amphibole sponge in arc crust pressure temperature time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes and hence the compositions of liquid fractionates here we use la yb and dy yb versus sio 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid lower crust where amphibole is stable because amphibole is rarely a phenocryst phase we term this cryptic amphibole fractionation if this process is as widespread as our investigation suggests then 1 amphibole cumulates may act as an effective filter for water dissolved in mantle derived magmas 2 amphibole cumulates may act as a fertile source of intracrustal melts and fluids and 3 recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle

crustal strain crustal stress and microearthquake activity in the northeastern japan arc abstract crustal movement and seismic activity in the northeastern japan arc are produced by a reaction of the pacific plate sinking under the continental plate we have investigated the time variation of crustal strain obtained from analysis of extensometer installed at eleven crustal movement observatories in the northeastern japan arc we then investigated using the finite element method the spatial distribution and time variation of the crustal strain and stress caused by plate subduction we finally compared the results with microearthquake activity observed by the seismic network of tohoku university the spatial distribution and time variation of microearthquake seismic energy were elucidated in order to know the seismic circumstances around the eleven observatories a contour map of the seismic energy distribution was obtained showing a lineation of the seismic activity the time variation of various strain components from 1967 to 1981 were revealed with the accumulation of additional data it is possible to recognize the propagation of the maximum shear strain in the direction s50 e at 40 km year from the pacific ocean side to the sea of japan a similar time variation of areal dilatation was found at the five observatories located at a distance of more than 100 km the directions of the compression axes derived from the composite focal mechanism solution of microearthquakes coincide well with those of principal strain obtained from data gathered using extensometers indicating e w compression in most of the observatories and n s compression around myk the spatial distribution and the accumulation of crustal stress caused by plate subduction were investigated using the finite element method and a comparison with a hypocenter distribution of microearthquakes was performed the distribution of the crustal stress is closely related to the crustal structure but the amplitude of the observed strain is affected by local structures around the stations

zircon u pb geochronology of the nyainqentanglha group from the lhasa terrane new constraints on the triassic orogeny of the south tibet abstract the studied amphibolite facies metamorphic rocks typical of the nyainqentanglha group occur as a tectonic block in the central lhasa terrane south tibet and consist mainly of gneiss amphibolite quartz schist and marble the amphibolite has a similar mineral paragenesis of plagioclase garnet biotite amphibole quartz muscovite and were metamorphosed under p t conditions of 0 63 0 66 gpa and 588 622 c zircons from three dated samples include two types first type of zircons has an inherited magmatic core and a metamorphic rim whereas the other type of zircons is of metamorphic origin la icp ms in situ u pb analysis indicates that the inherited magmatic cores of zircons yield widely variable 206 pb 238 u ages ranging from 1857 ma to 743 ma recording the magmatic thermal events of the material source of the metamorphic rock protoliths in contrast the metamorphic zircons yield concordant 206 pb 238 u ages of 225 213 ma the results indicate that the nyainqentanglha group has been subjected to triassic metamorphism instead of the precambrian metamorphic basement of the lhasa terrane as considered traditionally these rocks together with the coeval sumdo eclogite and pana blueschist form a permian to triassic metamorphic belt with an e w extension of up to 300 km therefore this work provides new constraints on the late paleozoic to early mesozoic orogeny in the central lhasa terrane south tibet

nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

dynamics of shallow late variscan gold mineralization the le châtelet au arsenopyrite quartz veins massif central france the le ch telet gold deposit is unique among the mined deposits of the french variscan basement in that the gold is not free but caught up in the arsenopyrite crystal lattice microstructural analysis shows that the au arsenopyrite quartz veins are hosted by north trending dextral strike slip faults resulting from nne ssw compression during the early stephanian around 300 ma textural study indicates that vein emplacement was due to sequential dynamics related to spasmodic strike slip movements along these brittle structures seismic regime the classic sequence begins with the slow crystallization of sulphide free comb quartz followed by a co deposition of microcrystalline quartz and needles of au arsenopyrite the rapid crystallization of the au arsenopyrite bearing microcrystalline quartz was due to a sudden drop in fluid pressure the veins correspond to dilational jogs formed by a suction pump mechanism during a late phase of the variscan orogeny they were trapped in migmatite about 1 km below surface

paleomagnetism of tertiary rocks from the oga peninsula and the rotation of northeast japan we have made a paleomagnetic study of tertiary rocks from the oga peninsula northern honshu island volcanic and sedimentary rocks were sampled from 26 sites in the peninsula the ages of the rocks range from 62 ma to the present the results indicate a counterclockwise rotation of northeast japan with respect to eastern asia around a pole at 58 n 149 e between about 22 ma and 15 ma the amount of the rotation is about 20 before the rotation northeast japan was situated along the east coast of the asian continent

paleomagnetic evidence for the miocene counter clockwise rotation of northeast japan rifting process of the japan arc paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of cretaceous to miocene age from northeast japan the remanent magnetizations of welded tuffs with age between 32 ma and 21 ma old from 17 widely distributed sampling sites in northeast japan are fairly well grouped with a mean ofd 41 2 i 56 5 95 7 2 the remanent magnetization of welded tuffs of 14 ma and 11 ma from 4 sites shows a northerly direction d 11 7 i 60 5 95 15 3 these data imply that northeast japan was apparently subjected to a counter clockwise rotation through 47 14 about a vertical axis on the representative point of northeast japan 141 e 39 n during the past 21 ma comparing with the period of the clockwise rotation of southwest japan the differential rotation between northeast japan and southwest japan is concluded to have occurred concurrently during the period between 21 ma and 12 ma a rifting model of the japan arc is proposed northeast japan rotated counter clockwise through 50 about a northern pivot at 146 e 44 n while southwest japan rotated clockwise through 54 about a southern pivot at 129 e 34 n the differential rotation about different rotation pivots gives rise to the opening of a diamond shaped back arc basin the japan sea

