metamorphism melting and extension age constraints from the high himalayan slab of southeast zanskar and northwest lahaul abstract we present evidence for two distinct stages of tertiary metamorphism m1 and m2 in the high himalayan slab of southeast zanskar and northwest lahaul as well as evidence for an older pre himalayan metamorphism pre m1 the m1 was a regional barrovian type event related to crustal shortening and thickening of the indian plate margin while m2 was associated with crustal melting and the emplacement of the gumburanjon leucogranite into the zanskar shear zone at the top of the slab u pb dating of metamorphic and magmatic accessory phases constrains the timing of m1 between 30 and 37 ma and the crystallization and emplacement age of the gumburanjon leucogranite at 21 22 ma inherited accessory phases in metamorphic and magmatic samples suggest that the protoliths of the slab are at least lower paleozoic in age and that they experienced a major pre m1 thermal perturbation at ca 450 500 ma whether this was associated with a regional barrovian type metamorphism or whether it was a thermal event rel

retrograde net transfer reaction insurance for pressure temperature estimates retrograde net transfer reactions significantly affect compositions of metamorphic minerals yet are rarely considered when determining pressure temperature p t conditions two natural amphibolite facies metapelites from the central himalaya of nepal exhibit extremely common compositional patterns including increases in mn and fe fe mg at the rims of garnets which are the result of retrograde garnet dissolution and fe mg exchange with biotite however typical thermobarometric approaches for these rocks result in errors of hundreds of degrees and 3 6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids these large errors result because dissolution of high fe garnet has strongly affected the fe mg ratio of matrix biotite x ray maps help evaluate the extent and chemical effects of retrograde reactions in these samples by identifying mineral regions that retain highest t compositions or through a new data processing approach by permitting correction of mineral compositions to original high t values these approaches ensure against retrograde net transfer reactions and should be applied routinely in thermobarometric studies they ultimately yield p t estimates that are more petrologically reasonable and permit rapid screening of samples for those least affected by retrograde reactions reconsideration of thermobarometry in the central and eastern himalaya indicates that retrograde net transfer reactions are extremely common therefore previous thermobarometric studies based on garnet major element compositions from that region should be reevaluated

contemporary crustal deformation in east asia constrained by global positioning system measurements global positioning system gps measurements collected since the early 90 s allow us to derive geodetic velocities at 16 permanent stations in east asia and 68 campaign mode sites in north china the resulting velocity field shows the following 1 contrary to the early inferences that the shanxi rift has accommodated significant right slip motion our results suggest that the rift system at least in its northern part in north china is under ese wnw extension at a rate of 4 2 mm yr the velocity field also suggests that an ese wnw trending left lateral shear zone deforming at a rate of 2 1 mm yr may exist along the north rim of north china at the latitude of 40 n separating actively extending north china in the south from relatively stable mongolia in the north 2 central and east china move at a rate of 8 11 mm yr east southeast with respect to siberia implying that the overall east southeastward motion is the dominant mode of deformation in east china 3 the india plate moves at a rate of 6 1 mm yr slower than the nuvel 1a model prediction relative to the eurasia plate 4 significant eastward motion 20 2 mm yr with respect to siberia occurs in southeastern tibet about half of this eastward motion 11 mm yr is absorbed by structures along the eastern boundary of the tibetan plateau

