a p t t d discontinuity in east central nepal implications for the evolution of the himalayan mid crust abstract rock specimens collected from lower middle and upper portions of the exhumed himalayan mid crust in the upper tama kosi region of east central nepal yield pressure temperature time deformation p t t d paths that demonstrate the presence of a structural metamorphic and geochronologic discontinuity a p t t d path from below the discontinuity defines a loading path with deformation at staurolite grade metamorphic conditions occurring at 10 8 ma p t t d paths from above the discontinuity in contrast record an earlier and protracted metamorphic history that includes decompressional heating monazite ages from specimens collected above the discontinuity constrain prograde garnet growth to be 21 ma with decompression related garnet breakdown initiating at c 19 ma and likely continuing until c 15 ma these contrasting p t t d histories separated by a tectonometamorphic discontinuity are consistent with an evolutionary model for the himalaya in which rocks above the discontinuity were metamorphosed in the deep hinterland of the orogen and deformed during lateral extrusion from beneath tibet while rocks below the discontinuity were deformed and metamorphosed later within the shallower foreland of the orogen these results demonstrate the spatial and temporal relationships and compatibility between lateral midcrustal extrusive flow and wedge taper processes within an evolving orogen

monazite xenotime garnet equilibrium in metapelites and a new monazite garnet thermometer prograde suites of pelitic rocks were examined with electron microprobe introduction and laser ablation inductively coupled plasma mass spectrometry to monazite ce la th po4 plays an important role in determine the systematics of element partitioning between coexisting studies of igneous and metamorphic petrogenesis foremonazite xenotime and garnet monazite grains that grew in most monazite is used to date specific events in a equilibrium with xenotime are enriched in y and dy compared with petrogenetic sequence e g parrish 1990 harrison et al monazite that grew in xenotime absent assemblages y and heavy 1997 hawkins bowring 1999 foster et al 2000 rare earth element contents of monazite coexisting with xenotime monazite may contain a large percentage of the sample increase with rising temperature monazite xenotime y gd and rare earth element ree budget and thus exert an y dy partitioning is systematic within a metamorphic grade and important influence on the evolution of melt composition increases slightly with increasing metamorphic grade suggesting wark miller 1993 bea 1996 the thermobarometric that monazite xenotime pairs approached partitioning equilibrium potential of monazite coexisting with xenotime has been garnet and monazite in both xenotime bearing and xenotime absent recognized leading to calibration of monazite geoassemblages show a strong r 0 94 systematic relationship thermometers gratz heinrich 1997 1998 heinrich between inverse temperature and ln keq for the net transfer equiet al 1997 andrehs heinrich 1998 in addition the librium yag oh ap 25 4 qtz 5 4 grs 5 4 an recognition that nearly all lead in monazite is radiogenic 3ypo4 mnz 1 2h2o suggesting that garnet and monazite parrish 1990 montel et al 1994 has led to development crystallized in compositional equilibrium the following temand application of electron microprobe monazite dating perature keq relationship for the equilibrium above has been derived techniques suzuki adachi 1991 montel et al 1996 williams et al 1999 t c 1 45p bars 447772 32052 567 40 rln keq 273 15 full realization of monazite as a thermochronometer requires detailed understanding of the specific reactions responsible for its formation previous studies of monazite with a precision of some 30 c for temperature estimates our petrogenesis have focused on monazite compositional observations suggest that major and accessory phases interact in a zonation zhu o nions 1999a 1999b monazite coupled fashion during metamorphism and also approach a state growth kinetics ayers et al 1999 behavior of monazite of compositional equilibrium as reactions proceed during hydrothermal alteration poitrasson et al 1996 crowley ghent 1999 and melting events watt 1995 or to a limited extent the influence of major phase mineral assemblage in monazite reactivity zhu

