deformation metamorphism and imbrication of the indian plate south of the main mantle thrust north pakistan south of the main mantle thrust in north pakistan rocks of the northern edge of the indian plate were deformed and metamorphosed during the main southward thrusting phase of the himalayan orogeny in the hazara region between the indus and kaghan valleys metamorphic grade increases northwards from chlorite zone to sillimanite zone rocks in a typically barrovian sequence metamorphism was largely synchronous with early phases of the deformation the metamorphic rocks were subsequently imbricated by late north dipping thrusts each with higher grade rocks in the hanging wall than in the footwall such that the metamorphic profile shows an overall tectonic inversion the rocks of the hazara region form one of a number of internally imbricated metamorphic blocks stacked after the metamorphic peak on top of each other during the late thrusting this imbrication and stacking represents an early period of post himalayan uplift

collision tectonics in the nw himalayas west himalayan tectonics involve the collision of microplates between the indian and asian plates the kohistan complex consists largely of tightly folded basic volcanics and sediments generated as late jurassic to late cretaceous island arcs these were intruded by post folding mid cretaceous eocene plutonics produced from continued subduction of the indian plate after closure of a suture between kohistan and the karakorum the himalayan structures show major thrust sheets and the kohistan arc is essentially a crustal pop up with southward upright and northward verging structures developed above a thick ductile decoupling zone the indus suture which can be traced for 100 km beneath kohistan on large reentrants this pop up formed by a two stage process closure of the northern suture followed by closure of the southern indus suture granitic rocks of the kohis tan ladakh batholith dated at 100 40 ma post date most of the structures related to the northern suture but were deformed and carried southwards on shear structures related to the indus suture postcollisional deformation carried this kohistan complex on deep decoupling zones over the indian plate on a series of imbricated gneiss sheets the thrusts climbing up section in the movement direction so that in the far s some override their own molasse debris folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms i e the nanga parbat and hazara syntaxes the nanga parbat syntaxis probably formed due to a shear couple near a branch line where one of the main himalayan thrusts joined the indus suture beneath kohistan crustal delamination to produce the imbricated gneiss sheets could not account for all the displacement of india into asia suggested by palaeomagnetic data there must also have been lateral displacement as demonstrated by the large oblique slip shear zone in the hunza valley n of kohistan

neotectonics of the nanga parbat syntaxis pakistan and crustal stacking in the northwest himalayas the northwest termination of the himalayan arc is marked by two large antiformal structures termed syntaxes and a nw se zone of intermediate depth earthquakes which have yielded both strike slip and thrust sense fault plane solutions this contribution presents new structural data from the nanga parbat area the northern syntaxis one of the fastest rising portions of the orogen 7 mm yr 1 our studies show uplift related to syntaxis growth to be accommodated by structures located along its western margin fault kinematics in the southern part of the margin indicate nw directed thrusting along a shear zone and high level cataclastic faults termed the liachar thrust zone this carries amphibolite facies basement rocks out onto quaternary sediments further north along the western margin active faults are dominantly dextral strike slip oriented north south e g the shahbatot fault overprinting relationships suggest that the strike slip fault zone has migrated southwards into the nw directed thrust zone these relationships are consistent with the northwest termination of the arcuate himalayan thrust belt at a lateral tip generating folding and radial thrust directions faulting patterns at nanga parbat suggest that this tip zone has migrated southwards active faulting is now concentrated in the northwest continuation of the hazara southern syntaxis along a seismogenic strike slip and thrust zone these deep level tips lie on the crustal stacking thrusts which cut through the higher level sse directed thrusts of pakistan

compositional differences in enstatite chondrites based on carbon and nitrogen stable isotope measurements carbon and nitrogen abundance and isotopic compositions from four eh4 one eh5 five el6 chondrites and one aubrite were determined by using stepped pyrolysis n only and combustion n and c extractions in attempts to distinguish the components present carbon contents range from 0 15 to 0 70 wt with no systematic relationship between carbon content and meteorite group or petrologic type whole rock 13c values range from 28 5 to 4 1 most c occurs as graphite and when temperature steps above 700 c are considered there is a difference between eh4 5 13c 9 1 to 5 8 and el6 chondrites 13c 6 7 to 4 2 carbon in bustee aubrite is isotopically lighter 13c 24 than in any enstatite chondrite nitrogen occurs as osbornite sinoite and in isostructural substitution for oxygen in silicate lattice sites nitrogen abundances and isotopic compositions are more variable than c due to the heterogenous distribution of n bearing minerals three el6 s containing osbornite have higher n concentrations than other type 6 enstatite chondrites sinoite where present is depleted in 15n relative to osbornite nitrogen in the bustee aubrite has a similar abundance and 15n value to those of el6 s again dominated by the presence of osbornite in addition to the refractory c and n bearing minerals there is also organic material largely terrestrial contamination and evidence for at least two exotic components the first is a host for xe hl and is characterized by 13c 47 and 15n 73 whereas the second is less well defined but is marked by 15n 269

dating of deformation phases using k ar and 40ar 39ar techniques results from the northern apennines abstract paleozoic to oligocene metasedimentary rocks present in the alpi apuane region of the northern apennines italy have been sequentially deformed during a tertiary progressive deformation in an attempt to date the individual deformation episodes over 50 conventional k ar and 1140 ar 39ar incremental gas release analyses have been carried out on fine grained white micas separated from samples whose structural settings were well known mineralogy x ray diffractometry and thin section analyses indicate that the constituent muscovite and phengite formed under metamorphic conditions of 3 4 kbars and 300 400 c during all deformational phases pre existing micas were variably crenulated during each subsequent deformational phase both k ar and 40ar 39ar analyses were carried out on 0 6 2 m 2 6 m and 6 20 m size separates of the phengitic white mica although the k ar apparent ages range from 11 to 27 ma and are consistent with available stratigraphic constraints the 40ar 39ar age spectra display variable internal discordancy these isotopic data indicate that 1 both the k ar and 40ar 39ar total gas ages decrease as the degree of crenulation increases 2 the k ar and 40ar 39ar total gas ages decrease as grain size decreases 3 for each sample characteristics of the 40ar 39ar age spectra depend upon grain size with fine sizes yielding discordant patterns which systematically increase in apparent age from low to high temperature and 4 phengitic micas associated with earliest structures yield generally older ages than micas associated with later structures the isotopic results are interpreted to indicate that the major deformation phase d1 occurred at approximately 27 ma with subsequent pulses ending by c 10 ma these results may be combined with finite strain data to suggest that the region was deformed at strain rates between 10 15 and 10 14 s 1 a 27 ma age indicates mid oligocene initiation of plate tectonic activity in the western mediterranean and concomitant deformation in the northern apennines