geology of the higher himalayan crystallines in khumbu himal eastern nepal abstract in this paper we present the current geological knowledge and the results of new geological and structural investigations in the cho oyu sagarmatha makalu region eastern nepal and southern tibet the tectonic setting of the middle and upper part of the higher himalayan crystallines hhc and tibetan sedimentary sequence is characterized by the presence of pervasive compressive tectonics with south verging folds and shear zones overprinted by extensional tectonics in the middle and upper part of the hhc two systems of folds f2a and f2b have been recognized affecting the s1 high grade schistosity causing kilometer scale upright antiforms and synforms the limbs of these upright folds are affected by f3 collapse folds top to se extensional shear zones and extensional crenulation cleavages linked to extensional tectonics the uppermost portion of the hhc and the lower part of the tibetan sedimentary sequence is affected by two major extensional fault zones with a top to ne direction of movement the lower ductile extensional shear zone brings into contact the north col formation with the high grade gneisses and micaschists of the hhc it is regarded as the main feature of the south tibetan detachment system the upper low angle fault zone is characterized by ductile brittle deformation and thin levels of cataclasites and brings the slightly metamorphosed ordovician limestones into contact with the north col formation extensional tectonics continued with the formation of e w trending high angle normal faults three metamorphic stages of himalayan age are recognized in the hhc of the sagarmatha makalu region the first stage m1 is eclogitic as documented by granulitized eclogites collected at the top of the main central thrust zone in the kharta region of southern tibet the second event recorded in the kharta eclogites m2 was granulitic with medium p 0 55 0 65 gpa and high t 750 770 c and was followed by recrystallization in the amphibolite facies of low pressure and high t m3 the first event has also been recorded in the overlying barun gneiss where m1 was followed by decompression under increasing t the m2 event producing the dominant mineral assemblage garnet sillimanite biotite and then by strong decompression under high t with growth of andalusite cordierite and green spinel also changes in phase compatibilities suggest an increase in metamorphic temperature t coupled with a decrease in metamorphic pressure p in some of the thrust sheets of the mct zone a telescoped metamorphic zonation ranging from the sillimanite to the staurolite and biotite zones is characteristic of the ductile extensional shear zone which is the lower part of the stds in the sagarmatha region evidence for decompression under increasing temperature anatexis and leucogranite emplacement accompanying extension in the hhc was found throughout the whole ductile shear zone particularly in metapelites both below and above the makalu leucogranite and in micaschists of the staurolite zone

etude petro structurale des chevauchements ductiles himalayens sur la transversale de l everest makalu nepal oriental tectonic and microtectonic data in eastern nepal indicate that the major observed thrusting 100 km on the main central thrust mct postdates the barrovian metamorphism of the high himalaya gneisses this result at variance with the famous reverse metamorphism model better explains the abnormal metamorphic superpositions in the himalayas and accounts for the lack of high pressure assemblages under the thick allochtonous high himalaya tibetan slab pressure and temperature estimates by microprobe analysis on plagioclase biotite garnet kyanite sillimanite and cordierite assemblages are presented for samples collected along the mct shear zone and across the gneiss slab in the everest makalu area since there is very little difference in pressure at the front of the slab kathmandu klippe and its root these estimates support the existence of important late metamorphic thrusting the decrease of pressure towards the top of the gneiss pile combined with a small temperature increase explains the k

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

metamorphic zonation migmatization and leucogranites along the everest transect of eastern nepal and tibet record of an exhumation history

the igneous crystallization history of an ancient martian meteorite from rare earth element microdistributions rare earth element ree and other selected trace and minor element concentrations were meas ured in individual grains of orthopyroxene feldspathic glass of plagioclase composition and merrillite of the alh 84001 martian meteorite unlike in other martian meteorites phosphate is not the main ree car rier in alh 84001 the ree pattern of alh 84001 bulk rock is dependent on the modal abundances of three ree bearing phases namely orthopyroxene which contains most of the heavy rare earth elements hrees feldspathic glass which dominates the eu abundances and merrillite which contains the majority of the light rare earth elements lrees variations in the ree abundances previously observed in different splits of alh 84001 can easily be explained in terms of small variations in the modal abundances of these three minerals without the need to invoke extensive redistribution of lrees at least some orthopyroxenes i e those away from contacts with feldspathic glass in alh 84001 appear to have preserved their original ree zonation from igneous fractionation an estimate of the alh 84001 parent magma composition from that of the unaltered orthopyroxene core i e zoned orthopyroxene with the lowest ree abundances indicates that it is lree depleted this implies that the martian mantle was already partly depleted within 100 ma of solar system formation which is consistent with rapid accretion and differentiation of mars although equilibration and exchange of rees between phases in particular transport of lrees into the interstitial phases feldspathic glass and merrillite cannot be ruled out our data suggest that the lree enrichment in melts in equilibrium with these interstitial phases is most likely the result of late stage infil tration of the cumulate pile by a lree enriched melt