microgranular enclaves as evidence of rapid cooling in granitoid rocks the case of the los pedroches granodiorite iberian massif spain microgranular enclaves from the los pedroches granodiorite lpg los pedroches batholith iberian massif spain have sr nd isotopic and mineral chemical compositions close to those of their host this similarity is not related to restite unmixing as indicated by the igneous textures of the enclaves a number of other geological and geochemical lines of evidence including the high ree and hfse contents of the microgranular enclaves relative to the host granitoid strongly suggest that this similarity cannot be explained by magma mixing alternatively a crystallization process by rapid cooling within the host granitoid magma could explain the geochemical and textural characteristics of the microgranular enclaves including shape grain size mineralogy texture chemistry and sr nd isotopic composition such a crystallization occurred at the walls of the magma conduits through which the granitic magmas were emplaced in the upper crust this process should be considered as an alternative hypothesis to magma mixing for the generation of some microgranular enclaves especially where no direct evidence exists for the presence of basic magmas coeval with granitoids and where there is a lack of isotopic contrast between hosts and enclaves as the process is favoured by feeder dyke related emplacement we suggest that abundance of microgranular enclaves can be related to the mechanism of emplacement of granitoid bodies

igneous garnet and amphibole fractionation in the roots of island arcs experimental constraints on andesitic liquids to evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs a series of experiments were performed on a synthetic andesite at conditions ranging from 0 8 to 1 2 gpa 800 1 000 c and variable h2o contents at water undersaturated conditions and fo2 established around qfm garnet has a wide stability field at 1 2 gpa garnet amphibole are the high temperature liquidus phases followed by plagioclase at lower temperature clinopyroxene reaches its maximal stability at h2o contents 9 wt at 950 c and is replaced by amphibole at lower temperature the slopes of the plagioclase in boundaries are moderately negative in text t text x text h 2 text o space at 0 8 gpa garnet is stable at magmatic h2o contents exceeding 8 wt and is replaced by spinel at decreasing dissolved h2o the liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1 2 gpa series but show substantial enrichment in feo mgo for the 0 8 gpa series related to the contrasting roles of garnet and amphibole in fractionating fe mg in derivative liquids our experiments indicate that the stability of igneous garnet increases with increasing dissolved h2o in silicate liquids and is thus likely to affect trace element compositions of h2o rich derivative arc volcanic rocks by fractionation garnet controlled trace element ratios cannot be used as a proxy for slab melting or dehydration melting in the deep arc garnet fractionation either in the deep crust via formation of garnet gabbros or in the upper mantle via formation of garnet pyroxenites remains an important alternative despite the rare occurrence of magmatic garnet in volcanic rocks

collation of age of ore hosting volcanics in xiongcun superlarge cu au deposit on basis of three zircon u pb shrimp ages the superlarge xiongcun copper gold deposit in the west central segment of the gangdese orogenic belt occurs in volcanic rocks of danshiting formation and its origin remains a problem of controversy in this paper zircon u pb shrimp dating was made for three whole rock samples collected separately from the dacite porphyry outside the ore body a granodiorite porphyry dyke within the deposit and the granodiorite porphyry at the depth of 366 m in a drill hole and the result shows that their formation ages are 195 4 6 ma with mswd 1 48 179 5 ma with mswd 2 6 and 175 5 ma with mswd 3 2 respectively these ages suggest that this volcanic suite was formed in early jurassic instead of formerly widely accepted late cretaceous if this age collation is valid for the whole rigaze forearc basin the generation age of the rigaze forearc basin as well as the collision history of the indian plate with the eurosian continent should be wholly reconsidered under the background that the ore forming age is already known 38 11 ma the age data of the early jurassic ore hosting volcanics naturally rule out the possibility that the xiongcun cu au deposit belongs genetically to the porphyry or seafloor exhalation type this cu au deposit is controlled by a nww trending fault and lacks the alteration mineralization association typical of an epithermal deposit its ore forming fluids are characterized by mixture of magmatic and meteoric water and its ore sulfides contain the same s and pb isotopic compositions as the ore hosting volcanics all these metallogenic features reflect great similarity to the jiaodong jiaojia style gold deposits the authors therefore tend to believe that the xiongcun superlarge deposit belongs probably to thealtered cataclastite type cu au deposit

counterclockwise rotation of northeast japan paleomagnetic evidence for regional extent and timing of rotation early to late miocene welded tuffs have been sampled from northeast honshu island and the oshima peninsula of hokkaido island japan for paleomagnetic study characteristic directions with a high unblocking temperature component above 560 c are isolated from 24 sites westerly declinations d 1 91 are identified in welded tuffs older than 17 ma from these areas when combined with previous paleomagnetic data from welded tuffs the early miocene paleomagnetic poles derived from northeast honshu island and the oshima peninsula are statistically identical and indicate that northeast honshu island and the oshima peninsula were subjected to counterclockwise rotation as a single tectonic block these results establish that the rotating block of northeast japan extends north to the oshima peninsula to the latitude 43 n the dimension of this rotating block is of the order of 500 km the paleomagnetic data provide a best curve for the rotation process with the counterclockwise rotation reaching a climax at 15 0 ma and a net amount of rotation of 46 4 the 500 km long northeast japan block was rotated counterclockwise through more than 45 and accompanied a clockwise rotation of the 600 km long southwest japan block at about 15 ma

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

biogeochemical weathering in sedimentary chronosequences of the rhone and oberaar glaciers swiss alps rates and mechanisms of biotite weathering abstract we analysed the composition of phyllosilicate minerals in sediments deposited by the rhone and oberaar glaciers swiss alps in order to identify processes and rates of biogeochemical weathering in relation to glacial erosion the investigated sediments are part of chronosequences consisting of a suspended fresh sediment in melt water b terminal moraines from the little ice age lia approximately 1560 1850 and c tills of the younger dryas interval yd approximately 11 500y bp secondary weathering products associated with the suspended sediment have not been observed we therefore exclude intermittent subglacial storage and weathering of this material and assume that the suspended sediment is directly derived from mechanically abraded bedrock this implies that biogeochemical weathering processes started once the glacially derived sediment was deposited in the proglacial area the combination of a developing vegetation cover the generally high permeability allowing the percolation of precipitation and the chemical reactivity related to the dominance of fine grained material biotite weathering rates are calculated using the difference in the biotite content between unweathered and weathered glacial sediments within the investigated profiles the reactive mineral surface area is estimated geometrically both with regards to the total relative surface wr t as well as to the relative edge surface wr e wr t biotite weathering rates are estimated as 10 13 10 15 mol biotite m 2 biotite s 1 wr e biotite weathering rates are on the order of 10 13 10 14 mol biotite m 2 biotite s 1 biotite weathering rates obtained by this study are in the order of one magnitude higher in comparison to other published field based weathering rates using biotite as an indicator we therefore suggest that glacially derived material in the area of the oberaar and rhone glaciers is generally subjected to enhanced biogeochemical weathering starting immediately after deposition in the proglacial zone and subsequently continuing for thousands of years after glacier retreat