documentation of neogene regional metamorphism in the himalayas of pakistan using u pb in monazite abstract in the himalayas investigators have constrained the timing of metamorphism associated with the eocene orogeny through the use of cooling ages on a number of minerals the majority of such studies give minimum metamorphic ages of 30 40 ma 1 2 3 4 5 6 largely confirming theoretical studies predicting peak metamorphic conditions occurring tens of millions of years after large scale collision e g 7 in northwestern pakistan however the nanga parbat haramosh massif nphm exhibits much more recent activity with rapid and accelerating cooling over the past 10 ma 1 2 the nphm also exhibits unexpectedly recent igneous activity three leucogranite dikes give minimum zircon u pb ages of 7 5 and 2 3 ma 8 the problem we address in this paper is whether the thermal structure suggested by recent leucogranite production at depth is discernible in the shallower metamorphic rocks of the nphm to date metamorphism we have used the u th pb system in monazite a common accessory mineral in high grade metamorphic rocks which can be a viable metamorphic geochronometer dating the formation of the mineral under moderate metamorphic conditions 9 10 monazite separated from schists and gneisses of the nphm gives u pb ages of 4 11 ma we interpret these young ages as evidence for neogene metamorphism affecting the nphm an event heretofore unsuspected in the greater himalaya and only intimated in the nphm this recent metamorphism roughly coincided with leucogranite production at depth 8 and with the onset of high denudation rates 1 2 as such our conclusions offer important constraints on further modeling of similarly anomalous behavior within tectonically active regions suggesting that any reasonable model should strive to explain the relationship between recent rapid uplift and coincident recent metamorphism

structural evolution and asymmetric uplift of the nanga parbat syntaxis pakistan himalya the nanga parbat syntaxis in the nw himalaya is a still growing crustal scale north trending antiformal structure in the core of which indian plate gneisses have been uplifted from beneath the overthrust rocks of the kohistan island arc isotopic and fission track geochronology show that uplift rates within the syntaxis have increased to present day rates of 6 mm yr uplift has been accommodated by a combination of initial northwest verging thrusting on the western margin of the syntaxis followed by crustal scale folding within the syntaxis and latterly by dextral reverse faulting on the western margin this thrusting folding and faulting is the effect of deformation at the north western lateral tips of the main himalayan thrusts where they interfere with the south southeast verging thrusts of the northwest himalaya

fluid enhanced deformation transformation of granitoids to banded mylonites western sierra nevada california and southeastern australia abstract deformation of tonalite and adamellite from the western sierra nevada california and southeastern australia has produced mylonite zones that show a broad range of microstructural development varying from strongly banded and or foliated to locally non foliated accompanying these fabrics are new mineral assemblages commonly rich in quartz and epidote whereas the banded foliated varieties appear to develop from progressive deformation of the granitic host they also occur in close proximity cm m to domains of non banded and or weakly to non foliated fabrics of similar mineralogy suggesting a complex timing and partitioning of deformation fluid flow and metamorphism in the mylonite zone whole rock major and trace element analyses of the tonalites show large enrichments of sio 2 cao fe 2 o 3 total fe and sr and strong depletions in na 2 o k 2 o feo mgo ba and rb with increasing stages of deformation although substantial chemical change appears to occur under static conditions locally chemical changes in the adamellite are much less dramatic all suites show systematic decrease in 18 o and convergence of oxygen isotopic compositions of whole rock and quartz with increasing deformation minimum fluid rock is estimated to lie between 0 1 and 10 increasing with intensity of deformation and depending on host composition isotopically exchanged seawater or connate water are the most plausible fluids chemical data from a number of mylonite zones in various orogenic belts show widely varying behavior gain loss of major and minor elements eleven factors control which and how much elements are gained lost during mylonitization of the granitoids the most influential of which are primary composition mineralogy and primary texture of the host as well as several aspects of fluid interaction active during deformation these influences may be a function of local rather than regional geological conditions

dating mylonitic deformation by the 40ar 39ar method an example from the norumbega fault zone maine strongly mylonitic rocks associated with the regionally extensive norumbega fault zone in south central maine provide an excellent opportunity for testing the effects of mylonitization on argon isotopic systems in muscovite 40ar 39ar muscovite age spectra from samples outside the zone of mylonitization are relatively undisturbed and have well defined early carboniferous plateau ages in contrast to these nonmylonitized samples all age spectra for muscovite from the mylonites are highly discordant they are characterized by young ages at low extraction temperatures which systematically increase to ages that equal the plateau ages for muscovite collected outside the mylonite zone detailed petrographic observations suggest that these systematic discordances reflect a mixing of argon components from older relict muscovite porphyroclasts and fine grained white mica aggregates that recrystallized during mylonitic deformation total gas ages of five different grain size fractions separated from the same mylonite sample become progressively younger with decreasing grain size indicating a larger component of the recrystallized grains in the finer grain size fractions although the three finest grain size fractions give different total gas ages and do not overlap in age for most of their release spectra their initial increments do coincide at approximately 290 ma this indicates a minimal older age contribution from the relict porphyroclasts in the initial increments and suggests the 290 ma age provides a good estimate for the time of mineral growth associated with mylonitic deformation these data combined with kinematic analysis reveal that the segment of the norumbega fault zone studied the sandhill corner fault is a late carboniferous early permian dextral strike slip fault a lack of significant offset in regional early carboniferous mineral age patterns across the fault suggests that displacement was probably less than 30 km this study demonstrates that 40ar 39ar dating methods can be used to date deformational events effectively as long as several important criteria are met first and foremost samples must be well characterized prior to analysis dynamic recrystallization must have occurred at or below the closure temperature of the mineral to be analyzed regional cooling patterns must also be established through detailed thermochronology so that mineral ages and age spectra from the deformed rocks can be compared to regional cooling ages of the same mineral finally the effects of excess argon must be negligible

