identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

experimental melting of biotite plagioclase quartz muscovite assemblages and implications for crustal melting abstract in order to understand the role of mica rich rocks as a source of granite magmas a series of melting experiments was performed on two different starting materials the first composition is a model biotite gneiss consisting of 30 wt biotite 30 wt plagioclase and 40 wt quartz the second composition is a model two mica pelites consisting of 15 wt biotite 15 wt muscovite 30 wt plagioclase and 40 wt quartz experiments were performed under vapor absent conditions at 1 0 gpa and between 750 o and 950oc with only biotite in the starting material the volume of melt is always less than 15 vol below 900oc and reaches 25 vol at 950oc in experiments that involve both biotite and muscovite in the starting material the melt proportion increases up to 28 vol at 825oc and reaches 60 vol at 950oc for the biotite plagioclase quartz bpq assemblage the solidus is located at 800oc at 1 0 gpa the melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet biotite orthopyroxene plagioclase quartz in addition the experiments show that at 1 0 gpa biotite can be stable above 950oc with both micas in the starting material bpqm the solidus at 1 0 gpa is located at 750oc the melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet sillimanite biotite quartz plagioclase kfeldspar in experiments below 900oc at 950oc the residuum consists of garnet orthopyroxene biotite plagioclase the melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz mineral modes of the source rocks particularly the amount of quartz are at least as important as the amount of available h20 in controlling the melt fraction generated during crustal anatexis

age and cooling history of the manaslu granite implications for himalayan tectonics abstract the manaslu granite is one of the high himalayan leucogranites which are believed to be derived by partial melting of quartzofeldspathic gneissess of the high himalayan crystallines hhc and emplaced at the contact between the hhc and the tethyan sedimentary series we have analyzed a suite of muscovite 15 biotite 3 and alkali feldspar 10 by the 40 ar 39 ar method to help constrain the age and cooling history of the granite 40 ar 39 ar ages ranges are 18 4 to 13 3 ma 17 0 to 14 7 and 16 4 to 3 4 for muscovite biotite and k feldspar respectively based on the muscovite analyses and a re interpretation of a previously published u pb monazite age we conclude that the crystallization age of the manaslu granite is everywhere 20 ma the postcrystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km these data place a minimum age of movement on the mct in this area of 20 ma we estimate the time of magma segregation and transport to be no more than 5 ma

contact metamorphism and depth of emplacement of the manaslu granite central nepal implications for himalayan orogenesis abstract the manaslu massif central nepal provides a well exposed example of a deeply eroded pluton its contact aureole can be followed from the base to the top along medium to low grade mesozonal to anchizonal tethyan metasediments contact metamorphic mineral assemblages and thermobarometric estimations suggest that the granite was emplaced at 18 21 km for the base and 9 13 km for the roof calculated temperatures within the aureole at 550 40 c are compatible with the intrusion temperature of a leucogranitic magma microstructural evidence shows that the temperature remained high 500 c at the base of the massif during and after granite emplacement whereas towards the top of the granite deformation proceeded rapidly at lower temperature heating of the abundant calcareous rocks in the contact aureole released a local co2 rich fluid whereas a h2o and boron rich fluid seems to have pervaded the whole zone this fluid is probably exsolved from the granitic melt during its crystallization the depth of emplacement of the massif has an important implication for the reconstruction of the himalayan geodynamic evolution implying a burial of the tethyan metasediments by a major refolding of the sedimentary cover or more probably by extensive development of the north himalayan nappes towards the south for more than 70 km before granite emplacement i e before the miocene the young age of the granite and its depth of emplacement suggest a rapid tectonic denudation in the order of 1 mm a 1 probably by normal faulting north of the massif during its cooling

trace element modelling of pelite derived granites the presence or absence of a vapour phase during incongruent melt reactions of muscovite and biotite together with the composition of the protolith determines the trace element characteristics of the resulting melt provided that equilibrium melting occurs for those phases that host the tracc elements of interest for granitic melts rb sr and ba provide critical constraints on the conditions that prevailed during melting whereas ree are primarily controlled by accessory phase behaviour mass balance constraints for eutectic granites that are formed by the incongruent melting of muscovite in pelites indicate that melting in the presence of a vapour phase will result in a large melt fraction and deplete the restite in feldspar hence the melt will be characterized by low rb sr and high sr ba ratios in contrast vapour absent melting will result in a smaller melt fraction and an increase in the restitic feldspar consequently high rb sr and low sr ba ratios are predicted vapour absent melting will also enhance the negative eu anomaly in the melt granites that result from the incongruent melting of biotite in the source will be characterized by higher rb concentrations than those that result from the incongruent melting of muscovite the himalayan leucogranites provide an example of unfractionated crustally derived eutectic melts that are enriched in rb but depleted in sr and ba relative to their metasedimentary protoliths these compositions may be generated by the incongruent melting of muscovite as a low melt fraction f 0 1 from a pelitic source under vapour absent conditions