u pb and re os geochronological evidence for the jurassic porphyry metallogenic event of the xiongcun district in the gangdese porphyry copper belt southern tibet prc abstract two large copper gold porphyry deposits no i and no ii were discovered in the last decade within the xiongcun district which is located along the south margin of the gangdese porphyry copper belt these two deposits are hosted by two porphyries that were emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation the porphyries are both quartz diorite bodies with one of these distinguished by its large quartz eyes our new molybdenite re os and zircon u pb dates for no i and no ii deposits and host intrusions reveal the presence of two middle jurassic ore forming episodes in the xiongcun district the no ii deposit is related to the 181 175 ma quartz diorite porphyry and formed ca 172 6 2 1 ma the no i deposit is related to the 167 161 ma quartz diorite porphyry with large quartz eyes and formed ca 161 5 2 7 ma geochronological and geochemical features of the jurassic ore bearing porphyries in the xiongcun district indicate that the porphyry copper gold mineralization formed in an island arc setting which is related to the northward subduction of the neo tethys oceanic plate these observations in combination with geochronological data reported at gangdese porphyry copper belt indicated four major metallogenic events existed in the gangdese porphyry copper belt at 172 161 ma 65 48 ma 40 23 ma and 20 12 ma we have identified two mineralization episodes comprise middle jurassic metallogenic event 172 161 ma in the xiongcun district which provides new constrains for the middle jurassic metallogenic event in the gangdese porphyry copper belt

zircon u pb chronology of the nyingtri group southern lhasa terrane tibetan plateau implications for grenvillian and pan african provenance and mesozoic cenozoic metamorphism abstract the nyingtri group of the lhasa terrane in southern tibet consists dominantly of metasedimentary rocks and orthogneiss these rocks have a similar mineral paragenesis of plagioclase k feldspar biotite quartz sillimanite garnet staurolite muscovite amphibole indicating amphibolite facies metamorphic conditions inherited detrital zircons from the metasedimentary rocks show magmatic features and yield widely variable 206pb 238u ages ranging from 3300 to 50 ma the data define two prominent age populations 1200 1000 and 600 500 ma indicating that the source of the nyingtri group preserves the records of both grenville and pan african magmatic thermal events inherited magmatic zircon cores from the orthogneiss yield a crystallization age of 496 ma limiting the depositional age of the metasedimentary sequence to cambrian or older overgrowth rims on the detrital zircons from one metasedimentary rock yield a metamorphic age of 32 ma on the basis of these results together with the

syn collisional adakitic granodiorites formed by fractional crystallization insights from their enclosed mafic magmatic enclaves mmes in the qumushan pluton north qilian orogen at the northern margin of the tibetan plateau the qumushan qms syn collisional granodiorite which is located in the eastern section of the north qilian orogen at the northern margin of the greater tibetan plateau has typical adakitic characteristics and also contains abundant mafic magmatic enclaves mmes this recognition offers an unprecedented insight into the petrogenesis of both the adakitic host granodiorite and the enclosed mmes the mmes and their host granodiorites share many characteristics in common including identical crystallization age similar to 430 ma same mineralogy similar mineral chemistry and whole rock isotopic compositions indicating their genetic link the mmes are most consistent with being of cumulate origin formed at earlier stages of the same magmatic system that produced the qms adakitic granodiorite subsequent replenishment of adakitic magmas could have disturbed the cumulate piles as mmes dispersed in the adakitic granodiorite host during emplacement the geochemical data and petrogenetic modeling of trace elements suggest that the qms adakitic host granodiorite is most consistent with fractional crystallization dominated by the mineral assemblage of the mmes the parental magma for the qms granodiorite is best explained as resulting from partial melting of the ocean crust together with recycled terrigenous sediments during continental collision which may have also experienced interaction with mantle peridotite during ascent c 2016 elsevier b v all rights reserved

geochemistry and sm nd and rb sr isotopic composition of eclogite in the lhasa terrane tibet and its geological significance abstract a new occurrence of eclogite has been discovered in the lhasa terrane northeast of lhasa tibet along the northern margin of the gangdese magmatic arc the eclogite occurs as a 500 1000 m wide belt that extends over 60 km in an e w direction and consists of coarse grained massive rutile eclogite quartz eclogite and phengite eclogite whole rock geochemistry and relatively high end t for the eclogites suggest that their protoliths were low k oceanic tholeiites similar to typical n morb derived from a depleted mantle source however all analyzed samples have negative nb anomalies in morb normalized trace element patterns suggesting a suprasubduction zone influence zircon shrimp u pb ages of the eclogite ranging from 242 15 ma to 291 13 ma are interpreted as the age of peak metamorphism a sm nd isochron age of 306 50 ma 2 likely reflects the protolith age these data suggest that the paleo tethyan ocean existed along the northern margin of the gangdese island arc until the early permian

timing of rotational motion of southwest japan inferred from paleomagnetism abstract paleomagnetic field directions for the period younger than 35 ma are obtained from igneous rocks distributed in the san in district inner zone of southwest japan the remanent magnetization of samples between 30 and 35 ma old are fairly well grouped with a mean direction of d 65 9 i 48 6 and 95 6 5 this result establishes that southwest japan rotated clockwise 56 12 during the past 30 m y a declination value of about 60 is observed in the rocks of 28 ma d 52 2 i 33 5 and 21 ma d 69 9 i 49 5 comparing this with results from dacitic rocks with an age of 15 ma in other areas of southwest japan suggests that rotational motion did not occur possibly until 15 ma these results require that the rotation of southwest japan occurred just after the shikoku basin had been created

