unpairing metamorphic belts p t paths and a tectonic model for the ryoke belt southwest japan southwest japan is divided into outer and inner zones by the median tectonic line mtl a major transcurrent fault the outer zone is composed of the sambagawa high pressure intermediate or high p t type metamorphism chichibu and shimanto belts in the inner zone the ryoke belt andalusite sillimanite or low p t type metamorphism was developed mainly within a jurassic accretionary complex this spatial relationship between high p t type and low p t type metamorphic belts led miyashiro to the idea that metamorphic belts were developed as paired systems textural relationships and petrogenetically significant mineral assemblages in pelites from the ryoke belt imply peak p t conditions of 5 kbar and up to 850 c in migmatitic garnet cordierite rocks from the highest grade metamorphic zone it is likely that the thermal anomaly responsible for metamorphism of the ryoke belt was related to a segment of the farallon izanagi ridge as it subducted under the eastern margin of the asian continent during the cretaceous the sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dp dt slope inferred to have been generated by a nested set of hairpin like clockwise p t paths these p t paths are characterized by limited prograde thickening minor decompression at peak t and near isobaric cooling features that may be typical of p t paths in low p t type metamorphic belts caused by ridge subduction a ridge subduction model for the ryoke belt implies that juxtaposition of the high p t metamorphic rocks of the sambagawa belt against it was a result of terrane amalgamation belt parallel ductile stretching recorded as syn metamorphic predominantly constrictional strain in both ryoke and sambagawa belt rocks and substantial sinistral displacement on the mtl are consistent with left lateral oblique convergence diachroneity in fast cooling of the ryoke belt is implied by extant thermochronological data and is inferred to relate to progressive sw to ne docking of the sambagawa belt thus an alternative interpretation of paired metamorphic belts in japan is that they represent laterally contemporaneous terranes rather than outboard and inboard components of a trench arc paired system amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of paired metamorphic belts in the geological record

la icp ms u pb zircon geochronology geochemistry and kinetics of the wenquan ore bearing granites from west qinling china abstract many hydrothermal cu mo au deposits related to granitoid intrusives were recently discovered in the west qinling orogenic belt wqob these deposits were mainly formed during the late indosinian epoch ca 214 ma and the regional geological setting of cu mo ore formation in wqob during this epoch is poorly understood until now this paper describes the geochronology and geochemistry of the wenquan ore bearing pluton a composite granite body to study the geologic background of magmatic emplacement and ore formation the mo mineralisation occurs at the contact between a fine grained biotite monzogranite and a medium to fine grained porphyritic monzogranite zircon 206pb 238u ages of 223 3 ma biotite monzogranite and 225 3 ma porphyritic monzogranite were obtained geochemical analyses show that the wenquan pluton is a high k calc alkaline to shoshonite series rock with relatively high lree and low hree and a moderate to weak negative eu anomaly relatively negative anomalies of ba ti p nb ta also exist these results imply that the wenquan pluton was emplaced during a transitional process from collision to extension between the yangtze craton and north china craton during the later indosinian epoch the east qinling orogenic belt eqob and wqob had similar tectonic settings and intensive magmatic activity and mo mineralisation occurred the eqob was then involved in the mesozoic subduction of the pacific plate and its subsequent tectonic evolution was different from that of the wqob

u pb zircon and chime monazite dating of granitoids and high grade metamorphic rocks from the eastern and peninsular thailand a new report of early paleozoic granite abstract in order to understand the age and tectonic framework of eastern to peninsular thailand from the viewpoint of basement metamorphic and plutonic geology the la icp ms u pb zircon dating and the chemical th u total pb isochron method chime monazite dating were performed in the khao chao hub kapong to pran buri and khanom areas in eastern to peninsular thailand the la icp ms u pb zircon dating of the garnet hornblende gneiss from the khao chao area gave 229 3 ma representing the crystallization age of the gabbro and that of the garnet biotite gneisses gave 193 4 ma representing the timing of an upper amphibolite facies metamorphism the chime monazite dating of pelitic gneiss from the khao chao gneiss gave scattered result of 68 22 ma due to low pbo content and rejuvenation of older monazite grains during another metamorphism in the late cretaceous to tertiary time the u pb ages of zircon from the hua hin gneissic granite in the hub kapong to pran buri area scatter from 250 ma to 170 ma on the concordia granite crystallization was at 219 2 ma followed by the sillimanite grade regional metamorphism at 185 2 ma monazite in the pelitic gneiss from this area also preserves early to middle jurassic metamorphism and rejuvenation by later contact metamorphism by non foliated granite or by another fluid infiltration event in the late cretaceous to tertiary time the khao dat fa granite from the khanom area of peninsular thailand gave a u pb zircon age of 477 7 ma this is the second oldest granite pluton ever reported from thailand and is a clear evidence for the sibumasu block having a crystalline basement that was formed during the pan african orogeny the khao pret granite gives u pb zircon concordia age of 67 5 1 3 ma which represents the timing of zircon crystallization from the granitic melt and accompanied sillimanite grade contact metamorphism against surrounding metapelites and gneisses metamorphic rocks in the doi inthanon area also share the similar plutono metamorphic history with the khanom and the hub kapong to pran buri areas this suggests that these three areas belong to the sibumasu block and the sibumasu block records similar plutono metamorphic history from northern to peninsular thailand relative abundance of oceanic components in the khao chao gneiss their late triassic magmatic ages and the early jurassic metamorphic ages prefer the interpretation that the khao chao gneiss belongs to the sukhothai arc