shisha pangma leucogranite south tibetan himalaya field relations geochemistry age origin and emplacement the shisha pangma pluton forming most of the xixabangma 8027 m massif in south tibet is one of the 20 larger leucogranite intrusives that mark the highest structural levels of the himalayan metamorphic core the pluton occurs immediately below the shisha pangma detachment a strand of the south tibetan detachment std system a low angle 30 north dipping normal fault placing paleozoic black slates atop sillimanite grade pelites and calc silicate rocks k feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal s c fabrics indicative of down to the north extension the shisha pangma leucogranite is a heterogeneous polyphase intrusion with an earlier foliated biotite rich phase and a later tourmaline muscovite rich phase typically containing the assemblage kfs pl qtz ms tur gt bt sil ap the highly peraluminous granites have high 87sr 86sr ratios 0 738 0 750 typical of pelite derived anatectites nd depleted mantle model

heat sources for tertiary metamorphism and anatexis in the annapurna manaslu region central nepal following the collision of india with southern tibet crustal rocks of the leading edge of india 1 underwent regional metamorphism to upper amphibolite grade 2 melted locally to produce anatectic granitoids and 3 were sheared and thrust onto lower grade rock along the main central thrust yielding an inverted metamorphic sequence this sequence is exemplified in the annapurna manaslu region we use simple physical calculations to examine the heat sources involved in the different phases of metamorphism the regional metamorphism apparently is due to the burial of the northern edge of india beneath the accretionary prism along the southern edge of tibet the observed temperatures and pressures of this first phase of metamorphism are consistent with the thermal relaxation during the 10 30 m y before slip on the main central thrust began of thickened continental lithosphere whose original surface heat flux was between 50 and 70 mw m 2 the release of water from the footwall of the main central thrust apparently facilitated melting of the overlying crust in the second phase such melting could have occurred in the first million years or so of thrusting if warm 550 650 c crust in the footwall contained the necessary water if melting did not occur in the earliest stages of slip on the main central thrust dissipative heating with shear stresses of 10 to 100 mpa is required for temperatures near the main central thrust to have remained high enough to generate melting above the fault during the underthrusting of cold material the thickness 6 8 km of the zone of inverted isograds associated with the fault if undisrupted and due solely to thermal diffusion implies that the time required to carry the rocks preserving the inverted metamorphism from the surface to depths of 30 40 km was 4 8 m y the apparent inverted temperature gradients about 10 25 c km in this zone can be understood as the combined result of two processes diffusion of heat from hot rock thrust over cold rock expunged the original temperature gradient near the fault and could have created an inverse gradient of 10 20 c km the peak temperatures in such a zone however would not have exceeded about 350 c without an additional source of heat dissipative heating at shear stresses of about 100 mpa can account for peak temperatures in excess of 600 c during slip on the fault and would have contributed as much as 13 c km to the inverse gradient although inversion of metamorphic isograds could have occurred as a result of deformation within the main central thrust zone the high temperatures during slip on this zone still require dissipative heating unless the duration of slip exceeded 25 m y

a study of strontium diffusion in k feldspar na k feldspar and anorthite using rutherford backscattering spectroscopy abstract sr chemical diffusion has been measured in orthoclase anorthoclase and anorthite under dry 1 atm conditions a strontium oxide aluminosilicate powder mixture was used as the source material with rutherford backscattering spectroscopy rbs used to measure diffusion profiles over the temperature range 725 1075 c the following arrhenius relations were obtained orthoclase normal to 001 d 5 97 10 0 0060 0 0030exp 67900 1600rtcm 2 s 1 measurements of diffusion perpendicular to 010 and 100 indicate little diffusional anisotropy in orthoclase anorthoclase normal to 010 d 4 51 10 1 358 5 6exp 89100 4800rt cm 2 s 1 normal to 001 d 2 25 10 2 1615 31exp 89300 4600rt cm 2 s 1 anorthite normal to 010 d 3 85 10 2 0 380 0 0039 exp 78800 5400rt cm 2 s 1 the correlation of sr uptake with a reduction of k and ca in orthoclase and anorthite respectively indicates that sr is exchanging with these species nuclear reaction analysis nra measurements of al in orthoclase suggests that the coupled exchange sr 2 al 3 k 1 si 4 is taking place as expected sr closure temperatures for feldspars calculated with the above parameters are relatively high compared to those of some other minerals e g biotite muscovite apatite however closure temperatures of feldspars may be significantly depressed if they are exsolved or twinned leading to small effective diffusion lengths feldspar crystals of millimeter size should retain 87 sr 86 sr ratios except when subjected to thermal events of extreme temperature or duration