nyainqentanglha shan a window into the tectonic thermal and geochemical evolution of the lhasa block southern tibet 1 in the nyainqentanglha nqtl massif southern tibet a late cenozoic se dipping normal fault exhumed an oblique section of crust in its footwall u th pb dating of zircon and monazite from footwall exposures reveals a collage of felsic intrusions including cretaceous early tertiary and miocene granitoids ages of the latter span 10 m y suggesting semicontinuous or episodic miocene magmatism geochemical and isotopic analyses show a gangdese arc affinity indicating significant mantle heat and mass transfer in their formation and semicontinuous calc alkaline magmatism throughout the cenozoic indo asian collision the undeformed nature of the footwall cretaceous and miocene granitoids suggests that mesozoic cenozoic lhasa block deformation was thin skinned being concentrated in supracrustal assemblages this coupled with the lack of migmatites exposed in the nqtl implies the exposed crust was not a partial melt zone nor involved in large scale channel flow some 40ar 39ar thermochronologic studies of footwall k feldspars reveal that samples collected within several kilometers below the normal fault cooled prior to emplacement of young leucogranites indicating little perturbation of the background thermal structure since 15 ma this plus high melting temperatures and the lack of penetrative granitoid deformation requires that the melts formed at lower crustal levels and were emplaced rapidly to the midcrust seismic reflection results showing high bright spot anomalies in the midcrust along the nqtl rift may have imaged the youngest magmatic episode or its associated hydrothermal system

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

mesozoic crustal evolution of southern tibet constraints from the early jurassic igneous rocks in the central lhasa terrane abstract phanerozoic growth of continental crust on our planet is one of the important research themes in earth science here we present the results of a systematic study of newly found and previously reported mesozoic igneous rocks including diorite cumulate granodiorite cumulate mafic magmatic enclaves mme and host granitoids in the central lhasa terrane southern tibet these igneous rocks give zircon u pb crystallization ages of 199 189 ma based on constituent mineral and bulk rock compositions the cumulates are best understood as resulting from amphibole plagioclase and titanite crystallization from a mafic andesitic magma the host granitoids also show compositional systematics consistent with amphibole plagioclase fractional crystallization from andesitic magma the mmes share many characteristics with their host granitoids in common including identical crystallization age similar mineralogy mineral chemistry and zircon isotopic compositions representing earlier cumulate derived from the same magmatic system as their host rocks the magma parental to the studied early jurassic igneous rocks is best explained as resulting from partial melting of hydrated ocean crust together with varying continental material the increasing zircon ehf t values of multiple plutons in the central lhasa terrane with time during 215 170 ma indicate its gradual increase in mantle contribution we present the early mesozoic crustal evolution in the lhasa terrane

zircon u pb geochronology and geochemistry of early jurassic granodiorites in sumdo area tibet constraints on petrogenesis and the evolution of the neo tethyan ocean abstract the gangdese magmatic belt is oriented e w and located between the indus yarlung tsangpo and bangong nujiang suture zones in tibet it records the subduction of the neo tethyan oceanic crust indian eurasian collision and uplift of the tibetan plateau here we present new zircon u pb geochronology hf isotopic compositions and whole rock geochemistry of the granodiorites in the sumdo area of the gangdese belt our aim was to identify the petrogenesis of granitoid rcoks and to clarify the early stage evolution of the neo tethyan ocean during the late triassic to early jurassic the granodiorites yield zircon u pb ages of 193 201 ma indicating an early jurassic magmatic event they are metaluminous with medium to high k calc alkaline geochemistry and show characteristics of i type granitoids they show enrichment in light rare earth elements lrees and depletion in high field strength elements hfses in primitive mantle normalized spider diagrams indicating that their initial magma was generated in a subduction related active continental margin they have negative ehf t values 11 3 to 4 1 and crustal model ages tc dm of 1 50 1 95 ga hf isotopes and whole rock geochemistry indicate that the granodiorites were derived from partial melting of ancient lower crust material mixed with mantle material our data in combination with published geochronological and geochemical data show that the late triassic early jurassic arc magmatism in the gangdese magmatic belt was formed by the northward subduction of the neo tethyan oceanic crust beneath the lhasa terrane

helium isotopes in paleofluids and present day fluids of the larderello geothermal field constraints on the heat source 1 the he isotope composition of paleofluids entrapped in fluid inclusions of hydrothermal minerals is compared with the present day fluid composition of the larderello geothermal field almost constant values of 3he 4he m 3he 4he air r ra over time indicate that no important changes have occurred in the deep source of gases at least during the last 3 8 million years on a regional scale a correlation has been found between the r ra spatial distribution heat flow and bouguer gravity anomaly high values of r ra and heat flow and low bouguer anomaly values indicate that the larderello field is an area of preferential escape for mantle derived fluids a positive correlation has also been found between the r ra spatial distribution and a major seismic reflector named the k horizon a deep magma source refilled by periodic gas input from the mantle is the most likely source of 3he enriched fluids and the anomalously high heat flow the nearly constant value of r ra clearly indicates that input of fresh mantle material has occurred up to recent times clear evidence of mixing between mantle and crustal fluids indicates that the high r ra is the lower limit of the actual mantle value which is suggested to be similar to the subcontinental european mantle the decrease of r ra over time in the peripheral part of the larderello field indicates that important changes in the feeding fracture system and or cooling rate have occurred in these areas

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

jurassic u pb and re os ages for the newly discovered xietongmen cu au porphyry district tibet prc implications for metallogenic epochs in the southern gangdese belt the gangdese belt in southern tibet includes the cretaceous to early eocene gangdese continental magmatic arc a newly recognized middle jurassic magmatic arc and post gangdese arc magmatism of miocene age that is well known for its association with important copper molybdenum deposits although the tectonic history of the gangdese continental arc is relatively well constrained the tectonomagmatic setting of the jurassic and miocene magmatic events was poorly constrained previously geochronological analysis of samples from the xietongmen copper gold porphyry district has identified a previously unknown metallogenic event related to middle jurassic arc magmatism the occurrence of several metallogenic epochs within the southern gangdese belt highlights the importance of systematic metallogenic studies of this belt