discovery of a crystalline basement and early ordovician marine transgression in the karakorum mountain range pakistan a granite pluton intruding low grade quartzites and migmatites lies north of the karakorum axial batholith of cretaceous to miocene age the pluton is covered by transgressive litharenite and slate that contain a fairly rich assemblage of acritarchs of arenigian age early ordovician belonging to the peri gondwana biogeographic province this is the first time that crystalline basement has been found in karakorum and acritarchs in central asia these discoveries add new evidence that the karakorum microplate together with the helmand block of central afghanistan and the lhasa block of tibet belongs to a cimmerian domain at the northern fringe of gondwana

building the hindu kush monazite records of terrane accretion plutonism and the evolution of the himalaya karakoram tibet orogen in situ u th pb la icp ms monazite ages from the hindu kush of nw pakistan provide new petrochronologic constraints on the tectonic evolution of the himalaya karakoram tibet orogen monazites from two adjacent garnet staurolite schist specimens yield multiple age populations that record the major mesozoic and cenozoic deformational magmatic and metamorphic events along the southern margin of eurasia these include the accretion of the hindu kush sw pamir to eurasia during the late triassic followed by the accretion of the karakoram terrane in the early jurassic younger jurassic and cretaceous ages record the development of an andean style volcanic arc along the southern eurasian margin which ended with the docking of the kohistan island arc and the emplacement of the kohistan ladakh batholith during the late cretaceous the initial eocene collision of india with eurasia was followed by widespread high temperature metamorphism and anatexis associated with crustal thickening within the himalaya system in the late oligocene and early miocene

reconnaissance geology in upper chitral baroghil and karambar districts northern karakorum pakistan during the summer of 1992 a geological expedition crossed the northern karakorum range in northern pakistan from the chitral to karambar valleys from the villages of mastuj to imit some of the areas visited were geologically unknown a number of structural units were crossed belonging to the karakorum block or to other crustal blocks north of it they are a the axial batholith in which three plutonic bodies have been identified and b the northern sedimentary belt nsb in which three major tectonostratigraphic units form thrust stacks dipping to the north their internal stratigraphy and structural style are partly different the most complete contains a crystalline basement transgressed by a marine succession during the early ordovician the youngest strata are represented by the reshun conglomerate of inferred cretaceous age the northernmost unit of the nsb is tightly folded whereas the central one forms a monocline vertical faults mainly strike slip dissect the thrusted slabs metamorphic deformation is absent or reaches only the anchizone in the studied sector of the karakorum nsb to the north of the karakorum proper there are several other tectonic units separated by vertical faults they are from south to north a the tas kupruk zone with metavolcanics of basaltic to latibasaltic composition b the atark unit mostly consisting of massive carbonate rocks of mesozoic age and c the wakhan slates which consist of a thick widespread succession of dark slates metasiltites and sandstones the fine grained elastic rocks are supposed to be palaeozoic to early triassic in age the wakhan slates are intruded by plutons belonging to the east hindu kush batholith from which a single k ar age on muscovite gave a jurassic age

on the role of buoyant flexure in glacier calving interactions between glaciers and the ocean are key for understanding the dynamics of the cryosphere in the climate system here we investigate the role of hydrostatic forces in glacier calving we develop a mathematical model to account for the elastic deformation of glaciers in response to three effects i marine and lake terminating glaciers tend to enter water with a nonzero slope resulting in upward flexure around the grounding line ii horizontal pressure imbalances at the terminus are known to cause hydrostatic in plane stresses and downward acting torque iii submerged ice protrusions at the glacier front may induce additional buoyancy forces that can cause calving our model provides theoretical estimates of the importance of each effect and suggests geometric and material conditions under which a given glacier will calve from hydrostatic flexure we find good agreement with observations this work sheds light on the intricate processes involved in glacier calving and can be hoped to improve our ability to model and predict future changes in the ice climate system

