange the depleted deep sea benthic foraminiferal 13c events indicate that the scs deep basin became progressively a stagnant environment in the last 16 ma due to either closure of the connection with the wp bottom water or temporary reduction of the wp deep water flowing into the deep scs both the taiwan accretionary prism and the luzon arc became the main tectono morphological barriers for the wp bottom water flowing into the scs deep basin when eastward subduction of the scs oceanic lithosphere beneath the philippine sea plate started from the middle miocene 18 16 ma this began a long term trend of depleted scs deep sea benthic 13c values in the last 16 ma the oblique arc continent collision since 6 5 ma uplifted the taiwan accretionary prism rapidly above sea level and further isolated the scs from the open pacific the collision simultaneously causes backthrusting deformations in the north luzon trough forearc basin and sequentially closes interarc water gates between volcanic islands from north to south the loho and the taitung interarc water gates in the advanced collision zone were closed at 3 0 ma and 1 2 ma coinciding with the very low scs deep sea benthic 13c events at 3 0 and 1 2 0 4 ma respectively the taitung canyon between the lutao and lanyu volcanic islands in the incipient collision zone is semi closed presently these closure events also lead to the result that the wp deep water intrudes westward into the scs principally through the bashi channel between the lanyu and batan volcanic islands in the subduction zone

barrier island evolution middle atlantic shelf u s a part i shoreface dynamics abstract in part i of this paper near bottom fluid velocity and sediment concentration measurements from the shoreface of a middle atlantic barrier long island coast are analyzed to provide insight into the dynamics of erosional shoreface retreat in part ii these data are combined with observations of coastal stratigraphy in order to construct a model for the evolution and behavior of middle atlantic barriers wave motions on the long island shoreface tend to drive sediment onshore calculations of the onshore sediment flux driven by asymmetrical shoaling waves show that for the wave states commonly encountered on the middle atlantic shelf values become strongly positive landward of approximately 10 m water depth upper shoreface but drop to very low values shortly seaward of that isobath on the lower shoreface and inner shelf floor fair weather wave current interaction tends to cause a landward creep of sediment even though the wave orbital component of the near bottom velocity field is nearly symmetrical this movement occurs because the wave approach direction generally lies within 90 of the direction up coast or down coast of the alongshore wind driven current therefore fluid shear stress acting on the sediments tends to be reinforced during the shoreward stroke of the wave orbital motion and partially cancelled during the seaward stroke during major storms the rate of sediment transport increases by at least an order of magnitude and the role of wind driven currents becomes important much more sediment is resuspended by the wave orbital current component because storm waves are more powerful than fair weather swells but the wind driven flow component is also much stronger and is now available for transporting the resuspended sediment storm flows over the shoreface occur in distinct dynamic zones surf zone friction dominated zone transition zone geostrophic zone the zones are defined by dynamical considerations and expand or contract with the intensity of the causative wind stress during peak flow events the characteristic length scales of the friction dominated and geostrophic zones tend to correspond with the morphologic zones of the shoreface and inner shelf the storm intensified wind driven alongshore flows of the lower shoreface are frequently jet like in nature and may be upwelling or downwelling wind driven coastal flows with a strong downwelling component occur during most storms on the long island coast and are particularly important to the coastal sand budget during these events sand is entrained by the storm intensified upper shoreface circulation system of wave driven alongshore currents and rip currents and is fed into the main wind driven coastal current because of the offshore component of bottom flow sand is swept down the lower shoreface and onto the adjacent inner shelf fair weather processes may be unable to return storm deposited sand to the beach from such an offshore position the shoreface transport regime of the long island barrier coast thus consists of long periods of time months during which sand moves slowly toward the beach punctuated by short intense periods hours or days during which sand is transferred from the shoreface to the adjacent inner shelf the long term sense of movement of the shoreface whether retreating the long island case or prograding must depend on the loss or gain of sand by shoreface with respect to the inner shelf the data shows that the coastal sand budget is controlled not only by the upper shoreface cycle withdrawl of sand from the beach prism storage in the breakpoint bar and its subsequent return to the beach but by a cycle of larger spatial and temporal scale in which sand is exchanged between the shoreface as a whole and the adjacent inner shelf