petrogenesis of early to middle jurassic granitoid rocks from the gangdese belt southern tibet implications for early history of the neo tethys abstract the gangdese belt tibet records the opening and closure of the neo tethyan ocean and the resultant collision between the indian and eurasian plates mesozoic magmatic rocks generated through subduction of the tethyan oceanic slab constitute the main component of the gangdese belt and play a crucial role in understanding the formation and evolution of the neo tethyan tectonic realm u pb and lu hf isotopic data for tonalite and granodiorite from the xietongmen nymo segment of the gangdese belt indicate a significant pulse of jurassic magmatism from 184 ma to 168 ma the magmatic rocks belong to metaluminous medium k calc alkaline series characterized by regular variation in major element compositions with sio2 of 61 35 73 59 low to moderate mgo 0 31 2 59 with mg of 37 45 these magmatic rocks are also characterized by lree enrichment with concave upward trend in mree on the chondrite normalized ree patterns and also lile enrichment and depletion in nb ta and ti in the primitive mantle normalized spidergrams these rocks have high zircon ehf t values of 10 94 to 15 91 and young two stage depleted mantle model ages tdm2 of 192 ma to 670 ma the low mgo contents and relatively depleted hf isotope compositions suggest that the granitoid rocks were derived from the partial melting of the juvenile basaltic lower crust with minor mantle materials injected in combination with the published data it is suggested that northward subduction of the neo tethyan slab beneath the lhasa terrane began by the late triassic which formed a major belt of arc related magmatism

plagioclase zoning as an indicator of magma processes at bezymianny volcano kamchatka back scattered electron bse derived zoning patterns of plagioclase phenocrysts are used to identify magma processes at bezymianny volcano kamchatka based on the 2000 2007 sequence of eruptive products the erupted magmas are two pyroxene andesites which last equilibrated at 915 c temperature 77 87 mpa pressure and a water content of 1 4 wt textural and compositional zoning of individual plagioclase phenocrysts typically includes a repeated core to rim sequence of oscillatory zoning an50 60 truncated by a dissolution surface followed by an abrupt increase in an content up to an85 which then gradually decreases rimward this zoning pattern is interpreted to be the result of frequent replenishments of the magma chamber which cause both thermal and chemical interaction between resident and recharge magmas the outermost 70 to 150 m wide zoning patterns of plagioclase phenocrysts are composed of dissolution surface with a subsequent increase in an and fe contents zoning patterns of the rims exhibit correlation among plagioclase phenocrysts within one eruption rims are interpreted as a result of crystallization of a batch of magma in the conduit after recharge event

neogene stress field in sw japan and mechanism of deformation during the sea of japan opening we present new structural data from sw japan and discuss the mechanism of deformation during the opening of the sea of japan we studied the miocene basins at the southern margin of the sea of japan northern coast of sw japan and of the median tectonic line mtl by means of fault slip analysis to determine stress field directions during the opening of the sea of japan in early and middle miocene time on the southern margin the stress field was extensional or transtensional with a nw se trending extension direction the mtl was a normal fault in shikoku during early middle miocene and probably a normal fault with a left lateral component in central japan before having been strained by the collisions of the tanzawa and izu blocks with central japan we combine our results with published data for the sea of japan area and show that early and middle miocene stress field directions are remarkably consistent on the eastern southern and western margins of the sea of japan with hmax trending ne sw and hmin trending nw se the stress regime is transpressional or transtensional on the eastern and western margin and almost purely extensional on the southern margin this stress field distribution is in agreement with the model of the opening of the sea of japan in an extensional transfer zone between two n s right lateral strike slip zones proposed by jolivet et al 1991 analogue modelings and kinematic reconstructions showed that this model accounts for 20 to 30 of clockwise rotation in sw japan while paleomagnetic rotations reached 40 to 50 during the opening we present structural data showing that sw japan was strained during the rotation and did not behave as a rigid block our observations suggest that rotations of small blocks may have occurred we propose that sw japan has been sheared between the right lateral strike slip zones which bound the sea of japan and that the missing 10 to 30 of rotation are to be found in its internal deformation in our model the mtl is a normal fault with a strike slip component rotating in a right lateral shear zone

metamorphic rocks of the 1985 tibet geotraverse lhasa to golmud two examples of uplifted basement have been studied in the lhasa terrane of the tibetan plateau the nyainqentanglha orthogneisses are bounded by staurolite garnet schists to the north which record prograde metamorphism at 5 0 1 3 kbar 610 70 degrees c garnet sillimanite xenoliths within the orthogneiss suggest that peak temperatures reached at least 700 70 degrees c at 5 1 2 5 kbar these p t fields reflect high t low p metamorphism during eocene subduction and indicate that the syntectonic nyainqentanglha orthogneiss was emplaced at depths greater than 10 km sillimanite bearing assemblages from the amdo gneisses in the northern lhasa terrane provide evidence of crustal anatexis at temperatures 680 degrees c this event is poorly constrained in time but is probably cambrian or earlier within the kunlun terrane biotite and garnet isograds north of the xidatan fault indicate an increase in metamorphic grade from north to south reaching peak metamorphism at 470 30 degrees c 4 3 1 5 kbar synchronous with the emplacement of the triassic batholith regional metamorphism was followed by uplift of at least 2 km before emplacement of post tectonic early jurassic granites

reworked precambrian metamorphic basement of the lhasa terrane southern tibet zircon titanite u pb geochronology hf isotope and geochemistry abstract due to the paucity of exposure the formation and evolution of the precambrian basement of the lhasa terrane remain poorly known here we report zircon and titanite in situ u pb ages bulk rock geochemical and zircon hf isotopic data on the orthogneisses from the dongjiu area of the southern lhasa subterrane slt southern tibet geochemical data suggest that the protoliths of the biotite amphibole gneiss and biotite gneiss are granodiorite and granite respectively inherited magmatic zircon cores from these orthogneisses give protolith crystalline ages of 1520 1506 ma whereas the overgrown zircon rims give metamorphic ages of 605 590 ma the mesoproterozoic granitic rocks have bulk rock end t values of 3 6 to 0 1 and zircon core ehf t values of 4 5 to 2 6 which give similar tdm2 ages of 2 35 2 05 ga and 2 54 2 10 ga respectively suggesting their derivation from partial melting of paleoproterozoic crustal material the granitic rocks are also local provenance for the mesoproterozoic detrital zircons in the paleozoic strata in the lhasa terrane titanite in situ u pb ages further indicate that the dongjiu orthogneiss experienced more recent metamorphism at 26 ma the mineral assemblage and thermobarometry calculations indicate that the oligocene metamorphism occurred under medium pressure mp amphibolite facies conditions 5 4 7 2 kbar 691 765 c we propose that the dongjiu gneisses represent the precambrian metamorphic basement of the lhasa terrane but have been intensively reworked by metamorphism in the slt in response to the continued india asia convergence since the collision