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

records of the evolution of the himalayan orogen from in situ th pb ion microprobe dating of monazite eastern nepal and western garhwal abstract in situ th pb monazite ages from rocks collected along two transects the dudh kosi everest eastern nepal and the bhagirathi river garhwal himalaya india perpendicular to the main central thrust mct suggest a striking continuity of tectonic events across the himalaya the youngest age reported in this study 5 9 0 2 ma mswd 0 4 from matrix monazite grains collected beneath the mct in the garhwal region is consistent with several age data from rocks at similar structural levels in central nepal providing support for widespread late miocene mct activity the lateral parallelism of orogenic events is further manifested by the 20 7 0 1 ma age of a high himalayan leucogranite from an injection complex that outcrops along the dudh kosi everest transect resembling ages of these bodies reported elsewhere the youngest monazite grain analyzed along the dudh kosi everest transect is 10 3 0 8 ma the absence of 7 3 ma monazite ages in eastern nepal may reflect a different nappe structure which obscures the reactivated ramp equivalent exposed in the garhwal and central nepal garnets from the mct hanging wall greater himalayan crystallines and footwall lesser himalaya display different major element zoning and the patterns are useful for constraining the location of the thrust system that separates the two lithologies pressure temperature paths for two upper lesser himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone along the dudh kosi everest transect upper lesser himalayan monazite grains from three rocks record a clear signal at 14 5 0 1 ma mswd 8 4 and the 23 ma age that characterizes the hanging wall is notably absent monazites collected within a large scale greater himalayan crystallines fold yield the 14 ma age consistent with the structure forming due to mct related compression paleo mesoproterozoic 1407 35 ma matrix monazite grains are found within an augen gneiss unit located beneath the mct whereas cambro ordovician 436 8 548 17 ma inclusions are preserved within garnets of the greater himalayan crystallines the presence of 45 2 2 1 ma grains from lower structural levels of the greater himalayan crystallines indicates the unit realized conditions conducive for monazite growth during the eocene

links between short term velocity variations and the subglacial hydrology of a predominantly cold polythermal glacier the surface velocity of a predominantly cold polythermal glacier john evans glacier ellesmere island canada varies significantly on both seasonal and shorter time scales seasonal variations reflect the penetration of supraglacial water to the glacier bed through significant thicknesses of cold ice shorter term events are associated with periods of rapidly increasing water inputs to the subglacial drainage system earlyseason short term events immediately follow the establishment of a drainage connection between glacier surface and glacier bed and coincide with the onset of subglacial outflow at the terminus a mid season short term event occurred as surface melting resumed following cold weather and may have been facilitated by partial closure of subglacial channels during this cold period there is a close association between the timing and spatial distribution of horizontal and vertical velocity anomalies the temporal pattern of surface water input to the glacier and the formation seasonal evolution and distribution of subglacial drainage pathways these factors presumably control the occurrence of highwater pressure events and water storage at the glacier bed the observed coupling between surface water inputs and glacier velocity may allow predominantly cold polythermal glaciers to respond rapidly to climate induced changes in surface melting

evolution of the himalaya abstract the compression and attendant deformation of a thick and vast sedimentary prism formed since early riphean times on the northern continental margin of the indian craton gave rise to the himalaya mountains as a result of convergence and collision of the indian and asian plates the oceanic trench sediments tectonically implanted with sea floor material and intimately associated with calc al kaline volcanics in the narrow sindhu tsangpo belt extending from kohistan through dras leh darchen mansarovar to shigatse and beyond represent the subduction island arc complex which developed south of the dynamic southern margin of the asian continent and was welded to the colliding indian plate during the late eocene to oligocene period this complex is fringed to the north by a wide zone of andean type granitic bodies the evolution of the himalayan orogen is closely connected with the development of the present day andaman nicobar indonesia island arc subduction system in the southeast and the makran ranges oman trench in the southwest the evolution of the himalaya was accomplished in four major phases of tectonic upheaval during the late cretaceous to palaeocene karakoram phase late eocene to oligocene malla johar phase middle miocene to pontian sirmurian phase and late pliocene to middle pleistocene siwalik phase while the karakoram phase marks the convergence of continents and the malla johar phase represents the collision and subduction it was during the sirmurian upheaval that the main tectonic features developed and the himalaya acquired its distinctive structural complexion

