structural geology and regional tectonic significance of the ramgarh thrust himalayan fold thrust belt of nepal 1 the ramgarh thrust is one of the major fault systems of the himalayan thrust belt in nepal and northern india the ramgarh thrust sheet is 0 2 2 0 km thick and can be traced along strike the entire length of the himalaya in nepal the fault generally places the oldest paleoproterozoic rocks in the lesser himalayan series upon younger lesser himalayan rocks or lower miocene foreland basin deposits regional balanced cross sections suggest that the ramgarh thrust had at least 120 km of initial south vergent displacement subsequently the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying lesser himalayan rocks ramgarh hanging wall strata are greenschist grade phyllite quartzite and augen gneiss all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top to the south sense of shear structural fabrics in the ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet regional and local mapping of the ramgarh thrust in nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat except where local lateral ramps complicate its geometry similarly the structurally overlying main central thrust always places a hanging wall flat in greater himalayan series rocks upon the regionally flat ramgarh thrust sheet these geometric relationships preclude kinematic and thermal models that elevate greater himalayan and lower lesser himalayan rocks along high angle thrust ramps in the vicinity of the present traces of the ramgarh and main central thrust faults instead the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the main central thrust the present steep dips of the ramgarh and main central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the lesser himalayan zone the ramgarh thrust sheet overlaps a bed length of at least 100 km in lower miocene foreland basin deposits indicating that a significant amount of displacement on the thrust must have occurred after 15 ma growth of the lesser himalayan duplex and additional slip on the frontal part of the ramgarh thrust occurred from 12 to 5 ma the presence of a major greenschist grade metasedimentary thrust sheet composed of lesser himalayan rocks directly below the main central thrust suggests that the famous inverted metamorphism in this region is a result of structural inversion similarly the concept of a broad zone of intense shear strain related exclusively to emplacement of the main central thrust sheet is probably invalid in nepal

origin ore forming fluid evolution and timing of the logrosan sn w ore deposits central iberian zone spain the logrosan sn w ore deposits in the metallogenic sn w province of the european variscan belt consist of endo and exogranitic greisen type and quartz cassiterite veins associated with a s type granite mineral characterization fluid inclusion study isotope geochemistry and ar ar geochronology have been combined in order to reconstruct the conditions for sn w mineralization the endo and exogranitic mineralization must have been developed in a relatively long lived system 308 303 ma during or soon after the emplacement of the logrosan related granite at ca 308 ma the mineralizing fluids are characterized by complex aqueous and volatile h2o n2 co2 ch4 nacl fluid inclusions microthermometry and raman analyses indicate that fluid composition evolved from n2 ch4 to n2 rich followed by co2 rich fluids with varying amounts of h2o the presence of n2 and ch4 suggests the interaction with fluids derived from the nearby metasedimentary host rocks a model of host rock interaction assimilation and mixing of metamorphic and magmatic fluids resulting in change of the redox conditions is proposed for tin deposition later sulfide minerals were precipitated as a result of pressure and temperature release