tectonic significance of 24 ma crustal melting in the eastern hindu kush pakistan northward directed subduction has continued beneath the hindu kush from the mesozoic when the region formed part of the southern margin of asia until today the kohistan arc was accreted at ca 100 ma followed by the indian plate at 54 50 ma after the india asia collision a regional crustal melting event occurred at ca 25 21 ma to the east of the hindu kush in the karakoram range producing the baltoro pluton in the eastern hindu kush range most of the granitoids appear to be subduction related plutons that predate the india asia collision however the gharam chasma pluton is a two mica garnet tourmaline leucogranite that has intruded into staurolite grade schists and sillimanite k feldspar grade migmatites u pb ages on monazite xenotime and uraninite from undeformed samples of the gharam chasma pluton and a leucogranite dike that crosscuts the migmatites indicate that crustal melting occurred at ca 24 ma synchronous with the baltoro melting event this age also provides an upper limit on the age of the regional staurolite grade metamorphism in the eastern hindu kush

cooling exhumation and deformation in the hindu kush nw pakistan new constraints from preliminary 40ar 39ar and fission track analyses abstract asian crust in the hindu kush region in northern pakistan records a protracted history of rifting subduction and collision not commonly preserved within the himalaya because of this it is key to understanding the development of the southern eurasian margin both prior to and after collision with india new mica 40ar 39ar and apatite fission track geochronologic data from this region provide constraints on the kinematics of the hindu kush 40ar 39ar muscovite and biotite ages from the late cambrian kafiristan pluton are 379 7 1 7 ma and 47 2 0 3 ma respectively the muscovite age may record cooling or partial resetting while the biotite age is interpreted to record a thermal disruption associated with the early stages of continental collision in the himalayan system a 111 0 0 6 ma muscovite age from the northern part of the tirich mir pluton 123 ma old u pb is interpreted to indicate a recrystallization event 12 myrs after its intrusion in addition a younger muscovite age of 47 5 0 2 ma was derived from the opposite side of the same pluton in the immediate hanging wall of the tirich mir fault this eocene age is interpreted to represent the time of recrystallization during fault re activation in the early stages of india asia continent continent collision 40ar 39ar biotite analysis from the buni zom pluton yields an age of 61 6 1 1 ma and is interpreted to reflect cooling at mid upper crustal levels subsequent to the pluton s emplacement in the middle cretaceous finally 17 1 21 3 ma 40ar 39ar ages from the garam chasma pluton and surrounding metapelites indicate cooling immediately following crystallization of the leucogranite body in the earliest miocene latest oligocene the younger cooling history is resolved by fission track dating of apatite aft in the vicinity of the bounding tirich mir fault the tirich mir pluton yields an aft age of 1 4 0 3 ma which is consistent with active exhumation associated with the surface uplift of the 7700 m tirich mir peak the garam chasma pluton has a young age of 3 5 0 2 ma which also records rapid rock uplift and exhumation in the area finally an aft age of 9 1 2 1 ma was extracted from a metapelite in the footwall of an east verging thrust fault separating it from the garam chasma pluton to the west the difference in ages pliocene vs late miocene reflect differential cooling exhumation paths across that structure

old origin for an active mountain range geology and geochronology of the eastern hindu kush pakistan prior to accretion of the kohistan island arc during the late cretaceous and final suturing of india with asia at ca 50 ma the hindu kush mountains along the border of afghanistan and northwest pakistan were situated on the active southern margin of asia geology and geochronology of the eastern hindu kush range in pakistan demonstrate that localized crustal melting and leucogranite intrusion took place in the gharam chasma area at ca 24 ma more regionally developed and widespread deformation metamorphism and magmatism took place before collision between both india and the kohistan island arc with asia ca 195 ma u pb monazite ages on a deformed leucogranite dike from the upper lutkho valley indicate an early jurassic phase of crustal melting u pb monazite ages of 135 126 ma on a staurolite schist from near gharam chasma are interpreted as a minimum age for staurolite grade metamorphism within the tirich mir fault zone pegmatite dikes crosscut the staurolite schists u pb dating of uraninites from one of these pegmatite dikes reveals an age of 114 2 ma monazites from the same rock give ages of 125 121 ma possibly due to inheritance of older cores these jurassic early cretaceous constraints on metamorphism and magmatism relate to subduction and accretion processes perhaps including the suturing of the karakoram and hindu kush terranes along the tirich mir fault in general high temperature low pressure metamorphism and subduction related granitoid magmatism in the eastern hindu kush suggest a high thermal gradient in an active margin setting from early jurassic to cretaceous time