miocene paleomagnetic results from southeastern korea paleomagnetic samples were collected from 25 sites of late oligocene to middle miocene age from southeastern south korea the mean direction obtained is d 39 4 i 46 2 with 95 9 5 which is statistically distinguishable from the present axial geocentric dipole field direction when combined with the previous reported results of eight sites from this region we get a mean direction d 40 1 i 47 4 95 7 9 for 33 sites of late oligocene to middle miocene age this direction yields an early neogene virtual geomagnetic pole of 56 5 n 215 2 e with an 95 of 7 9 discordant from the cretaceous reference pole for korea the paleomagnetic data suggest that portions of southeastern korea underwent a large clockwise rotation of about 40 during the neogene two large ne trending right lateral faults the yangsan fault and the dongnae fault fig 1 cut the study areas both these faults show evidence for recent activity in addition minor faults with similar trends to these major faults occur the observed deflections in paleomagnetic declinations were probably caused by rigid block rotations of crustal blocks in a right lateral shear system during the neogene the paleomagnetic directions for miocene aged rocks from southeastern korea and southwestern japan are similar in direction and magnitude this suggests that the cause of the rotations in both areas may be related to a similar tectonic event in this area during the neogene

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

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

discovery of an eclogite belt in the lhasa block tibet a new border for paleo tethys abstract a newly discovered eclogite belt in the eastern part of the lhasa block tibet is about 500 1000 m wide and at least 60 km long in an e w direction the eclogites occur as tectonic slices in garnet bearing mica quartz schist they are generally fresh and form thick massive layers that consist chiefly of garnet grt omphacite omp phengite phe rutile rut quartz qtz p t calculations based on the grt omp phe mineral assemblage yielded peak metamorphic conditions of 2 7 gpa and 730 c close to the phase boundary between coesite and quartz and thus the eclogites can be regarded as part of a very high pressure metamorphic belt petrochemical data suggest that the eclogite protoliths were typical morb basalts derived from depleted mantle shrimp u pb dating of zircons from the eclogite yielded metamorphic ages ranging from 242 15 to 292 13 ma with an average value of 262 5 ma the morb eclogites are interpreted to be remnants of paleo tethyan oceanic lithosphere the eclogites along with permian island arc volcanic rocks to the north are believed to mark a carboniferous permian suture zone dividing the lhasa block into a northern and southern segment this newly identified suture zone suggests that the border of the paleo tethyan ocean jumped southward from north of the bangong nujiang suture to within what is now the lhasa block

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

geochemical constraints on the petrogenesis of granitoids in the east kunlun orogenic belt northern tibetan plateau implications for continental crust growth through syn collisional felsic magmatism early triassic syn collisional granitoids with mafic magmatic enclaves mmes crop out along the entire east kunlun orogenic belt ekob at the northern margin of the tibetan plateau they are andesitic in composition and enriched in light rare earth elements lrees with a flat heavy ree hree pattern their average composition resembles that of the bulk continental crust the enclosed mmes have the same mineralogy as their host granitoids but contain a greater mode of mafic minerals amphibole and biotite and thus have higher hree abundances zircon u pb dating shows that both the granitoid hosts and mmes have the same crystallization age of similar to 250 ma and indistinguishable bulk rock sr nd pb hf isotope compositions i sr of 0 7080 0 7116 varying pb 206 pb 204 i of 18 53 19 32 essentially constant epsilon nd t of 5 3 to 2 1 and a small range of positive epsilon hf t mostly 1 7 5 2 the complete isotopic overlapping between the granitoid hosts and the mmes is understood to reflect that the mmes are disintegrated cumulates formed at an early stage of the granitoid magma evolution within the same magmatic system the isotopic data set reveals that the granitoids are variably evolved melts produced by partial melting of the subducted paleo tethyan oceanic crust with terrigenous sediments under amphibolite facies conditions in response to the continental collision c 2014 elsevier b v all rights reserved

paleomagnetic evidence for post late miocene intra arc rotation of south kyushu japan paleomagnetism of sedimentary rocks of the late miocene uchiumigawa group exposed in southeast kyushu yielded 17 tilt corrected paleomagnetic directions with significant westward declinations fourteen of these were of reversed polarity and three at the intervening horizons were of antipodal normal polarity the overall formation mean direction is d 322 0 i 48 6 with 95 6 0 this is statistically indistinguishable from the mean direction of d 331 2 i 41 3 with 95 9 9 for the middle miocene deposits in tanegashima island of the northern ryukyu arc the common mean direction is d 333 2 i 45 1 with 95 4 9 this indicates that both south kyushu and the northernmost ryukyu arc have experienced 27 6 of counterclockwise rotation with respect to the eurasian continent after the latest miocene or during the last 6 m y this counterclockwise rotation cannot be fully explained by either a conventional model which links an arc rotation with the formation of back arc oceanic crust or the collision of the kyushu palau ridge with southwest japan arc we propose that this intra arc rotation took place during the period of extension of the continental crust behind the north ryukyu arc this study demonstrates that arc rotation may generally occur when the back arc spreading is of a much less advanced phase

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

zircon u pb dating and in situ hf isotopic analysis of permian peraluminous granite in the lhasa terrane southern tibet implications for permian collisional orogeny and paleogeography article i nfo the lhasa terrane has long been interpreted as a simple tectonic block rifted from gondwana during the late paleozoic and then drifted northward before finally amalgamating with the qiangtang terrane during the early cretaceous in this paper we document permian peraluminous granites near pikang in the southern margin of the central lhasa terrane close to the recently documented songdo eclogite of permian age zircon shrimp and la icpms u pb dating for a pikang granite sample gives an identical crystallization age of about 263 ma and a wide age range of inherited zircons 283 2141 ma in situ hf isotopic analyses for 20 zircons of 263 ma yielded ehf t values of 4 5 to 1 9 the pikang granites have high a cnk values 1 08 and high normative corundum 1 3 2 0 indicative of peraluminous s type granite they are characterized by moderately negative eu anomalies eu eu 0 48 0 61 and strongly negative ba nb sr p and ti anomalies the granites have high end t values 6 4 to 6 0 and low initial 87 sr 86 sr ratios 0 7082 0 7096 relative to melts derived from mature continental crust these rocks are interpreted to have been generated by mixing between mantle melts and their induced melting of mature crustal materials we interpret the pikang peraluminous granite magmatism the regional angular unconformity between the middle and the upper permian and the eclogite of the same metamorphic age 262 ma from the same geotectonic location to represent different products of a common event in time and space we tentatively term this common event as syncollisional orogeny i e the permian gangdese orogeny we further hypothesize that the orogeny may be genetically associated with the collision between the lhasa terrane and the northern margin of australia following the closure of the paleo tethyan ocean south of the lhasa terrane