from magmatic to hydrothermal sn li nb ta w mineralization the argemela area central portugal abstract the argemela mineralized area ama central portugal exhibits a nearly continuous magmatic to hydrothermal ore forming sequence typical of variscan granite related rare metal deposits disseminated and vein type mineralization are distributed in two systems the argemela mine am and the cabeco da argemela ca disseminated mineralization includes montebrasite cassiterite and columbite tantalite dispersed in rare metal granites vein type mineralization occurs both in granites and country rocks the ca system exposes three generations of intragranitic veins with montebrasite in the two earliest and montebrasite wolframite cassiterite and columbite tantalite in the latest country rock veins include a swarm of cassiterite and montebrasite veins am and rare isolated wolframite veins ca disseminated cassiterites are enriched in ta2o5 and nb2o5 compared to vein cassiterites disseminated columbite tantalites evolve toward mn and ta rich chemistries that contrast with the more fe and nb rich compositions in intragranitic veins in the ca system both intragranitic and country rock veins crystallize early mn rich wolframites followed by late more fe rich compositions associated with nb ta poor cassiterite the later replaced by stannite field relations structural mineralogical and geochemical data suggest that the disseminated and vein type mineralization are expressions of a continuous metallogenic evolution initiated at the magmatic stage pursued during the magmatic hydrothermal transition and ended with hydrothermal circulations in country rock the two mineralized systems share similarities but they also show major differences structural analysis demonstrates that the intragranitic veins in the ca and the country rock veins in the am were emplaced under the same tectonic regime therefore the two systems are variants of the same local metallogenic evolution sn and w show markedly contrasted behaviors both have a magmatic source but only sn reaches concentrations leading to saturation of the melt with cassiterite w is deposited later and preferentially to sn in intragranitic veins at the hydrothermal stage cassiterite and wolframite deposition are disconnected and overall wolframite occurs only in minor amounts the metallogenic model proposed for the ama emphasizes magmatic rare metal concentration processes exsolution of magmatic fluids and selective deposition of metals during the magmatic hydrothermal and later hydrothermal stages the sn li nb ta dominated and w poor signature of the ama makes it similar to lct pegmatites of the central iberian zone but is distinctive from the world class w sn cu panasqueira deposit nearby

geochemical indicators of metalliferous fertility in the carboniferous san blas pluton sierra de velasco argentina in the sierra de velasco northwestern argentina undeformed lower carboniferous granitoids 350 334 ma intrude deformed lower ordovician granites and have been emplaced by passive mechanisms typical of tensional environments the semi elliptic about 300 km shallow emplaced san blas pluton is 340 330 ma old with 1ndt between 21 3 and 21 8 which indicates that different from the nearby famatinian ordovician granitoids the san blas pluton had a relatively brief crustal residence with an interaction between asthenospheric material and greywackes the cupola of the pluton was almost totally eroded down during the upper

higher himalayan shear zone sutlej section structural geology and extrusion mechanism by various combinations of simple shear pure shear and channel flow in shifting modes the higher himalayan shear zone hhsz in the sutlej section reveals 1 top to sw ductile shearing 2 top to ne ductile shearing in the upper and the lower strands of the south tibetan detachment system stdsu stdsl and 3 top to sw brittle shearing corroborated by trapezoid shaped minerals in micro scale in the proposed extrusion model of the hhsz the e1 phase during 25 19 ma is marked by simple shearing of the upper sub channel defined by the upper strand of the main central thrust mctu and the top of stdsu as the lower and the upper boundaries respectively subsequently the e2a pulse during 15 14 ma was characterized by simple shear pure shear and channel flow of the entire hhsz finally the e2b pulse during 14 12 ma observed simple shearing and channel flow of the lower sub channel defined by the lower strand of the main central thrust mctl and the top of the stdsl as the lower and the upper boundaries respectively the model explains the constraints of thicknesses of the stdsu and the stdsl along with spatially variable extrusion rate and the inverted metamorphism of the hhsz the model predicts 1 shear strain after ductile extrusion to be maximum at the boundaries of the hhsz which crudely matches with the existing data the other speculations that cannot be checked are 2 uniform shear strain from the mctu to the top of the hhsz in the e1 phase 3 fastest rates of extrusion of the lower boundaries of the stdsu and the stdsl during the e2a and e2b pulses respectively and 4 variable thickness of the stdsl and rare absence of the stdsu non parabolic shear fabrics of the hhsz possibly indicate heterogeneous strain the top to sw brittle shearing around 12 ma augmented the ductile extruded rocks to arrive a shallower depth the brittle ductile extension leading to boudinage possibly did not enhance the extrusion