sulfide mineralogy in the polymetallic cassiterite deposits of dachang p r china several important mineral deposits of sn zn cu pb and other metals associated with devonian sediments and yanshanian cretaceous granitic rocks are known in the dachang district guangxi early genetic hypotheses related the origin of the deposits entirely to the yanshanian granites recently it was suggested that in devonian times an earlier syngenetic metal concentration may have occurred later overprinted by the yanshanian metallogeny this contribution is aimed at placing constraints on the physicochemical conditions during the yanshanian ore formation remobilization by studying the sulfide chemistry arsenopyrite sphalerite stannite and fluid inclusion data on the two major deposits in the area i e the polymetallic cassiterite deposit of changpo and the zn cu skarn deposit of lamo sphalerite and arsenopyrite are quite abundant in both deposits stannite is minor but fairly widespread at changpo and quite rare at lamo they are accompanied by pyrite pyrrhotite galena chalcopyrite cassiterite fluorite and a large variety of other sulfides and sulfosalts the main compositional data for sphalerite and arsenopyrite are summarized as follows

mantle driven cretaceous flare ups in cordilleran arcs abstract continental arcs display episodic magmatism characterized by flare ups and lulls models published to explain these patterns invoke 1 upper plate crustal processes driven by internal feedback 2 episodic mantle melting processes or 3 external lower plate tectonic events this study addresses the role of mantle magmas during flare ups in mesozoic cretaceous continental arcs using geochronological and geochemical data for three cretaceous arc segments the western peninsular ranges batholith wprb the peruvian coastal batholith pcb and the chilean coastal batholith ccb in all three arc segments bedrock zircon age patterns defining a flare up from 125 to 90 ma characterized by gabbro to granite units with sri

progressive sequence of reactions of the ryoke metamorphism in the yanai district southwest japan the formation of cordierite the compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low to medium grade pelitic rocks of the ryoke metamorphism in the yanai district southwest japan the biotite and muscovite that coexist with k feldspar have a higher k component in an a kf diagram than those in rocks lacking k feldspar this fact reflects an increase in the k2o content in muscovite but in biotite it reflects an increase of not only the k2o content but also of the octahedral vacancy at higher metamorphic grade beyond the cordierite isograd where cordierite coexists with neither chlorite nor k feldspar the biotite shows an increase in illite k aliv xii 1 si 1 and tschermak components alvi aliv r 1 si 1 where xii and r denote the interlayer vacancy and fe mg mn respectively a reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite i e k al poor biotite phengitic muscovite k al rich biotite cordierite quartz water by treating this as a key reaction in medium grade metamorphism a set of reaction in a progressive metamorphism is established for the ryoke metamorphism a typical low pressure type metamorphism some textures in one of the high grade areas the k feldspar cordierite zone suggest that a further two prograde reactions have taken place i e andalusite biotite quartz cordierite k feldspar water and andalusite sillimanite quartz cordierite k feldspar water this implies that this zone probably has a p t path involving isobaric heating

phase equilibria and the pressure temperature path of the highest grade ryoke metamorphic rocks in the yanai district sw japan the garnet cordierite zone the highest grade zone of the ryoke metamorphic rocks in the yanai district sw japan is defined by the coexistence of garnet and cordierite in pelitic rocks three assemblages in this zone are studied in detail i e spinel cordierite biotite garnet cordierite biotite and garnet biotite all of which contain quartz k feldspar and plagioclase the mg fe mg in the coexisting minerals decreases in the following order cordierite biotite garnet and spinel two facts described below are inconsistent with the paragenetic relation in the k2ofeomgoal2o3sio2h2o kfmash system in terms of an isophysical variation first garnet and biotite in the last assemblage have mg fe mg higher than those in the second second the first two assemblages are described by the reaction

rock quality assessment using the effect of mud filtrate invasion on conflicting borehole resistivity measurements abstractunexpected borehole measurements are often inaccurately interpreted due to limited knowledge of the formation particularly in tight gas unconventional reservoirs a novel method was applied to determine reservoir quality and the reliability of borehole array induction resistivity measurements in a tight gas sandstone reservoir four exploration wells drilled with synthetic oil based mud showed conflicting resistivity profiles the discovery well showed a conductive invasion profile but the appraisal wells showed resistive profiles simulation of oil based mud filtrate invasion was coupled with forward simulation and inversion of array induction resistivity measurements to determine the difference in such resistivity profiles laboratory measurements on rock core and fluid samples were used to calibrate a log based petrophysical model that was necessary to simulate the physics of fluid flow mud filtrate invasion the dynamic process of invasion was simulated with a multicomponent formulation for