the genesis of diogenites and hed parent body petrogenesis abstract samples of clasts and whole rocks from eighteen of the twenty four known diogenites orthopyroxenite breccia meteorites have been studied by combined inaa empa in order to elucidate their petrogenetic history the incompatible elements a12o3 and tio2 are positively correlated in diogenite orthopyroxenes these elements determined by empa are also positively correlated with incompatible trace elements such as yb determined by inaa on bulk samples these correlations indicate that most diogenites may form a single igneous fractionation sequence the incompatible minor and trace elements are decoupled from the major elements for essentially constant mg tio2 varies by about 4 times and yb by about 8 times hence there is no strong evidence for igneous fractionation preserved in the major element composition of orthopyroxenes geothermometry calculations using two pyroxene and spinelorthopyroxene thermometers show that these phases in diogenites maintained equilibrium to temperatures in the range 650 880 c far below likely magmatic temperatures in some diogenites orthopyroxenes of differing composition are in breccia contact therefore the low equilibration temperatures were achieved in the diogenites prior to brecciation decoupling of major and trace elements was most likely caused by slow subsolidus cooling in the diogenite pluton which allowed ca fe and mg to reach diffusive equilibration incompatible trace elements likely have much lower diffusivities in orthopyroxene and therefore may have largely preserved their magmatic distributions the extensive variation in incompatible trace elements indicates that either the diogenites were formed through an extensive crystallization interval or that partition coefficients greatly increased during crystallization if the former were true one would expect to find substantial amounts of plagioclase in the diogenites if they are comagmatic with eucrites this is contrary to observations a model in which diogenites crystallized from one or more melts of approximately orthopyroxenitic composition can satisfy both the mineralogie and trace element constraints

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

a review of the geophysical constraints in modelling the gondwana crust in india abstract the gondwanic lithosphere of india has evolved by suturing of three tectonic blocks namely the south indian block sib the bundelkhand block bb and the trans aravalli block tab the bb became separated from the russian platform during the opening of the proto tethys and joined the sib during the satpura orogeny around 921 ma the tab represents a younger crustal block than bb and is possibly a detachment of the arabian nubian shield which became separated along the owen fracture zone chaman fault during the himalaya collision tectonics around 10 ma the paleo plate margins of the microplates constituting the gondwanic lithosphere of india are defined by gravity and magnetic highs in regional and global geophysical data the crustal thickening as brought out by gravity data analysis has been controlled by thrust stacking the intent of this communication is to highlight the significance of geophysical constraints in paleo plate reconstructions and crustal growth

in situ laser ablation icp ms analyses of dimict diogenites further evidence for harzburgitic and orthopyroxenitic lithologies trace element concentrations in pyroxene plagioclase and olivine were measured in five diogenite breccias previously identified as containing distinct harzburgitic ol opx and orthopyroxenitic opx lithologies dimict three samples show two distinct populations of pyroxene trace element abundances supporting their classification as dimict these three meteorites show increases in y yb and hree concentrations from harzburgitic to orthopyroxenitic pyroxenes supporting the hypothesis that the lithologies are related through fractional crystallization whereby harzburgite olivine and pyroxene crystallized from the magma first followed by orthopyroxenite pyroxene depletions in lree and eu concentrations in the orthopyroxenitic lithology are most likely due to equilibration with lree and eu rich phases likely plagioclase which is found primarily in that lithology two samples do not show evidence supporting a dimict classification large pyroxene trace element variation in one sample indicates that it is polymict while uniform trace element distribution in the other suggests that it may be a monomict breccia

crustal structure associated with gondwana graben across the narmada son lineament in india an inference from aeromagnetics abstract aeromagnetic data over an 80 km wide belt along the ene trending narmada son lineament nsl starting from baroda in the west and continuing to the south of jabalpur in the east has been studied to understand the structural and tectonic framework of the region the area is covered by generally e w trending steeply dipping and folded archean phyllites and quartzites as basement with bijawars upper precambrian upper vindhyans upper proterozoic and gondwanas upper carboniferous overlying them overlapping them all are the deccan trap cretaceous eocene flows aeromagnetic linements and their disposition and pattern in this region suggest major dislocations in the crust the region around hoshangabad which is the intersection point of the nsl and the northwestern extension of the godavari lineament appears to have been intensely disturbed spectral analysis of aeromagnetic profiles across the nsl belt brought out a deep magnetic interface within crust at depths varying from 4 km to about 20 km below the surface perhaps corresponding to the discontinuity characterized by the interface of granitic and basaltic rocks there is a significant downwarping of this interface under the hoshangabad region suggesting that this is perhaps related to the evolution of the gondwana basin structure in this area this warping of the magnetic interface may be a reflection of the crustal flexuring and rift faulting elsewhere in the world concentrations of carbonatite complexes and dike swarms are known to occur in areas of crustal flexuring and rift faulting the occurrence of carbonatite complexes in this region e g at amba dongar and barwaha and dike swarms in the dadiapada region gives credence to the present inferences from the aeromagnetic study