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

rbsr dating of the kohistan arc batholith in the trans himalaya of north pakistan and tectonic implications the kohistan arc batholith in northern pakistan is situated between the indus suture and the northern suture it belongs to the trans himalayan belt which continues eastwards as the ladakh arc batholith in northwest india and the kangdese batholith in southern tibet in kohistan the island arc consists upward sequence of the chilas stratiform complex of norites noritic gabbros and chromite layered dunites which formed in the sub arc magma chamber plutons of tonalite and diorite one of which has a rbsr isochron age of 102 12 ma the chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites and the yasin group sediments which formed in overlying intra arc basins and which contain albian aptian faunas all these rocks were deformed by major fold structures which are correlated with the formation of the northern suture ne sw trending basic jutal nomal dykes north of gilgit cross cut all the above structures and have a39ar 40ar hornblende age of 75 ma thus the northern suture must have formed in the period 102 12 to 75 ma with the island arc attached to the eurasian plate further northward subduction of the tethyan plate gave rise to an andean type batholith two plutons of which have rbsr isochron ages of 54 4 ma and 40 6 ma the dir utror group of calc alkaline lavas and sediments with eocene fossils are remnants of the volcanic cover of the andean type batholith we suggest that late cretaceous blueschists formed during subduction under the active continental margin and that continental collision and formation of the indus suture was in the eocene the batholith was intruded by layered aplite pegmatite sheets at 34 14 ma and 29 8 ma rbsr ages in post collisional times the87sr 86sr initial ratios of all the dated rocks range between 0 7039 and 0 7052

a jurassic cretaceous island arc in the ladakh himalayas abstract the ladakh mesozoic ophiolite belt western himalaya contains a pile of volcanic thrust sheets dras unit which differ significantly in structure and composition from the ophiolitic melange zones the dras unit is composed of pillow lavas doleritic sills very irregular basaltic basaltic andesites and dacitic flows intercalated with pyroclastics volcanoclastic sediments and radiolarian cherts according to fossil evidence this volcanism must have been active between upper jurassic and upper cretaceous the presence of relict primary minerals such as magnesiochromite clinopyroxene hastingsitic hornblende and ti magnetite as well as distinctive bulk chemistries suggests that the volcanics belong to island arc tholeiite and to calc alkaline rock series typical of present island arcs in the caribbean and pacific model calculations incorporating probed phenocryst phases indicate that in addition to olivine clinopyroxene and plagioclase amphibole and titanomagnetite are crucial fractionating phases in the development of the dacites from a primitive tholeiitic melt the latter process must have taken place at about 1000 c and at moderate depth of 5 15 km within or underneath the island arc today hornblende bearing mafic cumulates appear in the vicinity of kargil within and close to the dras volcanics in a sr evolution diagram the dras volcanics have yielded a pseudo isochron with a low initial 87 sr 86 sr ratio of 0 7035 0 0003 which is in the same range as the mean of modern island arc volcanics however a geologically unrealistic age of 263 m y is obtained from the slope of this isochron the upper mantle is regarded as the source material for the island arc tholeiitic magmas enrichment in k ba sr and lree supports the involvement of components derived from dehydration or incipient melting of subducted tethyan oceanic crust in the mantle

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

anatomy of a continental subduction zone the main mantle thrust in northern pakistan the himalayas form an ideal natural laboratory to study the deformation processes of continental crust during collision orogeny new information is presented concerning the structural evolution of the main mantle thrust zone in the himalayas of n pakistan in the region around nanga parbat the hanging wall lies at relatively high levels within the kohistan arc terrane which has been emplaced onto indian continental rocks this thrust probably originated as a breakback structure in the hanging wall to the pre collisional oceanic subduction zone the present hanging wall contains a shear zone of about 1 km width developed under amphibolite facies conditions simple shear dominant strains have developed new fabrics parallel to the main thrust zone however these structures are redeformed by discrete extensional and compressional shears within about 100 m of the thrust contact again developed under amphibolite facies conditions the footwall consists of an old basement complex the nanga parbat gneisses overlain by a distinct suite of metasediments now at amphibolite facies this cover assemblage of psammites pelites and marbles with local metabasites consistently lies directly against rocks derived from the kohistan arc along the mmt the absence of structures suggestive of consistent rheological contrasts within the cover assemblages suggests that the vast majority of the deformation features they contain were developed only once they experienced substantial tectonic overburdens prior to this the indian cover rocks will have been passively subducted beneath the kohistan arc until into amphibolite facies we discuss these inferences in terms of evolving shear zone width with time and comment on the implications for predicting the character of mid deep crustal shear zones particularly from seismic reflection profiles

rb sr isotopic and compositional retentivity of muscovite during deformation abstract permian metapegmatite muscovite from the upper austroalpine matsch unit in southern tyrol italy was investigated regarding its rb sr and compositional retentivity during cretaceous upper greenschist facies deformation the data imply that microstructurally relic permian magmatic muscovite largely maintained its major and trace element compositions during deformation whereas the rb sr geochronometer is strongly affected by a net loss of sr lower sr concentrations of muscovite correlate with higher 87rb 86sr and 87sr 86sr ratios in most samples the muscovite grain size and magnetic fractions with the lowest 87rb 86sr and 87sr 86sr ratios preserve a permo triassic muscovite whole rock rb sr apparent age interpreted as to reflect formation during or cooling after pegmatite emplacement contrastingly muscovite fractions with higher 87rb 86sr and 87sr 86sr ratios are arranged along a roughly linear array with a positive correlation of the 87rb 86sr and 87sr 86sr ratios in the 87rb 86sr vs 87sr 86sr space they yield successively lower muscovite whole rock rb sr apparent ages we explain the variations in the rb sr isotopic character of microstructurally relic muscovite by a presumably deformation related loss of sr during the cretaceous event contemporaneously only very limited amounts of isotopically different sr from the matrix reservoir might possibly have entered the muscovite consequently the rb sr of the relic muscovite is affected by a net loss of sr the results imply that at temperatures of