mechanical anisotropies and mechanisms of mafic magma ascent in the middle continental crust the sondalo magmatic system n italy

development of migmatites and the role of viscous segregation in high t metamorphic complexes example from the ryoke metamorphic complex mikawa plateau central japan abstract the ryoke metamorphic complex of the mikawa plateau central japan contains well preserved geological and thermal structures formed during cretaceous high temperature high t metamorphism these structures can be explained by a pervasive melt migration model however the estimated flux of melt is much higher than that expected for compaction flow of felsic melt an additional tectonic force and or melt segregation to network of veins or layers is therefore required this paper presents the results of numerical modeling of the structural evolution of immiscible viscous fluids under simple shear deformation and discusses the role of viscous segregation in enabling pervasive melt flow in hot crust the results of numerical modeling of simple shear deformation of immiscible viscous fluids reveal that low viscosity fluids segregate into lenses and layers this segregation is a type of viscous segregation that takes place to minimize the dissipation of energy in the system the rate of energy dissipation is proportional to viscosity therefore this process is expected to occur universally in immiscible viscous fluids with large viscosity contrasts and is an important process in focusing melt in partially melted rocks the structures produced by the numerical simulation are similar to those of migmatites the simulation results suggest that melts distributed randomly in partially melted rock will segregate into lenses or layers with progressive simple shear deformation during deformation enhanced segregation a melt network will form dynamically if this process occurs at small to large scales in the hot crust melts will be effectively transported upward due to buoyancy forces it is possible that pervasive migmatites at low structural levels within high t metamorphic complexes may represent fossil melt pathways formed by crustal scale viscous segregation

fluid induced disturbance of the monazite th pb chronometer in situ dating and element mapping in pegmatites from the rhodope greece bulgaria abstract the monazite th pb chronometer is widely used in a variety of geological contexts but may be affected by interaction with fluids in the central part of the rhodope massif greece bulgaria synfolial pegmatite veins were emplaced within two major synmetamorphic shear zones in the chepelare shear zone czs which displays no evidence of post metamorphic fluid flow consistent monazite th pb ages of 36 ma are interpreted as dating vein emplacement higher in the metamorphic pile zircon and monazite give u th pb ages of 42 ma for vein emplacement in the nestos shear zone nsz pegmatites show evidence of post emplacement interaction with fluids during greenschist facies ductile deformation evidence for this includes the precipitation of calcite in microscale tension gashes within feldspar porphyroclasts and the oxygen isotope disequilibrium between quartz and feldspar aqueous carbonic fluids deriving from carbonate lithologies in the footwall of the nsz are identifiable in the pegmatites by the high 18 o value of quartz up to 21 8 and the presence of calcium in calcite veinlets in these samples some of the monazite grains show large intragrain scattering of th pb age spanning up to 12 ma with intergrain scattering reaching 16 ma from 39 to 55 ma within individual grains age domains correlate with chemical heterogeneities and some show a characteristic ca excess these chemical and isotopic alterations are interpreted to be caused by interaction with fluids derived from carbonate lithologies complementary u pb data on zircons from the nsz pegmatites yield mostly mesozoic ages related to an older metamorphic cycle and an age of 48 ma for one grain with typical magmatic zoning the th pb ages of 49 3 1 6 to 54 9 1 7 ma probably relate to the emplacement of the pegmatites the spread of younger th pb ages from 38 6 1 1 to 46 2 1 6 ma probably reflects the period of fluid circulation during progressive cooling to greenschist facies conditions or variable perturbations of the monazite isotope system during fluid assisted greenschist facies deformation this is consistent with 39 ar 40 ar mica ages of 32 to 34 ma interpreted as a tighter constraint for the timing of the greenschist facies metamorphism this study illustrates the capacity of monazite to record distinct events in a single rock and highlights the need for identifying the potential involvement of fluids in order to interpret monazite ages

electron microprobe observations of pb diffusion in metamorphosed detrital monazites electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite sillimanite transition 620 15 c and sillimanite orthoclase 680 15 c zones of the cretaceous ryoke metamorphic belt southwest japan monazites from pelitic gneisses are of metamorphic origin euhedral to subhedral and chronologically homogeneous giving chemical th u total pb isochron chime ages of 98 8 3 3 98 0 3 2 ma two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca 1700 ma most detrital monazite grains are heterogeneous in the tho2 and uo2 concentrations and have multiple or single rims as young as ca 100 ma several detrital monazite grains are well rounded in form exhibit homogeneous th and u distributions and show a pb diffusion profile in the margin the width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss 18 22 m for 620 c and 48 58 m for 680 c assuming the isothermal diffusion of pb from homogeneous monazite spheres during a 5 ma duration of peak metamorphism we obtain diffusion coefficients of 1 9 0 3 10 21 and 1 5 0 3 10 20 cm2 s at 620 c and 680 c respectively these data derive an activation energy of 2 44 2 85 1 26 105 j mol and a frequency factor of 3 4 10 7 8 5 10 12 2 2 107 cm2 s taking account of uncertainties of 15 c in the temperatures and of 20 in the diffusion coefficients the diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for pb in monazite that has been poorly understood so far