upper crustal shortening and forward modeling of the himalayan thrust belt along the budhi gandaki river central nepal a balanced cross section along the budhi gandaki river in central nepal between the main central thrust including displacement on that fault and the main frontal thrust reveals a minimum total shortening of 400 km minimum displacement on major orogen scale structures include 116 km on the main central thrust 110 km on the ramgarh thrust 95 km on the trishuli thrust and 56 km in the lesser himalayan duplex the balanced cross section was also incrementally forward modeled assuming a generally forward breaking sequence of thrusting where early faults and hanging wall structures are passively carried from the hinterland toward the foreland the approximate correspondence of the forward modeled result to observe present day geometries suggest that the section interpretation is viable and admissible in the balanced cross section the trishuli thrust is the roof thrust for the lesser himalayan duplex the forward model and reconstruction emphasize that the lesser himalayan duplex grew by incorporating rock from the footwall and transferring it to the hanging wall along the main himalayan thrust as the duplex developed the lesser himalayan ramp migrated southward the movement of lesser himalayan thrust sheets over the ramp pushed the lesser himalayan rock and the overburdens of the greater and tibetan himalayan rock toward the erosional surface this vertical structural movement caused by footwall collapse and duplexing in combination with erosion exhumed the lesser himalaya

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

seaweed assisted benthic gravel transport by tidal currents abstract traction transport of pebbles and cobbles occurs subtidally at current velocities below 0 5 m s 1 when seaweed attached to clasts provides additional lift and drag to the clast in the juan de fuca strait british columbia canada the seaweed cymanthere triplicata commonly attaches to pebbles and provides sufficient additional surface area for tidal currents to drag the clast along the seafloor using in situ measurements of current velocities at 13 m water depth the threshold for initiation of motion of a 30 mm pebble with attached seaweed is 0 3 m s 1 this is approximately one order of magnitude less than the activation velocity for a 30 mm pebble without attached seaweed in addition to kelp rafted floated gravel seaweed assisted benthic gravel transport is possible in marine settings where unidirectional currents e g tidal currents storm induced bottom currents are sufficient to transport pebbles alongshore and into and across the offshore below fairweather wave base if preserved in the rock record deposits of algal enhanced gravel deposited via unidirectional subtidal currents will likely appear as isolated gravel clasts encased in sandstone reflecting the similar current velocities required to transport these two clast groups

seismotectonics of the nepal himalaya from a local seismic network the national seismological network of nepal consists of 17 short period seismic stations operated since 1994 it provides an exceptional view of the microseismic activity over nearly one third of the himalayan arc including the only segment between longitudes 78 e and 85 e that has not produced any m 8 earthquakes over the last century it shows a belt of seismicity that follows approximately the front of the higher himalaya with most of the seismic moment being released at depths between 10 and 20 km this belt of seismicity is interpreted to reflect interseismic stress accumulation in the upper crust associated with creep in the lower crust beneath the higher himalaya the seismic activity is more intense around 82 e in far western nepal and around 87 e in eastern nepal western nepal between 82 5 and 85 e is characterized by a particularly low level of seismic activity we propose that these lateral variations are related to segmentation of the main himalayan thrust fault the major junctions between the different segments would thus lie at about 87 e and 82 e with possibly an intermediate one at about 85 e these junctions seem to coincide with some of the active normal faults in southern tibet lateral variation of seismic activity is also found to correlate with lateral variations of geological structures suggesting that segmentation is a long lived feature we infer four 250 400 km long segments that could produce earthquakes comparable to the m 8 4 bihar nepal earthquake that struck eastern nepal in 1934 assuming the model of the characteristic earthquake the recurrence interval between two such earthquakes on a given segment is between 130 and 260 years

metallogeny of tin magmatic differentiation versus geochemical heritage tio 2 sn and rb sr sn variation diagrams for granitic fractionation series from various tin and non tin provinces do not support the concept of geochemical heredity of tin i e the assumption of a primary crustal sn anomaly in tin provinces the controlling factor for the generation of tin granites is a special bulk distribution coefficient of sn probably dependent on the oxygen fugacity of the melt