petrogenesis of the ultramafic zone of the stillwater complex in north america constraints from mineral chemistry and stable isotopes of li and o to investigate the petrogenesis of cyclic units in layered intrusions we examined chromitite dunite poikilitic harzburgite and bronzitite from the ultramafic zone of the stillwater complex and measured stable isotopes of li and o in their major minerals the li isotopes in olivine range from 4 to 26 in 7li with uniform li contents of 1 3 ppm whereas orthopyroxene and clinopyroxene have li contents of 0 5 5 ppm and 4 8 ppm and 7li ranges of 13 to 7 and 14 to 6 respectively the 18o values vary from 4 91 to 5 72 in olivine from 5 11 to 5 87 in orthopyroxene and from 4 64 to 5 86 in clinopyroxene for a given sample olivine displays more variable and higher 7li but lower 18o values than orthopyroxene indicating that olivine experienced more extensive compositional modification after crystallization relative to orthopyroxene the general li and o isotopic compositions are interpreted as the result of re equilibration between interstitial liquids from which pyroxenes crystallized and cumulus minerals the inter mineral and inter sample isotopic variations correlate with mineral assemblages crystal sizes and major and trace element compositions revealing that the interstitial liquids varied compositionally mainly due to mixing between fractionated magma and newly injected primitive magma abrupt mineralogical and geochemical changes from silicate rocks to chromitites imply that hydrous fluids which collected on chromite surfaces and were later released from chromite seams played an additional critical medium of chemical exchange between minerals in the chromitites

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

petrology and geochemistry of the elephant moraine a79002 diogenite a genomict breccia containing a magnesian harzburgite component the elephant moraine a79002 eeta79002 diogenite is a fragmental breccia with a subtle lightdark structure it is composed of orthopyroxene with minor olivine chromite and ubiquitous inhomogeneously distributed approximately 5 500 m sized troilite and metal grains these latter are present in the matrix and as inclusions in and as symplectic intergrowths with orthopyroxene and olivine trace amounts of silica and diopside are also present most orthopyroxene compositions typical orthopyroxenes are in the narrow range wo2 1 2 7en74 1 75 6fs22 2 23 8 like those of most diogenites a few magnesian orthopyroxenes are present with compositions of wo1 7 2 5en77 5 80 2 fs18 2 20 3 these are among the most magnesian orthopyroxenes known from diogenites a few ferroan orthopyroxenes have compositions of wo2 1 2 9en71 7 73 7fs24 2 25 5 differences in al2o3 tio2 and cr2o3 between the different orthopyroxene groups are inconsistent with a simple igneous fractionation relationship between them olivine compositions are fo75 0 76 9 the olivines could be in equilibrium with the magnesian orthopyroxenes but not with the typical or ferroan orthopyroxenes that form the bulk of eeta79002 metal grains exhibit a range of ni and co contents and ni co ratios their compositions indicate that they are primary igneous metal metal and troilite grains are more prevalent in the dark samples the trace incompatible lithophile element contents of 16 samples are remarkably uniform their yb concentrations are all within their 2s analytical uncertainties of the mean the uniformity and low content of light rare earth elements in eeta79002 indicate that negligible amounts of a trapped liquid component or foreign material mixed in the breccia could be present the siderophile and chalcophile element data show that the light dark structure is due to the distribution of metal and troilite grains dark samples contain higher ni co and se compared to light samples meteorite eeta79002 appears to contain material from three or more related plutons a magnesian harzburgite and two orthopyroxenites and is a genomict breccia