the ryoke plutonometamorphic belt crustal section of the cretaceous eurasian continental margin abstract the ryoke belt comprising low pressure regional metamorphic rocks and low level granitoids expresses a deep in the crust of the cretaceous eurasian continental margin before the opening of japan sea the petrological p t estimation of the metamorphic rocks indicates the condition of upper to middle crust level the zonally arranged areal distribution of the san yo shallow facies to ryoke deep facies granitic provinces from back arc side to fore arc side is concordant with the occurrence of the associated low grade to high grade regional metamorphic rocks they are regarded as an upper to middle crustal section of the cretaceous eurasian continental margin these metamorphic rocks are derived mainly from the jurassic accretionary complex in the migmatitic high grade part the metasedimentary rocks were just being transformed into continental crust isotopic ages of these metamorphic and granitic rocks show an along arc variation of the systematic eastward younging from 100 ma to 70 ma for 800 km it cannot be interpreted with the steady state subduction model and indicates that the orogeny was caused by an episodic event presumably the subduction of kula pacific ridge beneath the eurasian continent during cretaceous the present juxtaposition of the ryoke and the sanbagawa paired metamorphic belts inevitably needs a secondary displacement from their birthplace two possibilities should be examined along arc transcurrent movement or across arc overthrusting

timing of the late cretaceous ignimbrite flare up at the eastern margin of the eurasian plate new zircon u pb ages from the aioi arima koto region of sw japan abstract the kinki district in the inner zone of southwest japan is characterized by late cretaceous volcanic rocks of the aioi and arima groups and the koto rhyolites from west to east these rocks are dominated by pyroclastic flow deposits related to caldera forming events here we present new laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb ages for the aioi group four isolated caldera filling deposits the lower middle and upper parts of the arima group and one intrusive rock the kashihara quartz gabbro and the lower and upper parts of the koto rhyolites including the oldest volcanic unit within these rhyolites these analyses yielded ages of ca 86 82 ma for the aioi group ca 83 81 ma for the arima group 78 6 ma for the kashihara quartz gabbro and ca 74 73 ma for the koto rhyolites the aioi group represents a cluster of calderas and the ages obtained for individual units in this group differ from the ages of adjacent units the arima group and the koto rhyolites both consist of pyroclastic flow deposits associated with caldera forming events and the ages of these rocks are all the same within error this suggests that the aioi group represents a series of individual caldera forming eruptions that are distinct from the arima group and the koto rhyolites which formed during a single stage of caldera formation the u pb ages presented here indicate that the late cretaceous caldera forming eruptions in the study area occurred at intervals of 1 myr and represent individual events that lasted for

tourmaline breakdown in the migmatite zone of the ryoke metamorphic belt sw japan a sharp line delimitating the distribution of tourmaline termed as a tourmaline out isograd is defined in the migmatite zone of the ryoke metamorphic belt japan the trend of the tourmaline out isograd closely matches that of the isograds formed through the regional metamorphism suggesting that it represents the breakdown front of tourmaline during regional metamorphism this is confirmed by the presence of the reaction textures of tourmaline to sillimanite and cordierite near the tourmaline out isograd the breakdown of tourmaline would release boron into associated melts or fluids and be an important factor in controlling the behaviour of boron in tourmaline bearing high temperature metamorphic rocks near the tourmaline out isograd large tourmaline crystals occur in the centre of interboudin partitions containing leucosome in the melanosome of the intervening matrix reaction textures involving tourmaline are locally observed these observations imply that tourmaline breakdown is related to a melting reaction and that the boron in the leucosome is derived from the breakdown of tourmaline in the melanosome during prograde metamorphism boron released by tourmaline breakdown lowers both the solidus temperature of the rock and the viscosity of any associated melt considering that the tourmaline out isograd lies close to the schist migmatite boundary these effects might have enhanced melt generation and segregation in the migmatite zone of the ryoke belt the evidence for the breakdown of tourmaline and the almost complete absence of any borosilicates throughout the migmatite zone suggest that boron was effectively removed from this region by the movement of melt and or fluid this implies that the tourmaline out isograd can reflect a significant amount of mass transfer in the anatectic zones