the siwaliks of western nepal i geometry and kinematics abstract the siwalik group which forms the southern zone of the himalayan orogen constitutes the deformed part of the neogene foreland basin situated above the downflexed indian lithosphere it forms the outer part of the thin skinned thrust belt of the himalaya a belt where the faults branch off a major decollement md that is the external part of the basal detachment of himalayan thrust belt this decollement is located beneath 13 ma sediments in far western nepal and beneath 14 6 ma sediments in mid western nepal i e above the base of the siwalik group unconformities have been observed in the upper siwalik member of western nepal both on satellite images and in the field and suggest that tectonics has affected the frontal part of the outer belt since more than 1 8 ma several north dipping thrusts delineate tectonic boundaries in the siwalik group of western nepal the main dun thrust mdt is formed by a succession of 4 laterally relayed thrusts and the main frontal thrust mft is formed by three segments that die out laterally in propagating folds or branch and relay faults along lateral transfer zones one of the major transfer zones is the west dang transfer zone wdtz which has a north northeast strike and is formed by strike slip faults sigmoid folds and sigmoid reverse faults the width of the outer belt of the himalaya varies from 25 km west of the wdtz to 40 km east of the wdtz the wdtz is probably related to an underlying fault that induces a a change of the stratigraphic thickness of the siwalik members involved in the thin skinned thrust belt and particularly of the middle siwalik member b an increase from west to east of the depth of the decollement level and c a lateral ramp that transfers displacement from one thrust to another large wedge top basins duns of western nepal have developed east of the wdtz the superposition of two decollement levels in the lower siwalik member is clear in a large portion of the siwalik group of western nepal where it induces duplexes development the duplexes are formed either by far travelled horses that crop out at the hangingwall of the internal decollement thrust id to the south of the main boundary thrust or by horses that remain hidden below the middle siwaliks or lesser himalayan rocks most of the thrusts sheets of the outer belt of western nepal have moved toward the s sw and balanced cross sections show at least 40 km shortening through the outer belt this value probably under estimates the shortening because erosion has removed the hangingwall cut off of the siwalik series the mean shortening rate has been 17 mm yr in the outer belt for the last 2 3 ma

late cretaceous granites from the giant dulong sn polymetallic ore district in yunnan province south china geochronology geochemistry mineral chemistry and nd hf isotopic compositions abstract as a world class tin tungsten province south china is well known for its extensive mesozoic granitic magmatism the dulong district located in the western cathaysia block of the south china tin tungsten province is characterized by widespread mesozoic granitoids and accompanying sn polymetallic ore deposit 30 mt of sn it is one of the most important polymetallic tin ore districts in china in this study three mineralization related granite types were identified in the dulong district including the dulong coarse grained granite dcg the dulong fine grained granite dfg and the dulong porphyritic granite dpg detailed studies are presented on zircon u pb ages major and trace elements mineral chemical and nd hf isotopic compositions of the tin bearing granites from the dulong district la icp ms u pb dating of zircon grains from these three granite bodies yields ages of 90 1 0 7 ma 89 7 0 8 ma and 86 0 0 5 ma respectively geochemically the granites are strongly peraluminous with high contents of alkalis enrichment in p li rb cs ta sn w and u depletion in ti mg co ni sr ba zr hf th and rare earth elements fractional crystallization of plagioclase and k feldspar was the principal process of magmatic differentiation that controlled rb sr ba and eu concentrations whereas rare earth elements were fractionated by accessory minerals such as apatite and monazite the geochemical data suggest that the rocks are highly fractionated s type granites the granites show bulk rock e nd t values in the range of 12 2 to 10 8 and zircon e hf t values from 15 5 to 2 5 with meso paleoproterozoic t dm c ages for both nd and hf isotopes geochemical and isotopic data suggest that these highly fractionated s type granites dcg dfg and dpg were originated from the same episode of partial melting of the protolith which have analogous components of metamorphosed pelitic rocks from the meso paleoproterozoic continental crust extreme fractional crystallization resulted in enrichment of tin in the evolved granitic magma and the reduced magmas fo 2 below nno would efficiently remove sn into a hydrothermal fluid and lead to deposition of sn rich mineral phases the association of dulong and gejiu magmatism in the western cathaysia block may be likely to mark the onset of back arc extension or intra arc rift in the region during late cretaceous the upwelling of asthenospheric mantle may have triggered partial melting of metasedimentary rocks in the overlying crust to generate the s type granitic magmas