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

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

new garnets for old cautionary tales from young mountain belts major and trace elements in garnet may be used to characterize p t t paths in orogenic belts but they are also often used without age constraints to obtain p t estimates alone during investigations into the prograde phase of the alpine himalayan tertiary orogeny garnets with anomalously old sm nd ages were discovered in three different areas they occurred in identical parageneses to garnets that grew during tertiary prograde metamorphism ages 500 ma were determined for homogenized garnets from the garhwal himalaya while garnets with distinctive growth zonation from the betic cordillera s spain gave an age of 235 1 1 7 ma although the garhwal date coincides with a recognized magmatic and metamorphic event the 235 1 ma age from the betics at first sight appears one of the more unlikely times for garnet growth duplicate analyses precisely date growth of the garnet cores which x ray maps reveal to be entirely distinct from the rims implying a hiatus before resorption and rim development this initial garnet growth may be linked to early crustal extension during pangaea break up these results reaffirm the value of garnet in the integrated study of polymetamorphic orogenic belts at the same time the data show the potential for unravelling the history of these complex zones and strike a cautionary note for thermobarometry

pyroxene as a recorder of cumulate formational processes in asteroids moon mars earth reading the record with the ion microprobe igneous cumulate rocks provide an important record of planetary magmatism but there are pitfalls in their interpretation cumulus minerals may react with the trapped melt and other cumulus phases during sub solidus reactions thus losing a direct record of their igneous history one of the best approaches for estimating the melt compositions parental to the cumulates is to analyze the cores of cumulus phases for elements with slow diffusion rates e g ree in pyroxene because these most reliably retain a record of the mineralmelt partitioning we have conducted sims studies of pyroxenes from a variety of planetary cumulates including lunar norites martian orthopyroxenites asteroidal orthopyroxenites diogenites asteroidal pyroxene plagioclase cumulates cumulate eucrites and terrestrial orthopyroxenites and norites stillwater complex montana we emphasized the ree la ce nd sm eu dy er yb along with sr y and zr in these investigations our studies yielded the several conclusions i lunar mg suite norites crystallized from highly evolved kreepy melts that were emplaced into the lunar anorthositic crust one viable model suggests remobilization of kreep lithologies that formed late in the crystallization of the lunar magma ocean 2 orthopyroxenites from asteroid 4 vesta diogenites likely formed as cumulates from melts derived from depleted mantle which had previously experienced eucritic basalt removal the calculated parental melts show a limited range of major element compositions but an exceedingly large range of trace element variation that is difficult to explain by any simple crystallization or melting models 3 the diogenites of 4 vesta are cumulates from melts derived from many depleted mantle reservoirs that either formed numerous orthopyroxene plutons later mixed by impact brecciation or commingled by magma mixing to form a limited number of differentiated plutons we base this conclusion on a comparative investigation of a well studied terrestrial orthopyroxenite sequence bronzitite zone of the stillwater complex an important lesson to be learned from these studies in comparative planetology is that if simple petrogenetic models do not work for well studied terrestrial occurrences there is little reason to believe that they will work for planetary environments where we have little geologic control on sampling

rb sr geochronology of granites and gneisses from the mount everest region nepal himalaya nineteen samples of granites orthogneisses and paragneisses from the high himalaya basement nappe s of the mount everest region have been dated by the rb sr method the post metamorphic tourmaline leucogranites of the upper imja drangka nuptse lhotse glacier have high initial sr87 sr86 characteristic of an anatectic origin from crustal material a whole rock isochron age of 52 m y early eocene has been obtained for the samples from the granite body of lhotse glacier apparently sr isotopic homogenization was not reached throughout the much larger nuptse granite

thermal model for the zanskar himalaya abstract crustal thickening along the northern margin of the indian plate following the 50 ma collision along the indus suture zone in ladakh caused widespread high temperature medium pressure barrovian facies series metamorphism and anatexis in the zanskar himalaya metamorphic isograds are inverted and structurally telescoped along the main central thrust mct zone at the base of the high himalayan slab along the zanskar valley at the top of the slab isograds are the right way up and are also telescoped along northeast dipping normal faults of the zanskar shear zone zsz which are related to culmination collapse behind the miocene himalayan thrust front between the mct and the zsz a metamorphic anatectic core within sillimanite grade rocks contains abundant leucogranite granite crustal melts of probable himalayan age a thermal model based on a crustal scale cross section across the zanskar himalaya suggests that m1 isograds developed during early himalayan barrovian metamorphism were overprinted during high grade mct related anatexis and folded around a large scale recumbent fold developed in the hanging wall of the mct