possible thermal buffering by crustal anatexis in collisional orogens thermobarometric evidence from the nepalese himalaya one dimensional thermal models suggest that large thrust faults may exert first order control on the thermal structure of collisional orogenic belts and an increasing body of petrologic data may be interpreted as consistent with these models we present new thermobarometric data from the central nepalese himalaya which imply that large sections of the himalayan orogen may have been at roughly uniform temperatures during active thrusting a phenomenon that is not predicted by existing models we attribute this phenomenon to thermal buffering as a consequence of widespread in situ anatexis if this process is common in the intermediate and lower crust of active orogens and if it is localized spatially one dimensional thermal models may produce an extremely oversimplified picture of heat transfer processes during collisional orogenesis

new constraints on the age of the manaslu leucogranite evidence for episodic tectonic denudation in the central himalaya comment and reply

effect of oxidation state on bi mineral speciation in oxidized and reduced granitoids from the uetsu region ne japan the relationship between bismuth bi mineral speciation and redox state in three types of granitoids from the uetsu region northeast japan is investigated electron microprobe analysis of bi minerals sphalerite mg fe bearing carbonate minerals and muscovite as well as sulfur isotope analysis of sulfide minerals and microthermometric study of fluid inclusions reveal that bi mineral speciation varies according to the redox state of the granitoids for example native bismuth and bismuthinite are abundant and bi sulfosalts are rare in the lowest fs2 and fo2 mineralized zones of the reduced iwafune granite s type ilmenite series while bi sulfosalts bi3 are abundant and trace amounts of native bismuth bi0 and bismuthinite are found in the highest fs2 and fo2 mineralized zones of the oxidized wasada granodiorite i type magnetite series bismuthinite is a major bi mineral and native bismuth and bi sulfosalts occur in only minor amounts in the mineralized zones of the nishitagawa granodiorite i type ilmenite series which has intermediate fs2 and fo2 to that of the iwafune and wasada samples our study indicates that bi mineral speciation related to granitic intrusive activity is controlled by the redox state of the magmatism such that native bismuth is typical of reducing conditions whereas bi sulfosalts are typical of oxidizing conditions

age of crustal melting and leucogranite formation from u pb zircon and monazite dating in the western himalaya zanskar india new u pb data from leucogranites in the high himalayan slab in the western himalaya of zanskar reveal that crustal melting occurred at 21 19 5 ma and that anatexis occurred along the himalaya at least from kashmir zanskar to eastern nepal at approx 24 19 5 ma high u monazites from leucogranitic melt pods in migmatites within the 35 km wide sillimanite k feldspar zone at the deepest structural levels exposed umasi la have u pb ages of 20 6 19 5 ma higher level migmatitic melt pods from shafat in the suru valley also contain magmatic monazites with u pb ages of 20 8 0 3 ma zircons from the leucogranites contain an approx 460 ma inherited component as do some umasi la monazites showing that the protoliths were at least ordovician in age

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

u th pb systematics and ages of himalayan leucogranites south tibet abstract the age and origin of five leucogranites from the high and tethys himalaya and two country rock gneisses were investigated by u pb dating of zircon fractions and single grains and fractions of monazite additionally th u concentrations in whole rock powders and isotopic compositions of pb in leached k feldspars were determined monazites yield ages of 16 8 0 6 m y for the nialam migmatite granite 15 1 0 5 m y for the lhagoi kangri granite 14 3 0 6 m y for a granite from mt everest and 9 8 0 7 m y and 9 2 0 9 m y for two varieties of the maja granite these data together with monazite ages of 21 9 0 2 and 24 0 0 4 m y determined earlier on the makalu granite 1 substantiate a period of intracontinental granite emplacements from 24 to 9 m y ago i e from uppermost oligocene to late miocene times such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of india with eurasia furthermore the individual ages together with structural relationships between granites and country rocks suggest that granite formation and tectono metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m y inherited lead components present in all granite zircons point to large proportions of precambrian material in the magma source regions up to 2200 m y old th u systematics between monazite and country rocks indicate that u has been leached from most of the granites after crystallisation of monazite zircon dating of the kangmar granite gneiss which occurs in a window through the tethys himalayan sediments shows that this pluton transformed to a gneiss during the alpine orogeny crystallised in lowermost palaeozoic times 562 4 m y ago