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

isotopic constraints on the cooling history of the nanga parbat haramosh massif and kohistan arc western himalaya nineteen 40ar 39ar and 22 rb sr mineral ages are presented for metamorphic rocks and granites collected from the northern part of the nanga parbat haramosh massif nphm and adjacent parts of the kohistan arc western himalaya the majority of these ages are interpreted as exhumation related cooling ages and they therefore place important constraints on the exhumation history of the region the 40ar 39ar and rb sr mica cooling ages for areas of the nphm to the north of the indus river lie in the range 2 8 ma and are similar to previously published ages from the indus gorge itself cooling histories for these areas show little systematic variation from east to west suggesting that the basement may have been exhumed as a relatively rigid block after the main stages of folding mica ages increase dramatically from the nphm westward into the kohistan arc where ages lie in the range 13 30 ma the contrast in both cooling ages and the age differential between mineral pairs between the two terranes indicates rapid cooling of the nphm in the last 10 m y compared with the kohistan arc the relationship between rates of cooling and exhumation is poorly constrained in regions of rapid uplift where the characteristics of the evolving geotherm are not well understood however the results are consistent with an anomalously rapid exhumation history of the nphm during the last 10 m y compared with the adjacent kohistan arc the interpretation of some of the ages documented in this study is problematical two biotite samples from contrasting lithologies within the nphm have yielded higher 87sr 86sr ratios than the corresponding whole rocks but the cause of this isotopic disequilibrium remains unclear other biotite samples from the region have consistently yielded younger rb sr ages than 40ar 39ar ages which may be a reflection of either preferential leaching of 87sr during fluid infiltration on a regional scale or the presence of excess argon alternatively the relative ages indicate slightly lower closure temperature for strontium diffusion in biotite than for argon diffusion in biotite where samples have undergone rapid cooling

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

magmatism and metamorphism in the ladakh himalayas the indus tsangpo suture zone abstract ladakh india provides a complete geological section through the northwestern part of the himalayas from kashmir to tibet within this section the magmatic metamorphic and geotectonic evolution of the northern himalayan orogeny has been studied using petrographic geochemical and isotope analytical techniques the beginning of the himalayan cycle was marked by large basaltic extrusions panjal trap of permian to lower triassic age at the northern margin of the gondwana continent indian shield these continental type tholeiitic basalts were followed by a more alkaline volcanism within the triassic to jurassic lamayuru unit of the gondwana continental margin lower jurassic to cretaceous oceanic crust and sediments ophiolitic melange s s accompany the triassic to cretaceous flysch deposits within the indus tsangpo suture zone the major structural divide between the indian shield high himalaya and the tibetan platform so far no relic of paleozoic oceanic crust has been found subduction of the tethyan oceanic crust during upper jurassic and cretaceous time produced an island arc represented by tholeiitic and calc alkaline volcanic rock series dras volcanics and related intrusives accompanied by volcaniclastic flysch deposits towards the tibetan continental margin subsequent to the subduction of oceanic crust large volumes of calc alkaline plutons trans himalayan or kangdese plutons intruded the tibetan continental margin over a distance of 2000 km and partly the dras island arc in the ladakh region the collision of the indian shield and tibetan platform started during the middle to upper eocene and caused large scale still active intracrustal thrusting as well as the piling up of the himalayan nappes the tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle spongtang klippe in the high himalayas the tectonic activity was accompanied and outlasted by a barrovian type metamorphism that affected triassic sediments of the kashmir nun kun synclinorium up to kyanite staurolite grade and the deeper seated units up to sillimanite grade cooling ages of micas are around 20 m y muscovite and 13 m y biotite towards the indus tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist prehnite pumpellyite to zeolite grade remnants of possibly an eo himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone

cooling history of the nw himalaya pakistan fission track and 40ar 39ar cooling ages indicate that the late tertiary cooling history of the himalayan ranges of northern pakistan is largely a function of uplift and erosion interpretation of cooling ages which range from under 0 5 ma to over 80 ma suggests that during the late tertiary long term uplift rates at least doubled from under 0 2 mm yr to in some cases well over 0 5 mm yr uplift rates show strong and systematic regional variations as well which reflect the greater uplift of eastern and northern regions the association of very rapid uplift and erosion with the nanga parbat haramosh massif can be explained by a locally vigorous collision of india with eurasia near a promontory of indian crust the resultant rapid uplift of the nanga parbat haramosh massif reactivated the main mantle thrust melange zone with a reversed sense of motion discontinuities in the coolingage distribution along the main mantle thrust in the southern swat hazara region may be the result of the thermal effects of overthrusting

rb and sr diffusion in alkali feldspars with implications for cooling histories of rocks volume diffusion of sr and rb have been measured in orthoclase feldspar as well as sr diffusion in albite using isotope exchange hydrothermal solutions and analysis by ion microprobe for water pressures of 1000 bars the kinetics of diffusion parallel to the c axis are given by the arrhenius relations strontium transport in the or is isotropic to within a factor of four the measurements use a new procedure in which only the elements in question are present in the starting fluid cooling rates of igneous rocks can now be determined from closure temperatures material balance and the rb sr dating method based on the departures of the mineral data from the whole rock isochron for cooling rates less than approximately 100 c my

active hydrothermal systems during the recent uplift of nanga parbat pakistan himalaya during the last 10 m y the nanga parbat haramosh massif in the northwestern himalaya has been intruded by granitic magmas has undergone high grade metamorphism and anatexis and has been rapidly uplifted and denuded as part of an ongoing project to understand the relationship between tectonism and petrologic processes we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m y we suggest that the rapid uplift of the massif created a dual hydrothermal system consisting of a near surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic metamorphic volatiles meteoric fluids derived from glaciers near the summit of nanga parbat were driven deep into the massif along the transpressional faults causing 18 o and d depletions in the gneisses and marked oxygen isotopic disequilibriuui between mineral pairs from the fault zones the discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif at deeper levels within the massif infiltration of low 18 o magmatic fluids caused 18 o depletions in the gneisses within the migmatite zone these low 18 o fluids were derived from the young 4 ma relatively low 18 o granites 8 that are found within the core of the massif geochronological evidence in the form of fission track and 40 39 ar cooling ages and u pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined fluid infiltration in the migmatite zone rocks located along the tato traverse was coeval with metamorphism granite emplacement and rapid denudation in the interval 0 8 3 3 ma finally we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the nanga parbat haramosh massif