geochemical and isotopic constraints on the petrogenesis of early ordovician granodiorite and variscan two mica granites from the gouveia area central portugal abstract a biotite granodiorite and seven sn bearing two mica granites crop out in the gouveia area central portugal a shrimp u th pb zircon age from the granodiorite and monazite ages from four of the two mica granites show that they are of early ordovician 480 ma and permo carboniferous i e variscan 305 and 290 ma age respectively the variscan two mica granites are late and post d3 major and trace element variation in the granitic rocks and their biotite and muscovite indicate mainly individual fractionation trends the granitic rocks are mostly depleted in hree relative to lree the biotite granodiorite is probably derived from igneous lower crust as evidenced by low initial 87 sr 86 sr 0 7036 high e nd t 2 5 and moderate 18 o 8 8 the two mica granites are probably derived by partial melting of heterogeneous mid crustal metasediments mainly metapelite and some metagraywacke as evidenced by their high initial 87 sr 86 sr 0 7076 0 7174 18 o 10 7 13 4 and major element compositions however variation diagrams for major and trace elements from two of the muscovite biotite granites and their micas define fractionation trends rb sr whole rock analyses from the two granites are perfectly fitted to a single isochron and the rocks have sub parallel ree patterns the younger granite is derived from the older by fractional crystallization of quartz plagioclase biotite and ilmenite tested by modelling major and trace elements most of the sn bearing granites are derived from distinct magma batches they result from partial melting of a heterogeneous mid crustal metasediment they do not represent a crustal anomaly in tin fractional crystallization is responsible for the increase in the sn contents of the granites and their micas muscovite has a higher sn content than coexisting biotite and is the principal host mineral for sn in these rocks

origin of the granites and related sn and pb zn polymetallic ore deposits in the pengshan district jiangxi province south china constraints from geochronology geochemistry mineral chemistry and sr nd hf pb s isotopes the pengshan sn and pb zn polymetallic deposits are located in the south margin of the jiujiang ruichang jiurui district of the middle lower yangtze river metallogenic belt in south china four large deposits include huangjinwa zengjialong jianfengpo and zhangshiba the former three are sn dominant deposits which occur as stratiform orebodies in the contact zones of the pengshan granites and within the country rock strata whereas zhangshiba consists of stratiform pb zn orebodies within the precambrian metasedimentary strata in this study we present results on zircon u pb ages major and trace elements and mineral chemistry as well as sr nd hf isotope data of the granites pb and s isotopes of both the sn dominant and pb zn dominant deposits and u pb dating of cassiterite from the pengshan district shrimp and la icp ms zircon u pb dating shows that the pengshan granites were emplaced in the early cretaceous 129 128 ma which is in good agreement with the u pb dating 130 128 ma of cassiterite from the jianfengpo sn deposit the pengshan granites consist mainly of weakly peraluminous highly fractionated i type affinity granitic rocks detailed elemental and isotopic data suggest that the granites formed by partial melting of mesoproterozoic metamorphic basement materials with minor input of mantle derived melts the mineral chemistry of biotite demonstrates that the pengshan granitic magma had a low oxygen fugacity thereby precluding the tin dominantly partitioning into the rock forming silicate minerals and favoring accumulation in the exsolved residual liquid during magma crystallization stages sulfur isotopes show a relatively heavy sulfur isotopic composition from 5 8 to 17 6 and no difference for sulfur isotopes between the sn deposits 5 8 13 4 huangjinwa zengjialong jianfengpo and the pb zn deposit mostly 7 1 13 0 except for one 17 6 zhangshiba the sulfur isotope data of pyrite from the host sedimentary rocks show relatively higher 34s values of 22 7 28 7 lead isotopes indicate that the granites have higher 206pb 204pb 207pb 204pb and 208pb 204pb ratios than the host precambrian metasedimentary rocks and the sulfide minerals from the orebodies fall in between them again there is no difference for pb isotopes between the sn deposits huangjinwa zengjialong jianfengpo and the zhangshiba pb zn deposit the coincidence in s and pb isotope compositions for sn and pb zn deposits suggests that similar sources for these mineralization possibly derived from a mixed source of the granitic magmas and the precambrian metasedimentary rocks these data therefore favor of a magmatic hydrothermal origin for both the sn and pb zn polymetallic mineralization in the pengshan district