isotopic constraints on the age and provenance of the lesser and greater himalayan sequences nepalese himalaya the main central thrust of the central himalaya juxtaposes the lesser himalayan sequence footwall with the greater himalayan sequence hanging wall both of which are regarded as components of the northern passive margin of india considerable uncertainty surrounds their respective ages and sedimentary derivation u pb and sm nd isotopic studies of samples from the langtang area central nepal demonstrate important distinctions between the two rock packages the greater himalayan sequence had a sedimentary provenance that included a major source of 0 8 1 0 ga zircons implying a late proterozoic age the source of the precambrian part of the lesser himalayan sequence contained 1 87 2 60 ga zircons and its depositional age is probably middle proterozoic nd isotopic characteristics of the two sequences are different at 21 ma the greater himalayan rocks had end values between 14 6 and 18 5 while their lesser himalayan counterparts had values between 21 4 and 25 9 giving rise to major differences in their model ages the lesser and greater himalayan sequences at langtang are interpreted as correlative to the lower and upper parts of the vindhyan supergroup currently exposed south of the gangetic plain in northern india on the basis of available age data the exposed hanging wall of the main central thrust in central and eastern nepal contains no archean early proterozoic basement if the fault does root into the basement at depth it must do so farther north than recently published cross sections indicate this implies a total displacement on the fault that is substantially greater than the 140 210 km required by tectonic overlap in the eastern himalayas

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

successive zoning of amphiboles during progressive oxidation in the daito yokota granitic complex san in belt southwest japan abstract clinoamphibole present in shallow intrusive masses of quartz diorite in the daito yokota area of the san in belt southwest japan is strongly zoned successively from a magnesio homblende core to an actinolite rim systematically decreasing in fe 2 fe 2 mg towards the rim the features of compositional variation and cation substitution are similar to those of amphiboles which have been altered by fluid separated during secondary boiling in porphyry copper bearing granitoids mineralogie data and textural relations of amphibole and the accompanying minerals indicate that the zoned amphibole has crystallized or recrystallized from a subsolidus fluid phase under progressively oxidizing conditions truly magmatic amphibole is restricted to small patches in the core of strongly zoned amphibole grains progressive oxidation is responsible for reequilibration of early formed fe ti oxide to yield extensive development of secondary magnetite and sphene the decreasing fe trend of amphibole has resulted from progressive fe depletion of the fluid phase by abundant magnetite precipitation the amphibole grains usually show an alternation of chemically distinct zones with unconformable boundaries indicating repeated growth and corrosion by multiple circulation of fluids

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

when did the roof collapse late miocene north south extension in the high himalaya revealed by th pb monazite dating of the khula kangri granite th pb ion microprobe measurements made on 12 monazite grains from the khula kangri granite tibet bhutan frontier are interpreted to indicate that crystallization occurred at 12 5 0 4 ma the leucogranite is cut by the gonto la detachment part of the southern tibet detachment system that has allowed upper level north directed extension of the himalayan orogen significant orogen normal extension in southern tibet appears to have continued 8 10 m y later than previously recognized this is the first reported crystallization age for a leucogranite east of the yadong cross structure an apparent 70 km offset of the high himalaya and southern tibet detachment west of the yadong cross structure reliable ages for high himalaya events major main central thrust slip granite generation and emplacement attainment of critical topography and major detachment extension group between ca 24 and 19 ma we interpret the west to east change across the yadong cross structure to be due to either 1 an abrupt 10 m y younging of principal high himalayan events or 2 a deeper thus younger exposed part of the footwall of the southern tibet detachment near khula kangri the southern tibet detachment is cut by the highly oblique yadong gulu rift a manifestation of tibet plateau east west extension integrated estimates of magnitude and rate of detachment displacement suggest that the observed postcrystallization north directed extension lasted for 1 3 m y after which time the yadong gulu rift formed this interpretation is consistent with initiation of east west extension of tibet at ca 8 ma

a late miocene pliocene origin for the central himalayan inverted metamorphism abstract perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the himalayan main central thrust mct generally thought to have been active during the early miocene however in situ 208 pb 232 th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the mct footwall occurred in this portion of the central himalaya at only ca 6 ma the apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the mct which tectonically telescoped the young metamorphic sequence this explanation may resolve some outstanding problems in himalayan tectonics such why the mct and not the more recently initiated thrusts marks the break in slope of the present day mountain range it also renders unnecessary the need for exceptional physical conditions e g high shear stress to explain the generation of the himalayan leucogranites