an f rich sn bearing volcanic intrusive complex in yanbei south china the igneous complex that hosts the yanbei porphyry tin deposit is situated along the eastern margin of the southeast china caledonian fold belt it intrudes a late jurassic granitic batholith and was unconformably covered by late cretaceous red beds the complex consists mainly of volcanic ignimbrites and minor rhyolitic lavas tuffs and subvolcanic rhyolite porphyry and intrusive rocks topaz bearing granite porphyry and granite these rocks are generally peraluminous with f and vi al rich biotite as the main mafic mineral and f rich and oh poor topaz as an accessory mineral in the granite porphyry and granite and as an essential mineral in the topazite topaz in the granites is usually interstitial and crystallized during the late magmatic stage the volcanic rocks and granites are rich in si k f sio 2 75 wt na 2 o k 2 o 0 03 0 72 f 0 06 6 48 wt and incompatible trace elements rb zr nb w sn and poor in mg ti ca p and compatible trace elements sr eu the chondrite normalized rare earth element ree patterns for the granites generally have a distinct v shape with strong eu depletion eu eu super whitesunwithrays 0 04 0 20 the main rocks of the complex have low epsilon nd t values 9 3 to 8 9 for volcanic rocks 4 0 to 4 8 for granites and high delta 18 o values of quartz from dacite 9 5ppm and granites 9 6 10 1ppm the difference in epsilon nd t between the volcanic rocks and granites indicates that they are not comagmatic the source of the volcanic rocks probably contains a greater metasedimentary component than that of the granites the yanbei porphyry tin deposit is located along the contact zone between topaz bearing granite porphyry and rhyolite porphyry and consists principally of one orebody which makes up 99 percent of the total reserves of the metallic minerals 5 by volume cassiterite pyrite and chalcopyrite are dominant sphalerite wolframite and galena are less important and siderite molybdenite and stannite are minor cassiterite accounts for 96 percent of the total sn reserves and stannite is the subordinate sn bearing mineral that occurs as rims around chalcopyrite cassiterite occurs mostly in disseminated form and as veinlets the wall rock alteration is intense and can be roughly grouped into three zones spatially arranged around the granite porphyry namely topaz quartz chloritic and sericitic zones most of the orebody occurs in the topaz quartz and chloritic zones hydrothermal alteration caused a decrease in k and na and an increase in al fe ca sn w and cu the delta 18 o values of quartz and cassiterite from the mineralized zones vary from 6 0 to 11 per mil and the calculated delta 18 o sub h 2 o values of the ore forming fluid range from 4 2 to 7 8 per mil the altered rocks have elevated delta 18 o sub h 2 o values 14 5 9 7ppm relative to fresh rocks the pyrite from the ore has a limited range of delta 34 s values 0 3 1 5ppm these data suggest a magmatic source for the ore forming hydrothermal fluid

spatiotemporal evolution of magmatic pulses and regional metamorphism during a cretaceous flare up event constraints from the ryoke belt mikawa area central japan abstract the spatiotemporal relationship between granitoid intrusions and low pressure temperature type regional metamorphism in the ryoke belt mikawa area is investigated to understand the tectono thermal evolution of the upper to middle crust during a cretaceous flare up event at the eurasian active continental margin three plutono metamorphic stages are recognized 1 99 84 ma intrusion of granitoids 99 95 ma pulse into the upper crust and high t regional metamorphism reaching sillimanite grade 97 0 4 4 ma to 88 5 2 5 ma in the middle crust 2 81 75 ma intrusion of gneissose granitoids 81 75 ma ma pulse into the middle crust at 19 24 km depth and 3 75 69 ma voluminous intrusions of massive to weakly foliated granitoids 75 69 ma pulse at 9 13 km depth and formation of contact metamorphic aureoles cooling of the highest grade metamorphic zone below the wet solidus of granitic rocks is estimated at 88 5 2 5 ma at ca 75 ma the upper middle crustal section underwent northward tilting resulting in the exhumation of regional metamorphic zones to 9 13 km depth although the highest grade metamorphic rocks and the 99 95 ma pulse granitoids preserve similar u pb zircon ages the absence of spatial association suggests that the regional metamorphic zones were mainly produced by a transient thermal anomaly in the mantle and thermal conduction through the crust supplemented by localized advection due to granitoid intrusions the successive emplacement of granitoids into shallow deep and shallow levels of the crust was probably controlled by the combination of change in thermal structure of the crust and tectonics during granitoid intrusions