identification of early carboniferous granitoids from southern tibet and implications for terrane assembly related to the paleo tethyan evolution abstractthis article presents zircon u pb and hf isotope data together with the whole rock major and trace element composition of early carboniferous granitoids newly identified from the jiacha and langxian areas in the southern lhasa terrane southern tibet the jiacha rocks are monzogranites that yield zircon u pb ages of 347 345 ma and ehf t values from 5 4 to 4 9 the langxian rocks are granodiorites with slightly older zircon u pb ages of 355 352 ma and lower ehf t values from 6 8 to 6 5 our data suggest that these granitoids were generated largely by reworking of paleoproterozoic ga basement materials in conjunction with literature data it is further argued that the southern and central parts of the lhasa terrane separated by the sumdo eclogite belt should have been an integrated block before the late paleozoic our study supports the notion that the lhasa terrane was derived from the northern margin of gondwanaland in association with formation of at least two stages of tethyan oc

relationship between western porphyry and mineralization in qulong copper deposit of tibet and its enlightenment to further exploration any change in basic geological knowledge of the qulong giant porphyry copper deposit the most important and exploring deposit in the gandise porphyry copper belt gpcb will result in change of exploration plan in the past the western the original and porphyries and the eastern the original i porphyry porphyries were both considered to have been formed in eocene and related to cu mo mineralization however based on careful field observation the authors found that the two kinds of porphyries have different alteration mineralization and petrologic characteristics mineralization in the western porphyry characterized by abundant pyrite veins and quartz sericite alteration with minor local epidote alteration is obviously controlled by fractures sulfide veins in them are typical d type veins at all exposed levels and usually have sericite alteration halos concentric alteration mineralization systems usually developed in porphyries related to mineralization were not found at the western porphyry in addition ductile deformation was developed in the western porphyry but not seen in the eastern porphyry indicating a more complex evolutionary process of the western porphyry it is thus inferred that the western porphyry probably has an older crystallization age than the eastern porphyry and it only served as wall rock during the formation of the ore deposit to testify the inference the authors carried out detailed petrological rock geochemical and radioactive age studies of the western porphyry the analytical results indicate that the western porphyry has high k2o content and aluminum saturation index asi 1 6 2 8 belonging to the strongly peraluminous and high k calc alkaline series and is rich in lile and depleted in hsfe with enriched light ree zircons from a sample of the porphyry were selected and dated by shrimp and as a result an age of 182 3 1 5 ma was obtained in contrast to the eastern porphyry the western porphyry is not depleted in ta and has strong negative eu anomalies and obviously older age indicating that the western porphyry was probably formed by crust melting in early jurassic the authors thus hold that further exploration should focus on the eastern porphyry instead of the western porphyry which is only the wall rock it is also thought that detailed observation in the field by means of suitable scientific analysis can avoid unnecessary exploratory investment

crustal scale strike slip deformation in hokkaido northern japan abstract field surveys in hokkaido reveal that the entire tectonic belt in northern japan was a right lateral shear zone from eocene to middle miocene time this zone of deformation can be followed northwards in sakhalin for more than 2000 km and formed at the same time as the opening of the japan sea a thrusting component during the strike slip deformation and later deformation stages led to the uplift of the deep parts of the shear zone exposing a complete section through the upper crust the stress field inferred from the analysis of fault sets is consistent with the non coaxial deformation observed in the deeper ductile parts of the shear zone there is a transition from west to east across the width of the belt 100 km from en echelon folds thrusts and nappes at upper structural levels through the brittle ductile transition with an incipient vertical schistosity into ductile deformation with a vertical foliation and metamorphic recrystallization in the amphibolite facies deep parts of the crust granulites were uplifted and retrogressed during the strike slip movement fault set analysis together with data concerning the direction and sense of nappe emplacement imply that thrusting west of the ductile deformation zone occurred during the strike slip deformation minor flat lying structures coexist with vertical pure strike slip structures this study and previous ones reveal a crustal scale half flower structure with superficial thrusting associated with a deep narrow ductile zone with a high gradient of deformation and temperature

amount of clockwise rotation of southwest japan fan shape opening of the southwestern part of the japan sea abstract paleomagnetic measurements have been carried out on welded tuffs ranging in age between 58 ma and 112 ma from yamaguchi and go river areas in the central part of southwest japan the new data together with those of younger igneous rocks published previously define the change of paleomagnetic field direction during the late mesozoic cenozoic period for southwest japan the paleomagnetic direction from this area has pointed 56 3 clockwise from the expected field direction estimated from apwp apparent polar wandering path of the whole of eurasia during the period between 100 and 20 ma in comparison with the expected one from the eastern margin of eurasia korea china siberia the cretaceous field direction of southwest japan shows the clockwise deflection by 44 49 these results establish that while the eastern margin of eurasia including southwest japan was rotated more or less with respect to the main part of eurasia during last 100 ma southwest japan was rotated clockwise through more than 40 with respect to the eastern margin of eurasia since 20 ma the large amount of rotation for southwest japan implies that it is rotated by an opening of the southwestern part of the japan sea which widens northeastward fan shape opening the tectonic feature of southwest japan and the japan sea is analogous to that of corso sardinia and the ligurian sea in the mediterranean indicating that the fan shape opening is a specific feature of the rifting of the continental sliver at the continental rim