geochemical constraints on the bimodal origin of high himalayan leucogranites major and trace element and rb sr isotope systematics of the manaslu leucogranite central nepal have been examined to constrain the role of mineral fractionation and fluids in peraluminous granite petrogenesis biotite and tourmaline are for the most part mutually exclusive with a predominance of two mica leucogranites over tourmaline leucogranites the 87sr86sr initial isotopic ratios sri indicate that leucogranitic melts were derived from two different sources the two mica leucogranites having a metagreywacke origin with sri 0 752 nd 15 such a bimodal nature of the source zone does not directly influence the magmatic evolution except that probably the higher initial boron content in the metapelitic rocks may increase the na2ok2o ratio in contrast the amount of water present during melting principally controls in part anatectic processes and element behaviour water saturated conditions probably occured during melting of metagreywackeous rocks and favoured crystallization of two mica leucogranites whereas water absent conditions prevailed during melting of metapelitic layers and favoured biotite plagioclase and monazite fractionation in the source zone and tourmaline crystallization in the leucogranite

granite migmatite genetic link the example of the manaslu granite and tibetan slab migmatites in central nepal in central nepal the tibetan slab is made up of biotite gneisses metapelites and metagreywackes orthogneisses metagranites and migmatites melanosomes are generally biotite muscovite bearing but locally they may be tourmaline rich when associated with boron rich granitic material leucosomes occur as lenses conformable with the foliation veins patches or as fillings in shear zones and extensional structures field relationships and mineralogical and chemical data show that three processes may have contributed to the formation of the tibetan slab leucosomes metamorphic differentiation or disequilibrium partial melting low zr tonalitic leucosomes in situ equilibrium partial melting high zr leucosomes and some granitic leucosomes and injection of externally derived melts most granitic and some tonalitic leucosomes the manaslu pluton belongs to the high himalayan leucogranite belt and was emplaced at the top of the tibetan slab it corresponds to a muscovite biotite leucogranite that has been assumed to derive from melting of the tibetan slab gneisses formation i phase relationships a more magnesian chemistry of the ferromagnesian minerals from the tibetan slab migmatites as compared to the manaslu leucogranite the microtextures of accessory phases and trace element compositions lower u li f and higher sr eu y yb contents in the migmatite leucosomes show that the in situ himalayan migmatites at the crustal level presently exposed have not been produced under the same p t xh2o conditions as the manaslu leucogranite magma while the formation i was the probable source for the manaslu granite migmatites within the formation are not the remanants of a melting process from which the manaslu granite was derived both the tibetan slab migmatites and the manaslu leucogranite may be considered as evidence of dehydration and melting at deeper crustal levels and of percolation of melts and hydrothermal fluids through the crust

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

hydrogen and oxygen isotope variations in the high himalaya peraluminous manaslu leucogranite evidence for heterogeneous sedimentary source abstract the manaslu granite belongs to the high himalaya leucogranitic belt which was produced by melting of the crust during postcollisional thrusting d and 18o values have been determined for whole rock and coexisting minerals from the 8 km thick manaslu massif and its 50 km long dyke sheet its country rocks and the formation 1 f1 paragneisses which are the source of the granite for the granite dmusc range from 70 to 85 and 18o w r from 10 9 to 12 8 h and o isotope fractionations among minerals are consistent with high temperature equilibrium and for oxygen closed system evolution a few samples coming mainly from the dyke swarm have very low d values down to 188 and biotite muscovite h isotope fractionations indicative of disequilibrium the d h ratios of associated magmatic tourmaline are essentially unmodified from the distribution of d values in the granite and its country rocks circulation of very low deuterium meteoric hydrothermal waters was extremely localized because these waters are depleted in deuterium by up to 50 relative to modern meteoric waters the manaslu area was either at an altitude substantially higher than that of today 2500 6000 m for the analyzed samples or a mountain chain once existed to the south the f1 gneisses have 18oquartz between 12 and 14 3 which confirms that the granite was generated from f1 but d values are 20 higher than in the granite such a difference can be a result of degassing of the magma and or introduction of fluid in the melting zone infiltration of low d fluid 90 into the hot but dry f1 probably triggered partial melting these fluids could have come from the dehydration of the midlands sediments which are separated from the overlying f1 by the main central thrust the correlations among 18o 87 sr 86 sr 20 ma and nd values in both f1 and the granite indicate that the variations of these isotopic ratios in the manaslu are inherited from those in f1 at the time of melting in turn these ratios in f1 are related to the proportion of quartz and phyllosilicates for the isotopic ratios of nd and o and to the quantity of radiogenic sr generated within the sediment which is a function of age and rb content amount of phyllosilicate some other himalayan leucogranites require either other crustal source rocks or the 18o and 87 sr 86 sr ratios of f1 vary along the himalaya