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

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

concentrations of rare earth elements in silicate rocks of the lower critical and main zones of the bushveld complex concentrations of rare earth elements in the lower critical and main zones of the bushveld complex at union section show distinct variation with stratigraphic height in that lower and lower critical zone cumulates are lree and th enriched ce sm 10 25 ce smn 3 65 th smn 6 17 over main zone cumulates ce sm 4 10 ce smn approximately 2 th smn 1 57 the upper critical zone constitutes a transitional interval in terms of ree concentrations ce sm 9 17 this pattern is broadly coupled albeit in a contraposed manner to that in initial sr isotopic ratio and confirms earlier studies that the bushveld complex crystallized from at least two compositionally distinct parental magmas mixing between the two magmas occurred as early as in the lower zone and is sensitively reflected by variation in ce sm th sm modal plagioclase and chromite while ree contents of the lower and critical zones are approximately 15 those of the corresponding b1 and b2 marginal rocks the main zone cumulates have similar ree contents to the b3 marginal rocks this suggests that the main zone rocks either crystallized from a highly viscous crystal mush without appreciable fractionation or from magma strongly ree enriched over the marginal rocks which would imply that the main zone is not represented in the suite of marginal rocks

the origin of himalayan anatexis and inverted metamorphism models and constraints abstract the key to comprehending the tectonic evolution of the himalaya is to understand the relationships between large scale faulting anatexis and inverted metamorphism the great number and variety of mechanisms that have been proposed to explain some or all of these features reflects the fact that fundamental constraints on such models have been slow in coming recent developments most notably in geophysical imaging and geochronology have been key to coalescing the results of varied himalayan investigations into constraints with which to test proposed evolutionary models these models fall into four general types 1 the inverted metamorphic sequences within the footwall of the himalayan thrust and adjacent hanging wall anatexis are spatially and temporally related by thrusting 2 thrusting results from anatexis 3 anatexis results from normal faulting and 4 apparent inverted metamorphism in the footwall of the himalayan thrust is produced by underplating of right way up metamorphic sequences we review a number of models and find that many are inconsistent with available constraints most notably the recognition that the exposed crustal melts and inverted metamorphic sequences not temporally related the generalization that appears to best explain the observed distribution of crustal melts and inverted metamorphic sequences is that due to specific petrological and tectonic controls episodic magmatism and out of sequence thrusting developed during continuous convergence juxtaposing allochthonous igneous and metamorphic rocks this coincidental juxtaposition has proven to be something of a red herring unduly influencing attention toward finding a causal relationship between anatexis and inverted metamorphism

ductile shear as a cause of inverted metamorphism example from the nepal himalaya in many mountain belts metamorphic sequences occur in an inverted arrangement such that rocks of higher metamorphic grade overlie rocks of lower metamorphic grade in some locations such as the main central thrust mct zone of the himalaya the metamorphic inversion is coincident with the zone of ductile thrust deformation although many explanations have been suggested for the metamorphic inversion of the himalaya none are consistent with all of the observed features a new model based on observations from the eastern nepal himalaya consists of inversion of the metamorphic sequence due to ductile shearing of a pre existing zone of right way up metamorphism this simple structural explanation creates an inverted metamorphic sequence consistent with observations of 1 a zone of distributed deformation 2 metamorphic textures overprinted by deformation and 3 metamorphic isograds sub parallel to shear fabric these observations are not all well accounted for by previously proposed models in this st

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

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

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

origin of the inverted metamorphism of the lower himalayas central nepal abstract the southern slope of the central nepal himalayas is divided geologically by the main central thrust zone into the underlying midland metasediment zone lower himalayas in the south and the overlying himalayan gneiss zone higher himalayas in the north the former zone consists of a thick pile of various kinds of metasediments folded into an anticlinorium and the latter zone consists of high grade crystalline rocks with a north dipping homoclinal structure and is further overlain by the tethys himalayas composed of non or slightly metamorphosed tethyan sediments the main central thrust zone shows a characteristic lithology such as calcareous schist quartzite phyllitic schist and augen gneiss and is intensely mylonitized the mineral composition and paragenesis of the rocks in the present area indicate an inverted metamorphism from the midland metasediment to the main central thrust zones that is the metamorphic grade of the midland metasediment zone increases upward toward the thrust zone from the chlorite to the biotite zones of the greenschist facies and the greenschist amphibolite transitional zone is attained in the thrust zone the himalayan gneiss zone is apparently much higher in metamorphic grade than the thrust zone on the basis of the compositional zoning of garnets and muscovite compositions however it is concluded that the himalayan gneisses suffered a polymetamorphism the younger metamorphism resulted in a retrograde metamorphism of the himalayan gneisses together with the above mentioned inverted metamorphism and an anatectic melting in depth this was caused by shear heating due to the thrust movement along the main central thrust zone during the alpine tectonism the older metamorphism which seems to be precambrian in age was much higher in metamorphic grade than the younger one and formed the pyrope rich cores of garnets in the himalayan gneisses the maximum metamorphic temperature of 550 c estimated from the mineral paragenesis in the thrust zone is compatible with that caused by shear heating due to the thrusting with thrusting velocity of 5 10 cm yr maximum stress of 300 bars and thrusting distance of 250 km bird 1978