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

geochemistry of hybrid granitoid rocks and of their biotites from central northern portugal and their petrogenesis abstract in the area of franzilhal s lourenco and pombal central northern portugal occur rocks ranging from hornblende biotite tonalite to muscovite biotite granite the distribution of some major and trace elements in the rocks and in the biotite shows an apparent calc alkalic trend of differentiation the behaviour of k li rb cs and k rb cr 103 fe3 ni 103 mg li 103 mg cs 103 k ratios of the rocks and of f of biotite cannot be explained by mineral fractionation but can be described in terms of hybridization the tonalite granodiorites and biotite muscovite granite are hybrid rocks formed by assimilation of pelitic sediments and dolomitic marbles by the granite magma the fo2 values range from 10 14 8 to 10 16 3 fo2 attains higher values during the crystallization of hybrid rocks than in the uncontaminated muscovite biotite granite giii magma the fh2o and ph2o values generally increase and the fh2o fhf ratio decreases in the hybrid rocks from tonalite to biotite muscovite bearing granodiorite and is smaller than in the uncontaminated granite magma

the role of fractional crystallization in the genesis of early syn d3 tin mineralized variscan two mica granites from the carrazeda de ansiaes area northern portugal abstract a multiphase granitic complex g1 g6 in the carrazeda de ansiaes area central iberian zone was emplaced during syn kinematic stages of the variscan orogeny between 329 9 0 8 ma and 318 9 1 9 ma as determined by id tims u pb on zircon and monazite the complex intruded the metasedimentary douro group in a nw trending antiform formed during the d 1 and d 3 deformation phases but was itself affected by ductile and brittle deformation the granites have equal or higher amounts of muscovite than biotite and contain surmicaceous enclaves and more rarely xenoliths schlieren and microgranular enclaves they are s type mainly alkaline calcic and peraluminous with asi ranging between 1 21 and 1 45 and normative corundum of 2 63 4 77 they are enriched in lree relatively to hree with negative eu anomalies 87 sr 86 sr i 0 7097 0 7159 end 6 2 to 9 1 and 18 o 10 96 11 81 geochemical and isotopic characteristics show that granites g1 g2 g4 and g5 correspond to distinct pulses of magma formed by partial melting of heterogeneous metasedimentary material geochemical fractionation trends recorded by whole rocks and minerals permit to distinguish and model two fractional crystallization series g2 g3 and g5 g6 the latter also affected by crustal assimilation during fractionation granites g1 g2 g3 and g6 have 20 ppm sn and g5 16 ppm sn they are also associated with mineralized sn and w bearing quartz veins fractional crystallization was the responsible process for the increase in sn content in granites from the series and their micas

a late cretaceous felsic magmatic suite from the tengchong block western yunnan integrated geochemical and isotopic investigation and implications for sn mineralization the tengchong block within the sanjiang tethys belt in the southeastern part of the tibetan plateau experienced a widespread intrusion of a felsic magmatic suite of granites in its central domain during late cretaceous times here we investigate the guyong and xiaolonghe plutons from this suite in terms of their petrological geochemical and sr nd zircon u pb and lu hf o isotopic features to gain insights into the evolution of the neo tethys the guyong pluton 76 ma is composed of metaluminous monzogranites and the xiaolonghe pluton 76 ma is composed of metaluminous to peraluminous medium and fine grained syenogranite a systematic decrease in eu ba sr p and ti concentrations a decrease in zr hf and lree hree ratios and an increase in the rb ba and ta nb ratios from the guyong to xiaolonghe plutons suggest fractional crystallization of biotite plagioclase k feldspar apatite ilmenite and titanite they also show the characteristics of i type granites the negative zircon ehf t isotopic values 10 04 to 5 22 and high 18o values 6 69 to 8 58 and the negative whole rock end t isotopic values 9 7 to 10 1 and high initial 87sr 86sr ratios 0 7098 0 7099 of the guyong monzogranite suggest that these rocks were generated by partial melting of the precambrian basement without mantle input the zircon ehf t isotopic values 10 63 to 3 04 and 18o values 6 54 to 8 69 of the xiaolonghe syenogranite are similar to the features of the guyong monzogranite and this similarity suggests a cogenetic nature and magma derivation from the lower crust that is composed of both metasedimentary and meta igneous rocks the xiaolonghe fine grained syenogranite shows an obvious rare earth element tetrad effect and lower nb ta ratios which indicate its productive nature with respect to ore formation in fact we discuss that the sn mineralization in the region was possible due to sn being scavenged from these rocks by exsolved hydrothermal fluids we correlate the late cretaceous magmatism in the central tengchong block with the northward subduction of the neo tethys beneath the burma tengchong block