geochemistry of granitic melts produced during the incongruent melting of muscovite implications for the extraction of himalayan leucogranite magmas granites formed from the incongruent melting of muscovite preserve evidence of chemical disequilibrium both between restite and melt and within individual phases during crystallization miocene leucogranites from the himalayas exemplify melt formation from a pelitic source through muscovite breakdown trace elements rb sr and ba reside almost entirely in micas and feldspars in the protolith and therefore are appropriate for quantitative modeling equilibrium between melt and restite was established for sr under fluid phase absent conditions but some disequilibrium is suggested by rb abundances consistent with known diffusion rates for these elements abundances of zr and ce in the granites which provide essential structural constituents of the accessory phases zircon and monazite indicate that the himalayan granite melts were saturated in these elements if formed at temperatures of 670 700 c but slightly undersaturated if formed at somewhat higher temperatures ion microprobe traverses of y and heavy rare earth element abundances across single garnets from himalayan leucogranites provide evidence for disequilibrium between melt and phenocryst during garnet growth thus constraining the evolution of magmatic compositions during melt formation and crystallization in some cases progressive melting of the protolith and crystallization of garnet from the melt appear to have been concomitant indicating that minimum solidus temperatures were maintained throughout the melting event this suggests that thermal buffering rather than melt extraction provided the rate determining step in magma evolution

the contact between the higher himalaya crystallines and the tibetan sedimentary series miocene large scale dextral shearing space and time evolution of the synmetamorphic structures across the metamorphic pile have been studied in several areas along the himalayan belt east and central nepal garhwal zanskar from one area to the other the evolution is very similar 1 at the base of the pile in the main central thrust mct shear zone the stretching lineation penetrative and regularly oriented n0 e to n30 e indicates the mct transport direction very constant all along the belt from the eohimalayan main metamorphic development up to the late metamorphic movements 2 at the top of the pile at the contact between the crystalline unit and its sedimentary cover gravity driven structures are confirmed north vergent folds ductile normal faulting however there are numerous local indications of late miocene syn to late emplacement of the leucogranitic plutons dextral shearing 3 in between across the medium part of the pile the stretching lineation shows a conspicuous progressive regional clockwise rotation clearly indicated by the strain trajectories mapped in nepal and garhwal points 1 and 2 show that the crystalline sedimentary boundary despite its apparent structural and metamorphic continuity is not only a normal fault but also an important dextral shear zone which has acted since the upper miocene as the main southern limit of the eastward extruding tibetan block

rbsr microchrons in the manaslu granite implications for himalayan thermochronology abstract samples of minerals from the manaslu leucogranite in the central himalaya were microsampled and measured for rb sr mineral ages the ages obtained are used in conjunction with models for rb sr closure temperatures to reconstruct the thermochronological history of the manaslu granite the rb sr results indicate a rapid change in temperature between 21 ma and 17 ma of 60 70 c million yr which in turn indicates unroofing at 2 mm yr the temperature time curve inferred from rb sr data is in excellent agreement with existing ar ar data on the manaslu granite and its thermal aureole the results confirm that parts of the himalaya in addition to the gangdese belt were eroded rapidly in the 20 17 ma time period and this erosion could account for the observed rapid increase in seawater 87 sr 86 sr a tourmaline whole rock age of 29 5 ma was obtained on a sample from the chokkang arm but may be unreliable because of post crystallization alteration of feldspar microsampling approaches are demonstrated that allow high precision isotopic data to be obtained on minerals sampled at scales of 100 m or less microanalysis combined with models for sr redistribution can be a useful thermochronological tool that can augment the 40ar 39ar approach

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

gps measurements of present day convergence across the nepal himalaya the high elevations of the himalaya and tibet result from the continuing collision between india and asia which started more than 60 million years ago1 4 from geological and seismic studies of the slip rate of faults in asia5 it is believed that approximately one third of the present day convergence rate between india and asia 58 4mmyr 1 is responsible for the shortening uplift and moderate seismicity of the himalaya great earthquakes also occur infrequently in this region releasing in minutes the elastic strain accumulated near the boundary zone over several centuries and accounting for most of the advance of the himalaya over the plains of india the recurrence time for these great earthquakes is determined by the rate of slip of india beneath tibet which has hitherto been estimated indirectly from global plate motions6 from the slip rates of faults in asia7 8 from seismic productivity9 and from the advance of sediments on the northern ganges plain10 here we report geodetic measurements using the global positioning system gps of the rate of contraction across the himalaya which we find to be 17 52 2 mm yr 1 from the form of the deformation field we estimate the rate of slip of india beneath tibet to be 20 5 2 mmyr 1 strain sufficient to drive one or more great himalayan earthquakes with slip similar to that accompanying the magnitude 8 1 bihar nepal 1934 earthquake may currently be available in western nepal