multiscale magmatic cyclicity duration of pluton construction and the paradoxical relationship between tectonism and plutonism in continental arcs abstract the close relationship between crustal magmatism an expression of heat dissipation and tectonics an expression of stress dissipation leads to the question of their mutual relationships indeed the low viscosity of magmas and the large viscosity contrast between magmas and surrounding rocks favor strain localization in magmas and then possible magmatic initiation of structures at a wide range of scales however new data about 3 d pluton shape and duration of pluton construction perturb this simple geological image and indicate some independence between magmatism and tectonics in some cases we observe a direct genetic link and strong arguments for physical interactions between magmas and tectonics in other cases we observe an absence of these interactions and it is unclear how magma transfer and emplacement are related to lithospheric plate dynamics a simple explanation of this complexity follows directly from the pulsed incremental assembly of plutons and its spatial and temporal characteristics the size of each pluton is related to a magmatic pulsation at a particular time scale and each of these coupled time space scales is related to a specific process in small plutons we can observe the incremental process the building block of plutons in larger plutons the incremental process is lost and the pulsation which consists of a cycle of injections at different timescales must be related to the composition and thermal regime of the source region itself driving magmatic processes melting segregation and transfer that interact with tectonic boundary conditions the dynamics of pulsed magmatism observed in plutonic systems is then a proxy for deep lithospheric and magmatic processes from our data and a review of published work we find a positive corelation between volume and duration of pluton construction the larger a pluton the longer its construction time large fast or small slow plutons have not been identified to date one consequence of this observation is that plutonic magmatic fluxes seem to be comparable from one geodynamic setting to another and also over various geologic time spans a second consequence of this correlation is that small plutons which are constructed in a geologically short length of time commonly record little about tectonic conditions and result only from the interference between magma dynamics and the local geologic setting the fast rate of magma transfer in the crust on the order of cm s relative to tectonic rates on the order of cm yr explain why the incremental process of pluton construction is independent of but not insensitive to the tectonic setting however in large plutonic bodies which correspond to longer duration magmatic events regional deformation has time to interact with the growing pluton and can be recorded within the pluton wall rock structure magma transfer operates at a very short timescale comparable to volcanic timescales which can be sustained over variable periods depending on the fertility of the magma source region and its ability to feed the system the fast operation of magmatic processes relative to crustal tectonic processes ensures that the former control the system from below

denudation history of the high t p ryoke metamorphic belt southwest japan constraints from chime monazite ages of gneisses and granitoids chime chemical th u total pb isochron method monazite ages were determined for gneisses and granitoids from the eastern and western parts of the ryoke belt separated by about 500 km the monazite ages for the gneisses are concentrated between 102 and 98 ma and are interpreted as the time of monazite formation under lower amphibolite facies conditions the peak metamorphism seems to be contemporaneous with the emplacement of the geologically oldest plutons that are dated at c 95 ma in both the eastern and western parts in the eastern part plutonism continued from c 95 ma to c 68 ma at intervals of 2 10 ma whereas in the western part it ceased at c 85 ma the chime monazite ages agree well with the relative age of granitoids derived from intrusive relationships of granitoids in both parts these lines of evidence are incompatible with a current view that the plutonometamorphism in the ryoke belt becomes younger towards the east the chime monazite ages coupled with available data on the depth at which the ryoke metamorphism took place and the emplacement of individual plutons show that the western part was eroded more rapidly about 1 5 mm year 1 than the eastern part about 0 8 mm year 1 over the time span from 91 to 85 ma the denudation rates agree well with those in active orogenic belts like the alps and himalayas

migration of igneous activities related to ridge subduction in southwest japan and the east asian continental margin from the mesozoic to the paleogene abstract in the inner zone of southwest japan the northern part of the median tectonic line mtl granites and their volcanic equivalents were formed in large quantities during the late cretaceous to early tertiary migration of magmatic activity related to ridge subduction is proposed to explain the widely distributed magmatism and its tectonics for this purpose the ages of granitic rocks were carefully inspected according to the criteria of the dating methods and procedures concerned and compiled with the sampling localities they clearly show a trend in rock ages along the mtl in agreement with the general direction of strikes in southwest japan from the trend of the ages and the cooling histories of the bodies it was concluded that the magmatism migrated eastward along the mtl with a rate of 30 km m y this migration concept is extended to the mesozoic igneous rocks of the east asian continental margin there two magmatic belts ranging in age from mesozoic to paleogene are found to become younger northeastward along the margins which is ascribed to the subduction of the farallon izanagi and kula pacific ridges

granitic magma ascent and emplacement neither diapirism nor neutral buoyancy it is probable that granitic magma ascent does not result from the intrin sic properties of the magmas within the uppermost crust neither the reduced viscosity nor the density contrast between magma and surroundings are them selves sufficient to induce either low inertia flow diapirism or fracture induced magma propagation dyking igneous diapirism is intrinsically restricted to the lower ductile crust dyking is therefore the most probable ascent mechanism for granitic magmas that reach shallow crustal levels a neutral buoyancy level in the crust at which magma ascent should stall is never observed this is demon strated by coeval emplacement of magmas with different compositions and densi ties and the negative gravity anomalies measured over many granitic plutons we suggest that deformation through strain partitioning is necessary to magma ascent pluton formation is controlled by local structures and rock types rather than by intrinsic magma properties as a result of its intermittent character deformation both local and regional induces magma pulses and this may have important consequences for the chemical homogeneity of intruded magmas ascent of granitic leucogranitic to tonalitic magma in the continental lithosphere is the most potent means of mass transfer and chemical segregation between the lower and upper crust granitic magmatism contributes to crustal recycling and to a lesser extent continental growth in rapidly accreted crust granitic rocks can occupy up to one third of the volume of the middle crust meissner 1986 together with shear zones they are the most conspicuous effects of plate convergence and collision in orogenic zones consequently they are a key element in understanding the evolution of earth s mechanical workings as most felsic magmas are emplaced in a liquid or near liquid state at temperatures over 800 a major heat source must be involved in their genesis the existence of such a heat source also has consequences for the deformation regime of the crust com monly a large temperature rise in the lithosphere will also increase the capacity of crustal material to respond to stress by deformation such a thermal anomaly will contribute to the dissipation of energy and thus buffer the temperature effects the stress regime will be imposed externally by the tectonic processes operating at the time as a reaction to a significant increase in crustal heat flow the rheological capacity of rocks to sustain stresses decreases and this activates deformation therefore most departures from a normal heat distribution in the lithosphere will trigger deformation e g formation of extensional rift zones or shear zones in convergent situations