geodetic measurement of horizontal crustal deformation in eastern taiwan abstract four medium aperture trilateration networks in eastern taiwan have been surveyed three to four times since 1981 one is the ilan network crossing an elongated active seismic zone the others are the hualien yuli and taitung networks located at the northernmost middle and southernmost portions of the longitudinal valley respectively based on changes of the observed line length the three components of the surface strain rate tensor for each of the networks are obtained by a least squares adjustment technique then the principal strain rates are calculated the ilan network gives a principal strain rate of uniaxial extension at 2 3 strain yr in the direction of n45 w the hualien network has a principal strain rate of 1 2 strain yr extension in n62 e and 1 2 strain yr contraction in n152 e the yuli network yields essentially a principal strain rate of uniaxial contraction at 8 4 strain yr in n117 e whereas the taitung network has a principal strain rate of 1 5 strain yr extension in n24 e and 3 9 strain yr contraction in n114 e the directions of contractionof both the yuli and taitung networks are consistent with the direction of the maximum compressive stress of this area furthermore average velocities of the relative motion between geodetic stations in the central range and the coastal range are estimated from the average rates of changes in line length stations in the hualien network show a left lateral relative motion in a direction more or less parallel to the strike of the longitudinal valley while stations in the yuli and taitung networks move toward each other in the direction approximately perpendicular to the trend of the longitudinal valley

plate boundary strain partitioning along the sinistral collision suture of the philippine and eurasian plates analysis of geodetic data and geological observation in southeastern taiwan crustal deformation and strain partitioning of oblique convergence between the philippine sea plate and the eurasian plate in the southern longitudinal valley of eastern taiwan were characterized based on geodetic analysis of trilateration network and geological field investigation the longitudinal valley fault one of the most active faults on taiwan branches into two individual faults in the southern longitudinal valley these two active faults bound the plio pleistocene pinanshan conglomerate massif between the coastal range the luzon island arc belonging to the philippine sea plate and the central range the metamorphic basement of the eurasian plate a geodetic trilateration network near the southern end of the valley shows a stable rate of the annual length changes during 1983 1990 the strain tensors for polygonal regions including triangular regions of the taitung trilateration network reveal that there are two distinct zones of deformation a zone of shortening thrusting between the pinanshan massif and the central range on the west and a strike slip movement between the pinanshan massif and the coastal range on the east the analysis of a discontinuity model consisting of three rigid blocks separated by two discontinuities has been carried out the results show that the deformation in this region can be characterized by two major faults a reverse fault is located between the plio pleistocene pinanshan massif and the metamorphic basement of the central range with a shortening rate of about 12 mm yr in the direction n280 e a strike slip fault is located principally along the river between the pinanshan massif and island arc system of the coastal range with a purely strike slip component of about 22 mm yr in the direction n353 e the analysis of the geodetic data further suggests that substantial deformation probably strike slip in type occurs within the pinanshan massif geological evidence of deformation in the plio pliestocene pinanshan conglomerate includes regional folding a conjugate set of strike slip fractures at the outcrop scale and morphological lineaments related to fracturing all indicating that the pinanshan massif is being deformed within a transpressive stress regime regional kinematic data indicate that a significant portion of the 82 mm yr of motion between the eurasian plate and the philippine sea plate is absorbed in the southern longitudinal valley by the decoupling of two distinct major faults the geometry of the oblique convergence and the rheology of the rock units the well consolidated plio pleistocene conglomerate and the sheared melange formation play the two important roles in the partitioning of crust deformation

strain modes within the forearc arc and back arc domains in the izu japan and taiwan arc continent collisional settings in this study i examine the strain modes of the forearc arc and back arc domains in arc continent collisional settings leading to arc material subduction delamination and or accretion the study focusses on two well documented colliding island arcs the izu bonin mariana ibm arc in japan and the luzon arc in taiwan both carried by the philippine sea plate firstly there is a body of evidence that both the ibm and the luzon arcs were built on the same late jurassic to early cretaceous proto philippine sea plate crust their internal structure is thus more heterogeneous than expected from paleogene or neogene supposedly intra oceanic island arcs secondly those arc systems and proximal back arcs have similar seismic characteristics attesting either for the presence of a middle crust with continental velocities and or serpentinized uppermost mantle that facilitate crustal shortening slivering and subsequent decoupling from the rest of the subducting plate it is shown that the proximal back arc domain called rear arc in case of paleoarc activity overlying the mantle wedge and the subducting slab may lose its strength if slab derived hydration occur decoupling then occurs below the moho arc delamination likely occurs in mid crustal levels because middle crust heated by nearby magmatism becomes weak accretion of arc material onto the upper plate depends on the characteristics of the arc itself and the geodynamic configuration most of the accreted material is probably underplated rather than frontally accreted