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

primary magmatic biotite in granitoid intrusions associated with late mesozoic jinchanggouliang erdaogou lode gold deposits north china craton linkage to gold mineralization abstract the late mesozoic jinchanggouliang and erdaogou lode gold deposits in the central segment of the northern north china craton are mainly hosted in late archaean metamorphic rocks of the jianping formation and jurassic volcanic sedimentary rocks these deposits are spatially and temporally associated with a series of i type granitoid intrusions i e the 128 136 ma duimiangou granodiorite complex with a massive margin and a porphyritic center at jinchanggouliang the 161 4 1 1 ma loushang quartz diorite intrusion and numerous syenite porphyry dykes 160 9 1 0 ma at erdaogou and the 227 217 ma xitaizi monzogranite batholith compositions of primary magmatic biotite in these granitoid intrusions have been used to investigate the nature of granitoid magmas and the evolution of magmatic processes and to identify possible linkages between gold mineralization and granitoid magmatism all primary magmatic biotite grains are mg rich in composition xmg mg mg fe 0 41 0 69 the cl contents of biotite 0 09 0 42 wt in the loushang quartz diorite and the duimiangou massive granodiorite correlate negatively with xmg and are consistent with the mg cl avoidance rule the cl contents of biotite 0 01 0 14 wt in the duimiangou porphyritic granodiorite syenite porphyry and xitaizi monzogranite on the other hand remain relatively constant and may be affected by multiple variables such as cl rich fluids and subtle changes in temperature halogen fugacity of the parental magma calculated from biotite shows that the duimiangou granodiorite complex at jinchanggouliang and syenite porphyry dykes at erdaogou have the log fhf fhcl ranges of 0 71 to 0 27 and 2 16 to 1 08 respectively and have similar log fh2o fhcl values from 3 53 to 4 23 oxygen fugacities logfo2 calculated from biotite equilibria for the duimiangou granodiorite complex and syenite porphyry dykes are 13 7 to 7 2 straddling on the nickel nickel oxide and hematite magnetite buffers the xitaizi monzogranite and the loushang quartz diorite have lower log fo2 values from 14 9 to 12 4 between the nickel nickel oxide and hematite magnetite buffers the duimiangou granodiorite complex and syenite porphyry dykes with high oxygen fugacities are similar to oxidized magmas associated with porphyry cu mo au deposits these results suggest that multiple episodes of i type granitoid magmas in the study area were rich in cl and high in oxygen fugacity especially those associated with the duimiangou granodiorite complex and syenite porphyry dykes the i type melts represented by the duimiangou granodiorite complex and syenite porphyry dykes as well as their related cl rich oxidizing fluids were likely efficient in dissolving transporting and precipitating gold for the formation of the jinchanggouliang and erdaogou lode gold deposits

geochemical investigations of the origin of the manaslu leucogranite himalaya nepal the himalayan orogeny is characterized by an intraplate subduction zone the thrusting of the tibetan slab over the foreland is probably responsible for the origin of the manaslu type leucogranites this geochemical study tries to examine the crustal origin of the granite to verify whether the paragneisses of the tibetan slab are the most probable parent material and to determine the subsequent petrogenetic processes the rb sr systematics suggest that 1 1 the scatter of data points in the 87sr86srvs 87rb86sr diagram may be due to the heterogeneity of the initial 87sr86sr ratios 2 2 the initial isotopic ratios are very high 0 730 0 760 and are in agreement with those of the gneisses of the tibetan slab at the time of the melting pb isotopic ratios are homogeneous they are also consistent with a crustal prehistory the ree and th contents are unexpectedly low in comparison with the much higher contents of the parent material monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed ree and th abundances alternatively field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity in consequence the low ree and th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the sr and pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying tibetan slab

the metamorphism in the central himalaya all along the himalayan chain an axis of crystalline rocks has been preserved made of the higher himalaya crystalline and the crystalline nappes of the lesser himalaya the salient points of the metamorphism as deduced from data collected in central himalaya central nepal and kumaun are 1 the higher himalaya crystalline also called the tibetan slab displays a polymetamorphic history with a first stage of barrovian type overprinted by a lower pressure and or higher temperature type metamorphism the metamorphism is due to quick and quasi adiabatic uplift of the tibetan slab by transport along an mct ramp accompanied by thermal refraction effects in the contact zone between the gneisses and their sedimentary cover the resulting metamorphic pattern is an apparent diachronic inverse zonation with the sillimanite zone above the kyanite zone 2 conversely the famous inverted zonation of the lesser himalaya is basically a primary pattern acquired during a one stage prograde metamorphism its origin must be related to the thrusting along the mct with heat supplied from the overlying hot tibetan slab as shown by synmetamorphic microstructures and the close geometrical relationships between the metamorphic isograds and the thrust 3 thermal equilibrium is reached between units above and below the mct far behind the thrust tip there is good agreement between the maximum temperature attained in the hanging wall and the temperature of the tibetan slab during the second metamorphic stage but closer to the mct front the thermal accordance between both sides of the thrust is due to a retrogressive metamorphic episode in the basal part of the tibetan slab