late paleozoic intrusive rocks from the southeastern lhasa terrane tibetan plateau and their late mesozoic metamorphism and tectonic implications abstract the lhasa terrane in southern tibet experienced late paleozoic and mesozoic cenozoic composite orogenesis this work reports a study on the petrology geochemistry zircon u pb chronology and hf isotopes of late paleozoic and late mesozoic intrusive rocks from the southeastern lhasa terrane the late paleozoic intrusive rocks crystallized in the late devonian early carboniferous of 371 to 355 ma representing a bimodal igneous association formed in the back arc extensional setting the mafic end member originated from the enriched mantle and experienced contamination of crustal materials characterized by a slight enrichment of lree positive anomalies of u k and pb and negative anomalies of th nb ta and ti the felsic end member was derived from the partial melting of the ancient continental crust characterized by metaluminous positive anomalies of th zr and hf negative anomalies of ba sr nb ta and ti and negative e hf t values of zircon with t dm2 ages from 1 90 to 1 40 ga the late cretaceous ca 107 ma mafic intrusions along with the late paleozoic intrusive rocks underwent nearly syn intrusion amphibolite facies metamorphism under p t conditions of 0 56 to 0 69 gpa and 692 to 735 c during the andean type orogeny correlated with the subduction of the neo tethyan oceanic slab beneath the lhasa terrane this study provides a new insight into the pre cenozoic tectonic evolution of the lhasa terrane

collision of the izu block with central japan during the quaternary and geological evolution of the ashigara area abstract the ashigara area is located in the northern part of the izu collision zone and has been suspected as a possible location of the boundary between the eurasian and the philippine sea plates in order to analyse its evolution during the collision we have determined the age of the ashigara group and studied its depositional environments and its deformation the age of the ashigara group is early to middle pleistocene the lower part was deposited in a 1000 2000 m deep trough which lay between the izu peninsula and central japan at that time the sedimentary facies show subsequent upward coarsening and shallowing the beginning of the collision resulted in the uplift of the tanzawa mountains which are the source of the voluminous conglomerates deposited in this area soon after the deposition of the upper part of the ashigara group this area was over thrust by the tunzawa mountains along the kannawa fault and strongly folded under a nw se compressional stress field then about 0 3 ma ago the direction of compression drastically changed to n s or ne sw when the colliding izu peninsula was finally locked against central japan

differential rotation of the eastern part of southwest japan inferred from paleomagnetism of cretaceous and neogene rocks paleomagnetic directions have been determined on late cretaceous and miocene rocks in the nohi area in the eastern part of southwest japan in an effort to clarify within block deformation of the rotated part of southwest japan twenty two sites in the area are found to have a reliable primary magnetic component through progressive demagnetization tests using thermal and alternating field methods tiltcorrected paleomagnetic data in the nohi area are divided into three stages based on geologic age late cretaceous early miocene 23 15 ma and middle miocene 12 10 ma the formation mean direction of 10 sites obtained from late cretaceous nohi rhyolite is d 17 0 i 49 8 and 95 6 1 neogene rock units underlain by nohi rhyolite show two distinct formation mean directions d 13 0 i 52 7 and 95 7 2 for eight sites of early miocene and d 2 5 i 55 6 and 95 6 6 for four sites of middle miocene differential rotation is proposed between the eastern part of southwest japan including the nohi area and the central part of southwest japan which is known to have rotated clockwise through about 50 around 15 ma the spatial distribution of contemporaneous paleomagnetic data in southwest japan suggests that two differentially rotated parts are connected in the western neighborhood of the nohi area it is concluded that the nohi area had rapidly rotated counterclockwise through 48 relative to the central part of southwest japan between 15 and 12 ma together with previous paleomagnetic data from early miocene rocks in the kanto area which is situated to the east of the nohi area the present results suggest that differential rotation of the eastern part of southwest japan is attributed to the formation of northward cuspate shapes of pre neogene terranes which constitute the framework of the japan arc one probable explanation of the formation of cusps is a crustal indentation caused by the collision of landmasses on the philippine sea plate against the japan arc in the middle miocene

submarine geology and post opening tectonic movements in the southern region of the sea of japan abstract the submarine geologic structure of the southern region of the sea of japan was analyzed on the basis of seismic reflection profile data and bottom samples the seismic reflection patterns depict sedimentary layers with well developed onlap and toplap terminations and truncations by submarine erosion the planes marked by series of onlap toplap and erosional truncations enabled us to define three stratigraphic groups composed of six formations in this region they are the kasumioki group the hamasakaoki group and the tottorioki group in ascending order detailed analysis of the seismic reflection profiles reveals subtle structures of these sedimentary units the oldest kasumioki group is folded and faulted forming submarine ridges the hamasakaoki group mainly occurs in the depressions between these ridges the youngest tottorioki group covers widely both of the older formations with a pronounced marine onlap fossil evidence indicates that the depositional ages of the kasumioki group the hamasakaoki group and the tottorioki group are early miocene to earliest pliocene late early to late pliocene and late pliocene to holocene respectively these observations provide the basis to understand two main structural features in this region 1 e w trending folds and 2 northwestward tilting blocks the former construct a fold zone composed of two to four ranges of anticlines that have steeper southern flanks truncated by reverse faults the latter are shown by slightly inclined floors formed by submarine erosion step like asymmetric basins bounded by ne sw trending reverse faults and onlap toplap sedimentation patterns the lateral change in bed thickness sedimentation patterns and age of submarine sediments indicate that the e w trending folds were formed in the early middle or late miocene to the early pliocene by n s compression whereas the northwestward tilt blocks were formed in the late early to late pliocene by nw se compression the sedimentary records in this region dictate an abrupt change in tectonic setting in the late early pliocene this change is interpreted to show that the sea of japan has been closing since then due to either the reactivation of the amurian plate or the directional change of the philippine sea plate motion

the iugs systematics of igneous rocks in order to create a sustainable classification of igneous rocks which all geologists might use an international body was set up by the iugs the iugs subcommission on the systematics of igneous rocks in the course of creating the classification the subcommission has established ten principles for its construction and for defining an appropriate nomenclature the principles are 1 use descriptive attributes 2 use actual properties 3 ensure suitability for all geologists 4 use current terminology 5 define boundaries of rock species 6 keep it simple to apply 7 follow natural relations 8 use modal mineralogy 9 if mode not feasible use chemistry 10 follow terminology of other iugs advisory bodies these principles and their rationale have not previously been enunciated the classification separates and individually classifies the pyroclastic carbonatitic melititic lamprophyric and charnockitic rocks before entering the main qapf classification for plutonic and volcanic rocks which is based on the modal mineral proportions of quartz q alkali feldspar a and plagioclase p or of alkali feldspar a plagioclase p and feldspathoids f rocks with mafic content 90 have their own classification if the mineral mode cannot be determined as is often the case for volcanic rocks then a chemical classification of total alkalis versus silica tas is used the nomenclature for these classifications necessitates only 297 rock names out of the c 1500 that exist

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