chiavennite from syenite pegmatites in the oslo region norway chiavennite occurs as reddish orange spherulites in vugs of syenite pegmatite at four different localities in the southern oslo region of norway chemical analysis and tga give a formula very close to camnbe2si5or3 oh 2 2hzo the mineral is orthorhombic a 8 866 7 b31 34 2 c4 787q a z 4 thestrongestreflections dina tr ft d ofthe powder pattern diffractometer data with preferred orientation are 15 7 55x020 7 84 25x040 5 e4 15x140 3 917 100x080 3 260 15 201 2 889 15x251 chiavennite crystals have the following properties hemimorphic spear shaped flattened on i0l0 with pyramid 161 eood to perfect cleavages on 100 010 and 001h d meas 2 56 glcm3 d calc 2 65 g cm3 for the ideal formula optically biaxial positive a 1 596 2 f l 6n 2 y l 6le 2 in white lieht 2vz calc 50 weak pleochroism in pale yellowish brown x z and optical orientation x a y b z c

lithological control on the deformation mechanism and the mode of fault slip on the longitudinal valley fault taiwan the longitudinal valley fault lvf in taiwan is creeping at shallow depth along its southern half where it is bounded by the lichi melange by contrast the northern segment of the lvf is locked where it is bounded by forearc sedimentary and volcanoclastic formations structural and petrographic investigations show that the lichi melange most probably formed as a result of internal deformation of the forearc when the continental shelf of south china collided with the luzon arc as a result of the subduction of the south china sea beneath the philippine sea plate the forearc formations constitute the protolith of the lichi melange it seems improbable that the mechanical properties of the minerals of the matrix illite chorite kaolinite in themselves explain the aseismic behavior of the lvf microstructural investigations show that deformation within the fault zone must have resulted from a combination of frictional sliding at grain boundaries cataclasis responsible for grain size comminution and pressure solution creep responsible for the development of the scaly foliation and favored by the mixing of soluble and insoluble minerals the microstructure of the gouge formed in the lichi melange favors effective pressure solution creep which inhibits strain weakening brittle mechanisms and is probably responsible for the dominantly aseismic mode of fault slip since the lichi melange is analogous to any unlithified subduction melanges this study sheds light on the mechanisms which favor aseismic creep on subduction megathrust

crustal scale weak zone along a collisional suture revealed by spatial variations in velocity structures and seismicity abstract high quality first arrival data collected with a high density temporary seismic array and regional seismic network were used to construct a p wave velocity model and identify the precise location of earthquakes in the active collision zone of southeastern taiwan a crustal scale weak zone defined by high seismicity is characterized by a steeply east dipping lower velocity anomaly and represents a boundary between an uplifted upper mantle and the luzon volcanic arc the main features of the weak zone with its associated low velocity zone and seismicity pattern vary significantly along the collisional boundary of southeastern taiwan along the weak zone unlithified sediments or highly fractured materials are compacted as the philippine sea plate moves toward the eurasian plate in areas of more advanced collision soft material is more compacted and can therefore store significantly greater amounts of strain energy this energy is released through episodic earthquakes

paleomagnetic evidence for a diachronic clockwise rotation of the coastal range eastern taiwan abstract paleomagnetic results obtained from over 2100 cores sampled at 132 early pliocene to late pleistocene sites in the coastal range of eastern taiwan indicate that since the late pliocene the margin of the philippine sea plate has undergone a clockwise rotation of about 30 as a direct consequence of the plio pleistocene collision of this plate with the chinese continental plate the rotation is diachronic and started between 3 and 4 ma ago in the northern coastal range then propagated southward at an average speed of the order of 70 10 km ma this value is in very good agreement with estimates of the southward propagation of the collision between the philippine sea plate and the eurasian margin in taiwan it is suggested that the rotation reflects the deformation of the philippine sea plate at depth and does not results from the shallow deformations involved in the building of the coastal range orogen