new constraints on the age of the manaslu leucogranite evidence for episodic tectonic denudation in the central himalayas the manaslu leucogranite of central nepal transacts one segment of the south tibetan detachment system a major extensional feature that helped to moderate large topographic gradients in the himalayan orogen in middle tertiary time 40 ar 39 ar ages for hornblendes from the northeastern contact aureole of the pluton indicate that intrusion occurred prior to 22 23 ma providing a minimum age for formation of the detachment system in the manaslu region that is at least 2 m y older than previous estimates from other parts of the orogen 40 ar 39 ar mica ages from the aureole similar to previously published mica ages from the upper part of the leucogranite indicate an episode of rapid cooling at 19 16 ma we attribute this cooling event to tectonic denudation of the pluton by movement on structurally higher exten sional faults and we suggest that gravitational collapse of the orogenic front occurred episodically over the late oligocene early miocene interval in the manaslu area

geochemical comparison between himalayan and hercynian leucogranites abstract isotopic sr pb nd o ree and trace element data from three himalayan nepal and six hercynian brittany france leucogranites are compared for the himalayan granites makulu mustang and especially manaslu the 87sr 86sr versus 87rb 86sr scatter diagrams and hence variability of the initial 87sr 86sr ratios reflect heterogeneity of the source materials these ratios are very high 0 730 0 770 for manaslu the very high 207pb 204pb ratios 15 8 at the corresponding 206pb 204pb ratio of 18 8 the very low 143nd 144nd ratios i nd 13 to 17 and the very high 18o 16o ratios 12 confirm the crustal origin with the paragneisses of the tibetan slab formation i as the probable parent source for the six hercynian granites with an individual rb sr isochron established for many of them the initial 87sr 86sr ratios range from 0 706 to 0 717 the lead neodymium and oxygen isotope compositions confirm their crustal source of presumably palaeozoic age but not specifically identified the rare earth patterns and thorium contents are comparable to those for himalayan granites the similarities of the geochemical signatures isotopes and trace elements of the himalayan and hercynian leucogranites suggest that the petrogenetic processes were comparable and that the himalayan continental collision model can be applied to the hercynian the depletion in ree and th implies either interaction of the granitic liquids with fluid or more probably early precipitation of monazite the increase in sm nd in the granites compared to their parent materials implies that the neodymium model ages of primary mantle extraction are maximum rather than minimum ages in contrast to the hercynian leucogranites the extremely radiogenic sr of the himalayan leucogranites reflects the very long crustal residence time before the melting event the apparent uniformity of the initial 87sr 86sr ratios of the individual hercynian granites may be related to an aging effect where the straight line fitting improves with increase in age of the pluton

a thermal model for the distribution in space and time of the himalayan granites abstract in southern tibet crustal thickening due to the india asia collision has led to the formation of two granite belts one is located at the southern edge of the accretionary wedge of tethyan sedimentary rocks close to the contact with basement gneisses of the tibetan slab the other is found within the wedge itself close to the kangmar thrust trace available ages suggest that the granites appeared first in the southern belt and then in the kangmar belt this sequence seems to violate the chronology of thrusting another feature of the himalayas is that melting started only about 20 ma after the onset of thickening which is much less than the thermal time constant of thick crust we give a thermal model based on the assumption of conductive heat transfer which explains these features the model relies on the geometry of a sedimentary accretionary wedge bounded by low angle thrust faults and on the existence of a thermal conductivity contrast between old basement and young sedimentary rocks the wedge of sedimentary rocks acts as an insulating cap and its southern edge heats up along the contact with basement rocks on a horizontal cross section there is a temperature maximum along this southern edge which explains why melting starts there the early thermal evolution is sensitive to local conditions and granites first appear in the vicinity of the most radiogenic parts of the basement the distribution of granites in space and time is seemingly random reflecting different melting events in different radiogenic environments in the heterogeneous basement this model predicts a relationship between radioactivity and age which is compatible with available data the results emphasize that there are large horizontal temperature variations across a thickened region and that granite ages are not related simply to the timing of tectonic phases