ascent and emplacement of buoyant magma bodies in brittle ductile upper crust 1 the emplacement of silicic magma bodies in the upper crust may be controlled by density such that there is no buoyancy to drive further ascent or temperature such that surrounding rocks are too cold to deform significantly over geological timescales evidence for the latter control is provided by negative gravity anomalies over many granitic plutons conditions of diapir ascent and emplacement in this case are studied with a numerical model for deformation and heat transport allowing for ductile elastic and brittle behavior a large strain formulation is used to solve for temperature stress strain and strain rate fields as a function of time for a range of diapir sizes density contrasts and background geotherms the method allows for large viscosity contrasts of more than 6 orders of magnitude and determines the dominant deformation mechanism depending on the local values of temperature strain and strain rate emplacement depth and final deformation characteristics depend on diapir size and buoyancy small diapirs less than about 5 km in diameter cannot reach shallow crustal levels and do not involve brittle deformation in the ductile regime the diapir flattens significantly upon emplacement due to stiff roof rocks and to the free surface above late stage deformation proceeds by horizontal spreading with little upward displacement of roof rocks and is likely to be interpreted as ballooning large diapirs more than about 5 km in diameter rapidly rise to shallow depths 1 5 km and induce brittle faulting in the overlying rocks in this regime buoyancy forces may lead to faulting in roof rocks in this case late stage ascent proceeds by vertical intrusion of a plug of smaller horizontal dimensions than the main body buoyant diapirs keep on rising after solidification long after the relatively short lived high temperature magmatic stage this may account for some phases of late caldera resurgence in extinct volcanic systems

high t low p metamorphism in the palaeoproterozoic halls creek orogen northern australia the middle crustal response to a mantle related transient thermal pulse high t low p metamorphic rocks of the palaeoproterozoic central halls creek orogen in northern australia are characterised by low radiogenic heat production high upper crustal thermal gradients locally exceeding 40 c km 1 sustained for over 30 myr and a large number of layered mafic ultramafic intrusions with mantle related geochemical signatures in order to account for this combination of geological and thermal characteristics we model the middle crustal response to a transient mantle related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere this mechanism has the potential to raise mid crustal temperatures by 150 400 c within 10 20 myr following initiation of the mantle temperature anomaly via conductive dissipation through the crust the magnitude and timing of maximum temperatures attained depend strongly on the proximity duration and lateral extent of the thermal anomaly in the mantle lithosphere and decrease sharply in response to anomalies that are seated deeper than 50 60 km maintained for 5 myr in duration and or have half widths 100 km maximum temperatures are also intimately linked to the thermal properties of the model crust primarily due to their influence on the steady state background thermal gradient the amplitudes of temperature increases in the crust are principally a function of depth and are broadly independent of crustal thermal parameters mid crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly although pluton related thermal signatures significantly dissipate within 10 myr even for very large high temperature intrusive bodies the interaction of pluton and mantle related thermal effects has the potential to maintain host rock temperatures in excess of 400 450 c for up to 30 myr in some parts of the mid crust the numerical models presented here support the notion that transient mantle related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high t low p metamorphic belts especially in those characterised by low surface heat flow very high peak metamorphic geothermal gradients and abundant mafic intrusions

revisiting the high temperature metamorphic field gradient of the ryoke belt sw japan new constraints from the iwakuni yanai area abstract we revisit the origin of the metamorphic field gradient exposed in the ryoke belt sw japan the type locality for low pressure high temperature l p h t conditions based on combined structural petrological and geochronological data from the iwakuni yanai area w ryoke in this area metasedimentary rocks pass from schist in the north to partly migmatitic paragneiss in the south and are surrounded by numerous granitoid bodies three deformation phases are emphasized an accretionary stage or burial deformation of weak intensity d0 a vertical shortening d1 which generated a subhorizontal locally extensional foliation in the southern gneiss zone and e w upright folding d2 which reworked both the northern schist and southern gneiss zones crystallization deformation relationships indicate that in the schist zone most andalusite cordierite and mica porphyroblasts formed after d0 and before d1 syn d1 mineral assemblages define e w trending metamorphic zones with increasing grade from north to south and rare syn d2 parageneses reveal a similar pattern continuous magmatic activity lasted from ca 105 to 94 ma the oldest granitoids shimokuhara soo namera found in the western part of the belt were emplaced below schistose rocks at 105 100 ma subsequently syn to post d1 granodiorite gamano intruded concordantly with the foliation of southern high grade gneissic rocks from 100 to 94 ma a large syn d1 pluton kibe intruded the center of the belt at 98 ma whereas a younger granite iwakuni and its satellite dykes were emplaced to the northeast at 96 94 ma constraints on the timing of d1 103 99 ma and the likely diachronous d2 phase 100 94 ma suggest a deformation continuum during magma intrusion and metamorphism we conclude that the exposed metamorphic field gradient is an apparent but fortuitous alignment of p t data resulting from spatially and temporally distinct thermal events peak temperature conditions in the relatively shallow schist zone were attained during a first pre tectonic contact metamorphic event a second regional metamorphic overprint arose from the syn tectonic intrusion of granitoids in the deeper gneiss zone a younger contact metamorphic event locally affected the eastern part of the schist zone the revised tectono thermal history highlights the composite and diachronous character of the ryoke metamorphic field gradient