intra arc sedimentation and tectonism late paleozoic evolution of the eastern klamath terrane california rocks of the eastern klamath terrane redding section in northern california record deposition magmatism and tectonism in an island arc setting volcanism and sedimentation persisted for more than 220 m y spanning devonian to middle jurassic time and yielding thick successions of water laid volcanic debris and lavas with interstratified carbonate and clastic sequences this study focuses on the upper paleozoic rocks within the section the oldest rocks studied within the upper devonian and mississippian brag don formation contain epiclastic debris that was deposited in a submarine fan setting sandstone compositions indicate that quartzose sedimentary and metasedimentary rocks formed the source area for epiclastic debris and that volcanism persisted throughout sedimentation transitional shallow water facies in the upper part of the bragdon interfinger with the overlying baird formation which contains abundant volcanic debris volcanogenic shelf and strandline deposits characterize the lower part of the formation and volcaniclastic turbidites characterize the upper part the baird formation is locally interfingered with the overlying lower permian mccloud limestone which records two stages of early permian carbonate platform development early wolfcampian and late wolfcampian to early leonardian only the younger of the two platforms is present within the study area and it contains more distal facies than those preserved to the south the strata contain abundant fusulinids and corals which provide age constraints for carbonate platform development and form the basis for regional biogeographic correlation the time transgressive upper permian bollibokka group nosoni and dekkas formations succeeds the mccloud limestone comprising thick sequences of volcanogenic debris intercalated lava and in the upper part lenses of shallow water limestone the upper permian strata reflect volcanism and rapid subsidence within the are all upper paleozoic units were intruded by small gabbroic bodies and diabase dikes which may be partly cogenetic with upper permian basalts in the upper dekkas formation the upper paleozoic succession records local pulses of magmatism and variable subsidence rates within the eastern klamath arc mississippian epiclastic rocks reflect uplift of the volcanic arc basement which contained rocks similar to those now exposed in the adjacent yreka terrane or shoo fly complex pulses in volcanism were restricted to mid carboniferous and late permian time diminished volcanism during the early permian allowed development of carbonate platforms rapid fluctuations of sedimentation and sub sidence rates pronounced lateral facies and age variations accumulation of thick volcanic debris aprons during episodic volcanism and sporadic persistence of volcanism during apparent lulls characterized deposition within this succession and are common processes in modern volcanic arcs the stratigraphic record is composed predominantly of volcanogenic rocks and lesser carbonates reflecting events and processes within the arc itself evidence for nearby basins or landmasses is sparse the mississippian epiclastic rocks however which were derived from the arc basement reflect an ultimate sialic crustal source comparison of volcanic arc evolution with the late paleozoic evolution of continental margin and basinal rocks to the east indicates possible paleogeo graphic ties between the eastern klamath volcanic arc and rocks of the golconda allochthon or the north american continent

stability of genthelvite zn4 besio4 3s an exercise in chalcophilicity using exchange operators phase relations involving genthelvite are studied on the basis of physical chemical principles and natural occurrences stability diagrams and exchange operators are considered as well as an extended system natural associations and solid state relations it is noted that the extremely high chalcophilicity of zn explains in part why genthelvite is such as rare mineral it is only somewhat more stable than willemite and is therefore restricted to the extremely low s environments provided by alkaline granites and syenites

fertile granites of precambrian rare element pegmatite fields is geochemistry controlled by tectonic setting or source lithologies abstract a total of twenty fertile granites and their rare element pegmatite aureoles was examined to correlate their tectonic position relative to the preceding orogenic event with their geochemical signature namely li rb cs be ga sn nb ta y ree sc ti zr be th u f nyf the range of t time difference between metamorphism and or deformation and granite intrusion is 20 to 465 ma for the examined cases there is some tendency toward syn to late orogenic origin of lct s as opposed to post to anorogenic timing of nyf s but the correlation is poor compared to the previously accepted stereotype similarly both metaluminous and peraluminous fertile granites are about equally spread among syn late and anorogenic environments lct systems are largely associated with peraluminous granites whereas all metaluminous to peralkaline plutons bear the nyf signature tectonic control on geochemistry of granite pegmatite suites is poor and secondary to the fundamental influence of the source lithologies yielding granitic melts the lct type suites are derived from undepleted upper crustal lithologies suffering their first anatectic event which mobilizes the most volatile components into low temperature low percentage melts supracrustal sequences as well as ortho and para lithologies of their basement serve as protoliths with variable degrees of involvement the nyf type suites whose plutons have the geochemical signature of a granites may be derived by melting of depleted lower crustal sources residual after preceding anatectic events or possibly by melting of juvenile lithologies with short crustal residence differentiation from mantle derived magmas is highly improbable volatile extraction of nyf elements from the mantle and their pre enrichment in protoliths or melts seems to be required the few mixed systems with variable expressions of a nyf lct signature are derived basically as nyf suites but contaminated by the lct elements from undepleted upper crust via several possible mechanisms