geochemistry of tin bearing granitic rocks abstract the use of rb ba and rb sr ratios as guides to tin mineralization has only local interest because each of the correlations log rb ba log sn and log rb sr log sn of granitic series has a different slope in every area the ternary relation between rb ba and cs is useful to distinguish between sn bearing granitic rocks and non mineralized granitic rocks the sn bearing granitic rocks are late differentiates of anatectic melts s type with primary albite and muscovite and greisenized granites with metasomatic albite and muscovite they have high sn rb contents rb ba and rb sr ratios and low ba and sr contents the portuguese sn bearing muscovite granites aplites pegmatites and hypothermal quartz veins have muscovites richer in sn than those of greisenized granites f and sn of muscovites from portuguese sn bearing granitic rocks are positively correlated cassiterite crystallization seems to be independent of f h 2 o f in hf and favoured by a decrease in fo2

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

asymmetric growth of the pyrenees revealed through measurement and modeling of orogenic fluxes orogenic asymmetry is ubiquitous in terms of topography structure and metamorphism but the relative influences of climate and tectonics in determining such asymmetry remain contentious this study demonstrates that for the case of the european pyrenees orogenic asymmetry is not defined by inherited structural asymmetry nor by orographic asymmetry of precipitation but is the simple consequence of the inherent tectonic growth of a doubly vergent orogen the pyrenees record approximately 60 myr of upper crustal accretion erosion and sedimentation that developed from late cretaceous to early miocene times approximately 165 km of plate convergence was accommodated by four phases of orogenic growth that can be summarized in terms of their generic orogenic development as follows 1 early inversion of passive margin extensional faults initiating the pro wedge 2 growth by a combination of frontal accretion and underplating to the pro wedge symmetrical and steady record of erosion and foreland basin sedimentation 3 cessation of pro wedge frontal accretion with propagation of the retro wedge deformation front 4 accelerated growth of pro wedge antiformal stack induced by underplating accelerated erosion over antiformal stack and increase in sediment discharge to pro foreland basin stabilization of outer pro wedge by sediment blanketing discrete element model experiments are used to explore the natural tendency of the system towards asymmetry model output demonstrates that during active frontal accretion to the pro wedge of an orogen the maxima in erosional denudation is offset towards the pro wedge relative to the maximum depths of exhumation this asymmetry is fundamentally driven by the progressive reduction in advection velocities of material within the orogen from the pro wedge into the retro wedge hence for an efficient erosional regime a dynamic coupling between erosion and structural uplift of the pro wedge will reduce activity of the retro wedge and hence drive asymmetry in terms of low temperature thermochronometry erosion and sedimentation the models also demonstrate the tendency for pro wedge growth through frontal accretion to slow however the cessation of frontal accretion as recorded by the south pyrenean thrust belt phases 3 and 4 requires additional forcing it is proposed that the thick wedge top sedimentation recorded at this time increased taper angles and hence stabilized the wedge accretionary influx into the orogen at this time was accomplished through underplating these processes governing asymmetry do not require a spatially variable climate or a laterally heterogeneous underthrust plate