mafic injection as a trigger for felsic magmatism a numerical study 1 the origin of crustal scale silicic magmatism remains a matter of debate and notable uncertainty exists concerning the physical mechanisms that drive ascent and emplacement of felsic magmas in upper crustal regions a 2 d numerical model demonstrates that injection of mantle derived mafic magma into a partially molten hot zone in the lower crust can drive felsic magma ascent and intrusion into upper crustal levels the injection of mafic magma induces overpressure in the reservoir which increases crustal stresses and triggers development of brittle plastic shear zones and can drive significant surface uplift the emerging topography causes a nonuniform overpressure distribution in the reservoir and can trigger felsic magma ascent along crustal shear zones based on systematic numerical experiments we investigate the influence of crustal strength and injection rate the initial upper crustal strength controls the degree of crustal faulting and surface uplift and therefore whether felsic magma ascent can be initiated or not the final upper crustal strength influences the depth and final style of felsic intrusion the injection rate of mafic magma determines the time scale of overpressure growth and surface uplift stage in contrast the duration of the subsequent felsic ascent and intrusion emplacement stages remains nearly constant our results imply that mafic underplating and intrusion into the lower crust may not only be a prime control for the generation of felsic magmas in the lower crust but may also be an important physical driving mechanism for felsic magma ascent and intrusion into upper crustal levels

behavior of zircon in the upper amphibolite to granulite facies schist migmatite transition ryoke metamorphic belt sw japan constraints from the melt inclusions in zircon behavior of zircon at the schist migmatite transition is investigated syn metamorphic overgrowth is rare in zircon in schists whereas zircon in migmatites has rims with low th u that give 90 3 2 2 ma u pb concordia age between inherited core and the metamorphic rim a thin dark cl annulus containing melt inclusion is commonly developed suggesting that it formed contemporaneous with the rim in the presence of melt in diatexites the annulus is further truncated by the brighter cl overgrowth suggesting the resorption and regrowth of the zircon after near peak metamorphism part of the zircon rim crystallized during the solidification of the melt in migmatites preservation of angular shaped inherited core of 5 10 m in zircon included in garnet suggests that zircon of this size did not experience resorption but developed overgrowths during near peak metamorphism the ostwald ripening process consuming zircon less than 5 10 m is required to form new overgrowths curved crystal size distribution pattern for fine grained zircons in a diatexite sample may indicate the contribution of this process zircon less than 20 m is confirmed to be an important sink of zr in metatexites and ca 35 m zircon without detrital core are common in diatexites supporting new nucleation of zircon in migmatites in the ryoke metamorphic belt at the aoyama area monazite from migmatites records the prograde growth age of 96 5 1 9 ma using the difference of growth timing of monazite and zircon the duration of metamorphism higher than the amphibolite facies grade is estimated to be ca 6 myr

dating pseudotachylyte of the asuke shear zone using zircon fission track and u pb methods abstract zircon fission track ft and u pb analyses were performed on zircon extracted from a pseudotachylyte zone and surrounding rocks of the asuke shear zone asz aichi prefecture japan the u pb ages of all four samples are 67 76 ma which is interpreted as the formation age of ryoke granitic rocks along the asz the mean zircon ft age of host rock is 73 7 2 ma suggesting a time of initial cooling through the zircon closure temperature the pseudotachylyte zone however yielded a zircon ft age of 53 9 2 ma statistically different from the age of the host rock zircon fts showed reduced mean lengths and intermediate ages for samples adjacent to the pseudotachylyte zone coupled with the new zircon u pb ages and previous heat conduction modeling the present ft data are best interpreted as reflecting paleothermal effects of the frictional heating of the fault the age for the pseudotachylyte coincides with the change in direction of rotation of the pacific plate from nw to n which can be considered to initialize the nne ssw trending sinistral extensional asz before the miocene clockwise rotation of sw japan the present study demonstrates that a history of fault motions in seismically active regions can be reconstructed by dating pseudotachylytes using zircon ft thermochronology