mineralogy of beryllium in granitic pegmatites beryllium is one of the most widespread rare elements in granitic pegmatites these rocks have been historically the sole industrial source of this metal e g norton et al 1958 and they still contribute a significant proportion of the global output of beryllium ores hand cobbed beryl constitutes a substantial proportion of beryllium ore concentrates in africa asia and south america although non pegmatitic rhyolite related bertrandite ores are virtually the single source in north america petkof 1975 the mineralogy of beryllium in granitic pegmatites is strongly diversified but very imbalanced in terms of numbers of species per mineral class on one hand and of the paragenetic role distribution and abundances on the other only a very few be minerals form at the magmatic stage of pegmatite consolidation with beryl absolutely dominant among them phosphates constitute most of the late subsolidus phases with silicates a close second but the number of the phosphate occurrences is very low and volumes are negligible a few oxide hydroxide and borate minerals complement the spectrum so far no other mineral classes are represented although the occurrence of some arsenates is considered possible part of the reason for the pattern above is the crystal chemical behavior of be one of the classic amphoteric elements which acts as a cation in acidic environments but participates in complex anions under alkaline conditions thus beryllo alumino silicates and beryllo phosphates of alkali and alkaline earth cations are widespread in contrast to silicates or phosphates of beryllium with no other cations or al only be2 is always tetrahedrally coordinated with oxygen beo4 6 or with oxygen and hydroxyl beo3oh 5 the divalent charge on be renders substitutions for other tetrahedrally coordinated oxycomplex forming cations such as si al or p difficult it is possible only by charge balancing via additional alkali or alkaline earth

tectonic processes at the collision of the d entrecasteaux zone and the new hebrides island arc the d entrecasteaux zone dez a large submarine mountain chain composed of two east trending subparallel ridges collides with the north trending new hebrides island arc the direction of convergence of the oceanic plate and arc suggests that the dez is being subducted at a slightly oblique angle 10 15 so that the dez creeps slowly north parallel to the trench one possible consequence of this northward creep is that slope strata south of the dez should bear a structural overprint caused by ridge collision whereas strata north of the dez may show no such overprint migrated multichannel seismic data presented in this report reveal the probable form of this overprint within the collision zone strata forming the arc s accretionary wedge dip toward the trench possibly recording progradation of slope rocks across the ocean floor and neither outer arc ridge nor slope basin is present seismic data from south of the dez however show no evidence for this overprint because both an outer arc ridge and slope basin are present and the structure of slope strata on opposite sides of the dez appears broadly similar involving a nearly invariant stratal dip toward the arc nevertheless the slope s morphology shows that collision has modified the structure of rocks below the arc s slope and that the arc responded differently to collision of the dez s two ridges the north ridge is nearly flat topped and mid ocean ridge basalt has been dredged from its flanks isobaths of the slope bulge westward where the ridge is being subducted suggesting that the slope has been uplifted by the ridge in contrast the south ridge shows sharp relief and is made up of stratified possibly sedimentary rocks and the bougainville spur a sharp peak projects above the arc s lowermost slope along the trend of this ridge the spur may be a peak similar to one of those surmounting the south ridge that rammed the arc s slope

seismotectonics of taiwan abstract high quality seismicity data and focal mechanism solutions obtained during 1973 1983 by the permanent taiwan telemetered seismographic network and several temporary local seismographic networks are used for a detailed study of the seismotectonics of the taiwan area seismicity distribution in southern taiwan clearly reveals an east dipping benioff zone which has a thickness of about 30 km and begins to deepen along 121 e at a dip angle of 55 60 the leading edge of this benioff zone reaches a depth of about 180 km between 21 n and 22 n but tapers off to a shallower depth of about 100 km from 22 n to 23 n the presence of this seismic zone implies that subduction of the south china sea plate under the philippine sea plate extends from luzon northward to about 23 n the position of the northern boundary of the south china sea plate as tentatively defined according to the seismicity distribution passes through southern taiwan from the offshore area in the taiwan strait west of kaohsiung in an east northeast direction to the taitung area where a triple junction probably lies seismicity is found to disappear abruptly below a certain depth in many parts of taiwan this phenomenon may be attributed to the frictional to quasiplastic transition in the crust or upper mantle comparison of shallow seismicity with surface faults and fractures shows that all areas of active shallow seismicity are marked by densely developed faults and fractures however the converse is not necessarily true this may be partly due to the relatively short duration of seismicity data and partly due to excessive weakening of some of the severely faulted and fractured areas finally focal mechanism solutions for west central taiwan and the kuangfu fuli area in eastern taiwan predominantly show a maximum horizontal compression in the se nw direction which can be related to collision between the eurasian and philippine sea plates however focal mechanism solutions for both the hualien area in eastern taiwan and the tainan area in southwestern taiwan show remarkable irregularities which may result from local tectonic complexities