magmatic and tectonic evolution of the ladakh block from field studies abstract the ladakh block is in an intermediate position between the indian plate in the south and the karakorum tibetan plate in the north to the west it is separated from the kohistan arc by the nanga parbat syntaxis to the east it is cut off from the lhasa block by the gartok nubra fault present data together with previously published results show that the ladakh block consists of an island arc in the south and a calc alkaline batholith in the north with remnants of a continental crust migmatitic gneisses and metasedimentary sequences such as quartzites and metapelites interbedded with basaltic volcanics and overlain by thick platform carbonates were found as evidence of a continental crust remnants of megafossils megalodon and lithiotis within the high grade metamorphic marbles indicate a probable age of late triassic to early jurassic these sediments were intruded by a faintly layered hornblende gabbro which preceded the calc alkaline magmatic episode gabbro and gabbronorites are found as roof pendants and large inclusions within diorites and granodiorites the major part of the batholith consists of granodiorite and biotite granite plutons ranging from late cretaceous to tertiary associated with the intrusives are volcanic rocks with trachyandesite to alkalibasalt and basalt andesite to rhyolite compositions garnet bearing leucogranites succeeded the emplacement of the major plutons the magmatic stage ended finally by intense fracturing and injections of ne sw striking andesitic dykes the southernmost unit of the ladakh block is formed by oceanic crust with serpentinized peridotite and hornblende gabbro and is covered by volcanics of an island arc type dras volcanics these units are intruded by gabbronorite as well as middle and upper cretaceous granodiorite and coarse grained biotite granite in a plate tectonic view the ladakh block represents a transitional sector between the pure island arc of kohistan in the west and the andean type margin of the lhasa block in the east

k ar and ar ar geochronology of the himalayan collision in nw pakistan constraints on the timing of suturing deformation metamorphism and uplift mica and hornblende k ar and ar ar data are presented from each of the three crustal components of the himalayan collision zone in north pakistan the asian plate the kohistan island arc and the indian plate together with u pb and rb sr data published elsewhere these new data 1 date the age of suturing along the northern suture which separates kohistan from asia at 102 85 ma 2 establish that the basic magmatism in kohistan which postdates collision along the northern suture predates 60 ma and that the later granite magmatism spanned a range of 60 25 ma 3 show that uplift amounts within kohistan are greater toward the nanga parbat syntaxis than away from it and that rate of uplift near the syntaxis increased over the last 20 ma to a current figure of about 5 5 mm a year 4 show that much of southern kohistan had cooled to below 500 c by 80 ma and that the major deformation which imbricated kohistan probably predated 80 ma and certainly predated 60 ma and was related to the kohistan asia collision rather than the kohistan india one 5 imply that uplift along the hunza shear in the asian plate together with imbrication of the metamorphics in its hanging wall took place at about 10 ma and was associated with breakback thrusting in the hanging wall of the main mantle thrust 6 suggest that the indian plate has a lengthy pre himalayan history with an early metamorphism at about 1900 ma major magmatism at 500 550 ma and early jurassic lithospheric extension or inversion and 7 show that the indian plate rocks were metamorphosed shortly after the collision within kohistan which occurred at circa 50 ma and subsequently cooled back through 500 c at circa 38 ma and 300 c at 30 35 ma with ages of cooling through 200 and 100 c as determined by fission track data locally controlled by nanga parbat related uplift tectonics

tectonic controls on the uplift of the nanga parbat massif pakistan himalayas cenozoic mountain belts like the himalayas contain regions that experienced rapid uplift in comparison with the areas around them1 whether this uplift occurred by buoyant rebound of previously thickened crust2 or during the crustal thickening process itself is uncertain3 4 here we show however that the 7 mm yr 1 recent uplift5 of the nanga parbat massif pakistan himalayas was accomplished during nw directed thrusting located along its western margin this 3 km wide liachar thrust zone operated from amphibolite facies to the present topographic surface and accommodates substantial crustal shortening the adjacent indus river valley itself cut by seismogenic faulting completely transects the massif showing that erosion keeps pace with geologically rapid thrust uplift6

geochronologic constraints on the tectonic evolution and exhumation of nanga parbat western himalaya syntaxis revisited abstract we examine the timing of deformation and exhumation of the nanga parbat haramosh massif in the western syntaxis of the himalaya this study presents geochronologic and thermochronologic data obtained from basement shear zone and intrusive units within the massif to reveal the cenozoic tectonic evolution of the massif and to document the extent of the plio pleistocene tectonic activity these results combined with structural and petrologic observations indicate that the western himalayan syntaxis was tectonically active for a protracted length of time and that the deformational succession was punctuated by three episodes since the beginning of the india asia collision 1 the western syntaxial indian plate rocks future nanga parbat underwent metamorphism melting and deformation during the initial collision of india with asia and the associated island arc the result of which is recorded as eocene to early miocene metamorphic and magmatic ages and oligocene cooling preserved in the indian co

petrogenetic and tectonic significance of young leucogranites from the northwestern himalaya pakistan in the himalaya of northwestern pakistan small discordant bodies of leucogranite intrude metamorphosed basement of the indian crust as the first step in assessing the tectonic significance of these granites we have used the shrimp ion microprobe to obtain u pb dates on zircon zircons of igneous appearance are scarce in the leucogranites and with one exception are associated with xenocrystic components which occur as discrete grains and as cores rimmed by young zircon zircons of igneous appearance are high in u 5000 to 40 000 ppm low in th u 0 01 to 0 05 and frequently colored a distinct blue xenocrystic components give a range of proterozoic ages consistent with ages previously reported for indian crust in sharp contrast zircons of igneous appearance yield clusters of concordant tertiary ages two leucogranite dikes from southern localities yield rather old emplacement ages of about 35 ma swat and about 50 ma naran together with previously determined 40ar 39ar and fission track cooling ages these ages document immediate and rapid metamorphism and denudation following eocene collision three leucogranite dikes from northern localities within the nanga parbat haramosh massif are very young having emplacement ages of about 2 3 ma 5 ma and 7 ma the formation and emplacement of these leucogranites during the rapid late tertiary denudation of the nanga parbat region suggests to us that decompression melting may be a viable mechanism for leucogranite genesis