tectono metamorphic evolution of the karakorum metamorphic complex dassu askole area ne pakistan exhumation of mid crustal ht mp gneisses in a convergent context the south karakorum margin east of the himalayan syntaxis consist of an e w elongated zone of young 10 3 ma high grade metamorphic rocks m2 and related migmatitic domes this late tectono metamorphic event post dates the palaeogene 55 37 ma phase of thickening of the belt featured by nw se structures and associated m1 amphibolite facies metamorphism 0 7 gpa 700 c this m2 metamorphism is characterised by low pressure high temperature conditions coeval with migmatite formation in response to a thermal increase of c 150 c compared to m1 culminating at a temperature of c 770 c and a pressure of 0 5 0 6 gpa rapid exhumation of migmatitic domes at a rate of 5 mm yr 1 was accommodated by vertical extrusion in the core of e w crustal scale folds these crustal scale folds formed in response to n s syn collisional shortening and were enhanced by thermal weakening of the migmatised continental crust m2 metamorphism is spatially and temporarily associated with granitoids showing a mantle affinity firmly suggesting that this could be the advective heat source for the granite and syenite generation and the subsequent migmatisation of the mid crustal level such relationships between a mantle related magmatism and a high temperature metamorphism in a convergent shortening context are suggestive of the breakoff of the subducted indian slab since 20 ma

metamorphic history of the south tibetan detachment system mt everest region revealed by rscm thermometry and phase equilibria modelling this study combines microstructural observations with raman spectroscopy on carbonaceous material rscm phase equilibria modelling and u pb dating of titanite to delineate the metamorphic history of a well exposed section through the south tibetan detachment system stds in the dzakaa chu valley of southern tibet in the hanging wall of the stds undeformed tibetan sedimentary series rocks consistently record peak metamorphic temperatures of 340 c temperatures increase down section reaching 650 c at the base of the shear zone defining an apparent metamorphic field gradient of 310 c km 1 across the entire structure u th pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 ma deformation within this 1 km thick zone of distributed top down to the northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo isotherms along the upper margin of the greater himalayan series ghs we interpret the preservation of such a high metamorphic gradient to be the result of a progressive up section migration in the locus of deformation within the zone this segment of the stds provides a detailed thermal and kinematic record of the exhumation of footwall ghs rocks from beneath the southern margin of the tibetan plateau

anatomy age and evolution of a collisional mountain belt the baltoro granite batholith and karakoram metamorphic complex pakistani karakoram abstract geological mapping and u th pb geochronology from the karakoram metamorphic complex and baltoro granite batholith has resulted in more detailed timing constraints on tectonic evolution of asian crust during and following collision and accretion of the kohistan arc and the indian plate to the southern margin of asia crustal thickening along the karakoram resulted in polyphase deformation metamorphism and melting u pb zircon and monazite ages show that kyanite and sillimanite grade metamorphism in the baltoro region peaked during oligocene early miocene c 28 22 ma times at structurally deeper levels sillimanite grade migmatites of the dassu dome have pliocene c 5 4 3 5 ma ages new u pb ages from crustal melt biotite muscovite garnet leucogranites from the trango towers cathedral peak and paiyu peak across the baltoro batholith range from 19 8 0 5 ma to perhaps as young as 14 ma using data from both the hunza and baltoro regions we suggest that regional high grade metamorphism was diachronous in space and time and could have lasted continuously for at least 37 ma c 50 13 ma or as much as 50 ma c 63 13 ma along the southern margin of the asian plate in the karakoram crustal melting along the baltoro granite batholith spanned at least 13 ma 26 4 13 ma supplementary material analytical methods and description of u th pb data of samples are available at http www geolsoc org uk sup18382