a lead isotopic study of the stillwater complex montana constraints on crustal contamination and source regions analyses of the pb isotopic compositions of plagioclase from 23 samples covering the stratigraphic thickness of the stillwater complex indicate a narrow range of apparent initial isotopic compositions 206pb 204pb 13 95 207pb 204pb 14 95 15 01 208pb 204pb 33 6 the uniformity of our data is in contrast to but not necessarily contradictory to other recent investigations which give indications that the complex formed by repeated injection of magmas with at least two distinct compositions that were presumably derived from different source regions samples from the basal series of the complex have consistently higher 207pb 204pb ratios suggesting either minor contamination from adjacent country rocks or a slight distinction between parental magmas apparent initial pb isotopic compositions of the complex are very radiogenic compared to late archean model mantle values but are nearly identical to initial pb isotopic compositions found for the the adjacent slightly older 2 73 2 79 ga late archean crustal suite in the beartooth mountains contamination of magmas parental to the stillwater complex by the late archean crustal suite is rejected for two reasons 1 th and u concentrations in stillwater rocks and plagioclase are very low about 0 08 and 0 02 ppm respectively yet th u ratios are uniform at about 4 in contrast to the highly variable 2 26 but often high th u ratios found for the late archean crustal complex 2 it seems improbable that any contamination process would have adjusted the isotopic compositions of the diverse magmas entering the stillwater chamber to near identical values the preferred hypothesis to explain the pb isotopic data for the stillwater complex and the associated late archean crustal suite involves a major late archean crust forming event that resulted in a compositionally complex crust mantle system with relatively homogeneous and unusual pb isotopic compositions the parental magmas of the stillwater complex were generated at different levels within this crust mantle system before isotopic contrasts could develop by radioactive decay within compositionally discrete reservoirs this situation limits the utility of all isotopic tracer systems in discriminating among the various mantle and crustal reservoirs that may have affected the final isotopic character of the stillwater magmas the late archean crustal complex and the stillwater complex melts were ultimately derived from the same distinct mantle without obvious direct interaction with the middle to early archean crust present in the region

trace and minor elements in minerals of nakhlites and chassigny clues to their petrogenesis abstract ion microprobe measurements of ree rare earth element and other selected trace and minor element concentrations were made in clinopyroxene olivine feldspar and chlorapatite in chassigny and the three nakhlites nakhla governador valadares and lafayette in chassigny we have found pigeonite occurring in a poikilitic habit it is the first reported occurrence of this mineral in this meteorite the most significant implication of this finding is that estimates of the major element composition of the chassigny parent melt will have to accommodate pigeonite as a primary mineral crystallizing before plagioclase chlorapatite contains the highest ree abundances in all meteorites studied here the ree patterns of chlorapatite plagioclase and augite in nakhlites and chassigny are similar although the downward trends of ree patterns are somewhat steeper for these minerals in chassigny than in the nakhlites pyroxenes in nakhlites and chassigny are extensively zoned in rees and other trace and minor elements such as y zr ti and al which appear to be resistant to diffusive reequilibration thus primary igneous zoning of these elements has been preserved in the pyroxenes of these meteorites and can be used to infer their crystallization histories the calculated ree patterns of the parent melts of nakhlites and chassigny are lree enriched and parallel to their whole rock ree patterns indicating that once crystal accumulation occurred the intercumulus trapped melt fractionally crystallized in an essentially closed system and that infiltration i e metasomatism by lree enriched magmas need not be invoked to explain the lree enrichment in bulk samples of these meteorites fractional crystallization model calculations for nakhla also confirm these conclusions the distributions of trace and minor elements in augites in nakhlites and chassigny suggest that all three nakhlites could be samples from different horizons of the same lithologic unit but that chassigny was not comagmatic with the nakhlites