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

geo tectonic framework of the himalaya of n pakistan in the karakorum range there is a structurally complicated cretaceous are comprising the kohistan sequence on its northern side the northern suture consists of a mega melange and is bounded to the s by tightly folded pillow bearing volcanics and sediments to the s the kohistan plutonic belt consists of southwards a early foliated and late post tectonic tonalites and diorites b aplites and pegmatites up to 30 of rock volume c basic dykes up to 10 m thick d the chilas complex a stratiform cumulate body over 300 km long and 8 km thick chromite layered dunites gabbros and norites with a low pressure granulite facies mineral fabric of tectonic origin e an amphibolite belt with a complex mixture of other rocks and f the jijal complex a 200 km 2 tectonic wedge of high pressure granulites and chromite layered dunites cumulate graded units in the chilas complex show that it is folded by an isoclinal anticline f 1 the mid upper crust of the are is folded by a 50 km half wavelength f 2 syncline the whole kohistan sequence with its two phases of isoclinal folds was tilted during himalayan collision so that the structures are now subvertical the southern suture main mantle thrust has a wedge of glaucophane schists the indian plate contains a basement of psammites and schists intruded by cambrian granites and overlain by isoclinally folded and metamorphosed carbonates and shales

evidence for a large magnitude eruption from campi flegrei caldera italy at 29 ka the 40 ka caldera forming eruption of campi flegrei italy is the largest known eruption in europe during the last 200 k y but little is known about other large eruptions at the volcano prior to a more recent caldera forming event at 15 ka at 29 ka a widespread volcanic ash layer termed the y 3 tephra covered 150 000 km2 of the mediterranean the glass compositions of the layer are consistent with campi flegrei being the source but no prominent proximal equivalent in the appropriate chrono stratigraphic position had been previously identified here we report new glass chemistry data and 40ar 39ar ages 29 3 0 7 ka 2 that reveal the near source y 3 eruption deposit in a sequence at ponti rossi and a nearby borehole s 19 in naples the dispersal and thickness of the deposits associated with this eruption herein named the masseria del monte tuff were simulated using a tephra sedimentation model the model indicates that 16 km3 dense rock equivalent of the magma erupted was deposited as fall this volume and the areal distribution suggest that the masseria del monte tuff resulted from a magnitude m 6 6 eruption corresponding to volcanic explosivity index vei 6 similar to the 15 ka caldera forming neapolitan yellow tuff m 6 8 eruption at campi flegrei however the lack of coarse thick traceable nearvent deposit suggests peculiar eruption dynamics our reconstruction and modeling of the eruption show the fundamental role that distal tephrostratigraphy can play in constraining the scale and tempo of past activity especially at highly productive volcanoes

high pressure metamorphism in the high himalayan crystallines of the stak valley northeastern nanga parbat haramosh syntaxis pakistan himalaya

synchronous anatexis metamorphism and rapid denudation at nanga parbat pakistan himalaya the nanga parbat haramosh massif of the western himalaya is a north trending half window of indian crust that provides spectacular exposures of precambrian basement gneisses that have been overprinted by himalayan metamorphism we report here petrologic data and u pb dates on zircon and monazite which document that nanga parbat gneisses underwent a pliocene pleistocene episode of high grade metamorphism and anatexis during an interval in which the nanga parbat massif was undergoing rapid denudation at mean rates of 5 mm yr we speculate that by initiating decompression melting this denudation may be at least partly responsible for the anatexis and high grade metamorphism

compositional changes of minerals associated with dynamic recrystallizatin the rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously the enhanced exchange rate may result from a reduction in grain size shorter distance for volume diffusion dissolution and growth of grains by diffusion creep pressure solution or the movement of high angle grain boundaries through strained grains during recrystallization in the dislocation creep regime the migration of high angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization the operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases an1 and an79 at 900 c 1 gpa confining pressure and a strain rate of 2x10 6s 1 the polygonal recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition at the conditions of these experiments the volume interdiffusion rate of nasi caal is too slow to produce any observable chemical change and microstructural chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine grained 2 10 m aggregates these results indicate that strain induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars pyroxenes and amphiboles

constraints on the tectonic evolution of the northwestern himalaya from geochronologic and petrologic studies of babusar pass pakistan petrologic geochronologic and structural studies of indian plate rocks in the footwall of the main mantle thrust indicate that peak metamorphism occurred in thrust imbricates well before the final assembly of the kohistan island arc with the indian plate as the northern margin of the indian plate was thickened by this early thrusting the metamorphic rocks evolved along increasing pressure temperature paths and in some regions reached temperatures sufficient for partial melting it is difficult to constrain the timing of this early deformation and metamorphism but 40 ar 39 ar hornblende ages and u pb ages of crosscutting leucogranite dikes suggest that in most areas peak metamorphism was earlier than 40 ma and in some areas earlier than 50 ma after peak metamorphism in the interval from 40 to 25 ma rocks of the indian plate continued to be thrust southward creating sharp metamorphic discontinuities at thrust boundaries from 25 to 20 ma rocks of the indian plate underwent rapid cooling

textural and rb sr isotopic evidence for late paleozoic mylonitization within the honey hill fault zone southeastern connecticut a petrographic and rb sr isotopic study of rocks within and near the honey hill fault zone places important constraints on its history of movement rb sr apparent ages for micas and plagioclase from these rocks have been reset and range from permian to triassic considerably younger than the minimum stratigraphic age ordovician of the rocks studied or of acadian devonian regional metamorphism permian rb sr ages of dynamically recrystallized muscovite date the development of mylonite fabric an older age is precluded by the excellent preservation of unrecovered quartz which indicates that these rocks did not experience temperatures high enough to anneal quartz or thermally reset rb sr isotopic systems in muscovite since the time of mylonitization metamorphic mineral assemblages and mineral apparent ages in rocks north of the fault zone indicate recrystallization under similar upper greenschist lower amphibolite grade conditions during permian to triassic time collectively these results indicate that the honey hill fault zone was active during the late paleozoic and that ductile deformation and metamorphism associated with the alleghanian orogeny extend well into southern connecticut an alleghanian age for mylonitization within the honey hill fault zone suggests it should be considered as a possible site for the major late paleozoic strike slip displacementsmore inferred from paleomagnetic studies for parts of coastal new england and maritime canada less