pre mesozoic alpine basements their place in the european paleozoic framework prior to their alpine overprinting most of the pre mesozoic basement areas in alpine orogenic structures shared a complex evolution starting with neoproterozoic sediments that are thought to have received detrital input from both west and east gondwanan cratonic sources a subsequent neoproterozoic cambrian active margin setting at the gondwana margin was followed by a cambrian ordovician rifting period including an ordovician cordillera like active margin setting during the late ordovician and silurian periods the future alpine domains recorded crustal extension along the gondwana margin announcing the future opening of the paleotethys oceanic domain most areas then underwent variscan orogenic events including continental subduction and collisions with avalonian type basement areas along laurussia and the juxtaposition and the duplication of terrane assemblages during strike slip accompanied by contemporaneous crustal shortening and the subduction of paleotethys under laurussia thereafter the final pangea assemblage underwent triassic and jurassic extension followed by tertiary shortening and leading to the buildup of the alpine mountain chain recent plate tectonic reconstructions place the alpine domains in their supposed initial cambrian ordovician positions in the eastern part of the gondwana margin where a stronger interference with the chinese blocks is proposed at least from the ordovician onward for the visean time of the variscan continental collision the distinction of the former tectonic lower plate situation is traceable but becomes blurred through the subsequent oblique subduction of paleotethys under laurussia accompanied by large scale strike slip since the pennsylvanian this global collisional scenario has been replaced by subsequent and ongoing shortening and strike slip under rising geothermal conditions and all of this occurred before all these puzzle elements underwent the complex alpine reorganization

five generations of monazite in langtang gneisses implications for chronology of the himalayan metamorphic core monazite grains from greater himalayan sequence gneisses langtang valley nepal were chemically mapped and then dated in situ via th pb ion microprobe analysis correlation of ages and chemistry reveals at least five different generations of monazite ranging from c 9 to 300 ma petrological models of monazite chemistry provide a link between these generations and the thermal evolution of these rocks yielding an age for the melting of greater himalayan rocks within the main central thrust sheet c 16 ma and for the timing of thrust sheet emplacement that are younger than commonly viewed chemical characterization of monazite is vital prior to chronological microanalysis and many ages previously reported for monazite from the greater himalayan sequence are interpretationally ambiguous

stratigraphic evidence for an early collision between northwest india and asia the collision of india with asia1 had a profound influence on late cretaceous and cenozoic oceanography2 climate3 fauna extinctions4 and the motion of at least some of the earth s litho spheric plates5 as the collision ended a period of rapid indo asian convergence6 a precise knowledge of its timing when the crust of the neo tethys ocean was completely subducted7 at some point along the plate boundary is important for understanding its wider consequences but current estimates of the collision age range from 65 to 38 myr before present6 8 11 here we report the results of extensive biostratigraphic analyses from waziristan and kurram in northwest pakistan which show that accretionary prism and trench strata were first thrust onto the northwest indian passive margin after 66 myr but before 55 5 myr after this time volcanic arc fragments the accretionary prism trench material and imbricates of the north indian slope were raised to shallow water depths and overlapped by upper palaeocene shallow water carbonates and shales12 14 indicative of post collision thrusting in this region finally both the suture and the indian craton were overlapped by continuous unconformable upper lower eocene shallow marine strata demonstrating that suturing was complete by 49 myr

the cimmerian geopuzzle new data from south pamir based on its permian triassic stratigraphic and biotic evolution we correlate the se pamir to the karakoram terrane and we consider them equivalent along with the central pamir to the qiantang terrane of tibet all of palaeozoic gondwanan ancestry we prove the occurrence of a marked cimmerian unconformity documented by lowermost jurassic deposits suturing intensively faulted and folded permian and triassic units which suggests that the south pamir collided around the t j boundary with the central pamir along the rushan pshart suture collision of the karakoram to the south pamir happened slightly later along the wakhan tirich boundary zone progressive time shifting of deformation can be related to the complex setting of the cimmerian belt which was subdivided into minor blocks by incipient oceanic basins providing strong crustal mobility terra nova 25 352 360 2013

major intrusive stages in afghanistan typology age and geodynamic setting afghanistan straddles the gondwana eurasia boundary most of its major structural areas comprise large plutonic belts based on some 700 major element analyses the typology of the eleven main plutonic belts has been determined these belts differ from each other on the whole they display a very broad range of petrographic types from nepheline syenites and gabbros to leucocratic muscovite tourmaline granites and a wide diversity of magmatic associations from cafemic or alumino cafemic associations of various types tholeiitic calc alkaline subalkaline i e monzonitic alkaline peralkaline to various aluminous ones taking into account the typology of the belts and chronological data four major intrusive stages have been recognized and related to the geodynamic evolution of afghanistan and central asia 1 aluminous lower palaeozoic ca 500 ma stage related to linear crustal thinning at the gondwana scale 2 cafemic and alumino cafemic triassic ca 230 ma stage related to southward dipping subduction of oceanic crust beneath an island arc involving a large continental strip detached from the southern asia margin 3 cafemic and alumino cafemic cretaceous ca 100 ma stage dominantly related to subduction of oceanic crust under marginal gondwana fragments 4 eocene oligocene stage ca 35 ma induced by the india eurasia collision including two quite different groups of intrusives a a cafemic and alumino cafemic group related to extensional phenomena dominantly associated with the westward extrusion of central afghanistan along the herat fault these phenomena occurred on both sides of the gondwana palaeozoic asia boundary b an aluminous group resulting from crustal thickening in marginal gondwana fragments

cretaceous evolution of the central asian proto paratethys sea tectonic eustatic and climatic controls the timing and mechanisms of the cretaceous sea incursions into central asia are still poorly constrained we provide a new chronostratigraphic framework based on biostratigraphy and magnetostratigraphy together with detailed paleoenvironmental analyses of cretaceous records of the proto paratethys sea fluctuations in the tajik and tarim basins the early cretaceous marine incursion in the western tajik basin was followed by major marine incursions during the cenomanian ca 100 ma and santonian ca 86 ma that reached far into the eastern tajik and tarim basins these marine incursions were separated by a turonian coniacian ca 92 86 ma regression basin wide tectonic subsidence analyses imply that the early cretaceous sea incursion into the tajik basin was related to increased pamir tectonism we find that thrusting along the northern edge of the pamir at ca 130 90 ma resulted in increased subsidence in a retro arc basin setting this tectonic event and coeval eustatic highstand resulted in the maximum observed geographic extent of the sea during the cenomanian ca 100 ma the following turonian coniacian ca 92 86 ma major regression driven by eustasy coincides with a sharp slowdown in tectonic subsidence during the late orogenic unloading period with limited thrusting the santonian ca 86 ma major sea incursion was likely controlled by eustasy as evidenced by the coeval fluctuations in the west siberian basin an early maastrichtian cooling ca 71 70 ma potentially connected to global late cretaceous trends is inferred from the replacement of mollusk rich limestones by bryozoan and echinoderm rich limestones

terminal fold belt deformation relationship of mid carboniferous megakinks in the tasman fold belt to coeval thrusts in cratonic australia megakinks in the meridional tectonic grain of the tasman fold belt were formed by north south compression in the mid carboniferous and appear to be related to the same stress system that formed intracratonic thrusts in central australia during the alice springs orogeny

age of crystallization and cooling of the k2 gneiss in the baltoro karakoram the mountains of k2 8611 m and broad peak 8047 m in the baltoro karakoram northern pakistan are composed of plagioclase homblende and biotite hornblende k feldspar orthogneisses and amphibolite facies paragneisses intruded by garnet biotite muscovite tourmaline leucogranitic veins a u pb zircon age of 115 120 ma was obtained on an orthogneiss from the south face of k2 40 ar 39 ar analysis on hornblende yields a plateau age of 90 6 1 8 ma consistent with a mid cretaceous phase of magmatism concomitant with early subduction related components of the karakoram batholith muztagh tower unit hunza plutonic unit we interpret the k2 gneiss as representing a culmination of mid crustal rocks along a discontinuous but wide zone north of the karakoram batholith

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