petrogenetic implications of neodymium isotope data from the kohistan batholith north pakistan neodymium data are presented for five granitoid trondhjemite granite and leucogranite plutonic units from the kohistan batholith aged 102 ma to 29 ma these have present day l43 nd 144 nd ratios of between 0 512980 and 0 512734 calculated initial 143 nd 144 nd ratios of between 0 512861 and 0 512705 and nd values of between 6 91 and 2 04 there is a decrease in nd with time which is inversely correlative with a similar increase in sr three plutonic units matum das gilgit and shirot formed from a source enriched in sm and depleted in rb or radiogenic sr relative to bulk earth whilst a fourth unit the indus confluence acid sheets is only slightly enriched in radiogenic sr these four units define a temporal trend from the least evolved matum das pluton to the most evolved indus confluence acid sheets this trend was produced as a result of subducting oceanic sediment or seawater altered oceanic crust which melted or dehydrated and increasingly modified the isotopic composition of an original mantle protolith situated above the subducting plate the data preclude any significant input to the magmatism from ancient crust new data presented here together with other published data indicate an immature metavolcanic crustal source for the matum das pluton and a plutonic basement crustal source for the indus confluence acid sheets these crustally derived units retain a mantle isotopic signature indicating that the geochemical signature of kohistan evolved by remobilization of recently formed arc crust in addition to new inputs of mantle derived magma a fifth plutonic unit the parri acid sheets shows a clear compositional break from the other units being significantly enriched in radiogenic sr and nd with respect to bulk earth the source to the parri acid sheets is interpreted as metasedimentary rocks with a high rb sr ratio and a crustal residence time of c 70 ma

changing source regions of magmas and crustal growth in the trans himalayas evidence from the chalt volcanics and kohistan batholith kohistan northern pakistan the kohistan batholith and chalt volcanics form a major part of the kohistan island arc terrane in northern pakistan they record some 70 80 ma of magmatism within the northwestern himalayan orogen the chalt volcanics are albian aptian mid cretaceous and currently available rbsr age data for the batholith indicate an intrusive age span of 102 to 30 ma the volcanics are composed of medium k calc alkaline andesites and low k high mg tholeiites some of which have boninitic and basaltic komatiitic chemistry three intrusive stages are recognised in the batholith 1 110 90 ma comprises low k trondhjemites and medium high k calc alkaline gabbro diorites with associated hornblendite cumulates 2 85 40 ma comprises low high k calc alkaline gabbro diorites with hornblendite cumulates granodiorites and granites and 3 circa 30 ma comprises biotite muscovite garnet leucogranites 87sr 86sr0 for the batholith range between 0 7039 and 0 7052 four magmatic source regions sce 1 sce 4 have been identified sce 1 is a variably metasomatised mantle wedge situated above an active subduction zone during stages 1 and 2 sce 2 is a harzburgite depleted with respect to major elements and incompatibles whilst retaining subduction related trace element ratios sce 2 melted during stage 1 within a fore arc position during the subduction of young hot oceanic crust to provide the necessary extra thermal input for harzburgite anatexis melts from sce 2 produced the high mg tholeiitic volcanics sce 3 melted during stage 1 to produce the low k trondhjemites and it is envisaged as incompatible element depleted primitive arc crust with a similar composition to the depleted chalt volcanics sce 4 melted at 30 ma during stage 3 as india underthrust eurasia frictional heating and dehydration of the indian plate caused crustal anatexis within the deep kohistan arc and these melts formed the stage 3 leucogranites the low 87sr 86sr initial ratios for the great bulk 95 of the batholith indicate that it represents a major juvenile addition to the continental crust this conclusion has important implications for crustal growth theories of island arc accretion and batholith underplating

geology of the northern part of the nanga parbat massif northern pakistan and its implications for himalayan tectonics the northern outcrop termination of indian continental crust lies at the nanga parbat massif from where this contribution presents new field data the tectonic contact with the structurally overlying kohistan island arc is concordant and ductile is associated with interleaving of nanga parbat and kohistan lithologies and may be correlated with the main mantle thrust found elsewhere in the nw himalayas this ductile shear zone is locally overprinted by cataclastic faults associated with exhumation of the massif but overall the northern outcrop termination of the massif is controlled by erosion through a gently northward plunging antiformal structure this folds both the main mantle thrust and the underlying concordant 9layered unit9 of the nanga parbat basement thus there is no indication that the massif acted as a promontory to the indian continent during collision nor that it is a structure entirely bound by neotectonic faults ductile shear fabrics associated with the 9main mantle thrust9 are cross cut by leucogranite sheets and pegmatites these may represent the stockwork to a significant crustal melt granite body described here in the northern massif this jutial granite shows many geochemical characteristics in common with similar bodies in the high himalayas which are consistent with anatexis of the buried indian continental basement rocks however the granite is enriched in heat producing elements particularly th 22 ppm and shows extremely high 87 sr 86 sr ratios 0 88 the high concentrations of radiogenic sr are also an attribute of a suite of hitherto enigmatic leucogranitic pegmatites that laces the nanga parbat massif suggesting that these may represent a stockwork to a largely buried body of which the jutial granite is a small exposure the enrichment in heat producing elements within the granite may reflect similarly high heat production in the source indian continental crust requiring in turn a fundamental re examination of the thermal evolution of this crust during himalayan metamorphism and exhumation

unroofing history of a suture zone in the himalaya of pakistan by means of fission track annealing ages abstract the uplift history of the swat valley and hazara region of northwestern pakistan has been established using 22 fission track dates on apatite zircon and sphene a major fault the main mantle thrust mmt strikes east west across the swat valley separates regions of markedly differing fission track age regimesm and may be a suture zone separating an extinct island arc terrane on the north from the indian plate to the south fission track ages ranging from about 55 to 58 m y for sphene 18 to 53 m y for zircon and 9 to 17 m y for apatite were obtained from the region north of the mmt to the south the fission track age ranges are 20 to 25 m y for sphene 17 to 26 m y for zircon and 16 to 23 m y for apatite disparate zircon and sphene ages on each side of the mmt imply different cooling histories for each side of the fault prior to 15 m y similar apatite ages on both sides of the fault imply similar cooling histories during the past 15 m y this may indicate that faulting ceased by 15 m y mean uplift rates have been derived from the fission track data using mainly the mineral pair method uplift rates in the region north of the mmt increased from 0 07 to 0 20 mm yr during the period 55 to 15 m y south of the fault uplift rates averaged in excess of 0 70 mm yr for the period 25 to 15 m y during the past 15 m y uplift across the mmt in the swat valley showsno discontinuities ranging from 0 16 mm yr in the south to 0 39 mm yr in the north a plausible interpretation for the fission track uplift data has the mmt verging to the south with overthrusting taking place at a depth between 3 5 and 6 0 km juxtaposing two terranes that were originally separated by a substantial but unknown distance in this model regional uplift followed cessation of faulting just prior to 15 m y

metamorphism and melting of the lithosphere due to rapid denudation nanga parbat massif himalaya the nanga parbat haramosh massif of the western himalaya has undergone a complex metamorphic and denudational history over the past 1 8 ga metamorphism in the tato region is characterized by a barrovian type metamorphism followed by partial melting and recrystallization along shear zones during nearly isothermal decompression while the massif was still at temperatures 550 c mineralogical and textural evidence for this path includes garnet zoning patterns late stage migmatization abundant cordierite both in schists and leucogranite dikes replacement of high pressure assemblages by low pressure assemblages and abundant low density inclusions thermobarometry on the nonmigmatized gneisses reveals conditions of 540 740 c at 7 1 13 1 kbar thermobarometry in the migmat ized rocks reveals final equilibration at 608 675 c at 3 9 6 8 kbar early fluid inclusions occur in quartz inclusions within garnet porphyroblasts that grew during decompression the last fluid inclusions to be trapped occur in microfrac

argon retentivity and argon excess in amphiboles from the garbenschists of the western tauern window eastern alps 22 hornblende k ar ages and 10 39ar 40ar spectra were obtained for hornblende garbenschists from the western tauern window the post kinematic amphiboles were produced during the late alpine prograde metamorphism 6 10 kb and 500 570 c two nearly potassiumfree cummingtonites rimming hornblende yield k ar ages of 120 ma while the 20 tschermakitic hornblendes scatter between 17 and 37 ma the reason for this scatter is excess ar possibly incorporated into amphiboles during healing of fractures now traceable by trails of fluid inclusions excess ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with k ar isochrons it can be quantified in 4 out of 10 hornblendes by 39ar 40ar stepwise heating experiments ages of 18 20 ma result for corrected hornblendes the retentivity of 40ar after correction for excess shows no correlation with chemistry within the narrow compositional range observed rather it shows intriguing correlations with irregularities in ca k spectra pointing to a microstructurally controlled mechanism for ar loss this observation leads to a critical evaluation of the closure temperature constant which apparently depends on an incompletely known number of mineralogical and environmental parameters in particular those 39ar 40ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure this gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media

rejuvenation of rbsr mica ages during shearing on the northwestern margin of the nanga parbat haramosh massif abstract the shear zone bordering the western margin of the nanga parbat haramosh massif nphm separates terranes with distinct but well documented cooling histories this study was undertaken in order to examine isotopic behaviour and if possible to date deformation or cooling within the contact zone itself rb sr muscovite and biotite ages and petrographic descriptions are presented for a range of metamorphic lithologies and granite sheets which have been affected to varying degrees by greenschist facies extensional crenulation this deformation has been kinematically linked to the differential uplift of the nphm ages for metamorphic lithologies and granite sheets are similar and are considered together determined muscovite ages mostly lie in the range 9 22 ma whilst the majority of biotite ages lie in the range 2 11 ma mica ages from the shear zone are significantly more variable than ages determined for unsheared lithologies from outside the shear zone even closely spaced samples yield variable mica ages and in some cases there is a significant difference between muscovite and biotite ages determined for the same sample muscovite ages yield a bimodal distribution with groups of ages in the range 17 21 ma and 8 12 ma the older group is thought to reflect the original purely thermostatic cooling of lithologies through the muscovite closure temperature prior to formation of the nphm structure in contrast the younger group of muscovite ages reflects variable degrees of isotopic resetting during shearing at temperatures below the muscovite closure temperature associated with the relative uplift of the nphm the majority of biotite ages also reflect partial to complete resetting during shearing however biotite samples from structurally competent amphibolite horizons preserve older 16 ma ages indicating that there was locally a strong lithological or fluid control on the degree of resetting the flanks of the nphm cooled through the muscovite closure temperature significantly earlier than the deeper structural levels presently exposed within the core of the nphm greenschist grade deformation was occurring within the shear zone at the same time that deeper levels of indian crust were passing through the muscovite closure temperature suggesting that the shearing reflects the relatively passive extensional movement of rocks sliding off the flank of the actively growing nanga parbat structure the results highlight the isotopic complexities of long lived shear zones present in orogenic belts but show that useful radiometric age information may be obtained if the regional tectonic history is well understood and if the samples for dating come from well constrained structural settings

geochemical and k ar isotopic behaviour of alpine sheet silicates during polyphased deformation abstract several deformational events with several generations of phengites occur in the schistes lustres from the northern cottic alps the phengites belong mainly to 1 1 old forms from a s 1 schistosity 2 2 deformed forms from s 2 cleavage tightening zones 3 3 neoformed forms from second generation stretching zones the si tetrahedral substitutions of these minerals which lie between 3 28 and 3 46 depend on their microstructural site this chemical variation cannot only be related to p t conditions it is also induced by deformation related solution deposition processes the k ar apparent ages of the phengites are widely scattered between 38 and 65 ma the data when plotted in an isochron diagram suggest a two step response at 58 54 ma and 50 49 ma of the phengites to deformation this could happened during the same event which produced the second generation of microstructures