palaeomagnetism of late cretaceous sediments from southern tibet evidence for the consistent palaeolatitudes of the southern margin of eurasia prior to the collision with india abstract twenty sites were drilled in the late cretaceous shexing formation for palaeomagnetic studies in the lhasa terrane near the locality of maxiang 29 9 n 90 7 e the stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions the tilt corrected mean direction is d i 350 8 32 1 with 95 8 1 and n 20 sites corresponding to a paleopole at 75 0 n 306 7 e with a 95 6 8 positive fold tests indicate a primary origin for the characteristic remanence based on previous cretaceous data mainly from the takena formation and paleocene data from the linzizong volcanic rocks near the city of lhasa the latitude of the southern margin of asia is located at about 15 n and yields a stable position of the lhasa terrane during cretaceous and paleocene compared with expected paleomagnetic directions from the stable india and eurasia blocks the collision palaeolatitude further implies the total latitudinal convergence was accommodated by 1700 800 km 16 2 7 6 between southern tibet and stable eurasia and 1500 830 km 14 4 7 9 between southern tibet and stable india since the collision of india and eurasia a collision age between c 54 and 47 ma was determined using the results for the southern margin of eurasia according to our new data and the extent of greater india

thermo rheological shear heating model for leucogranite generation metamorphism and deformation during the proterozoic trans hudson orogeny black hills south dakota abstract this paper evaluates thermotectonic models for metamorphism and leucogranite generation during the proterozoic trans hudson orogeny as recorded in rocks exposed in the black hills sd intrusion of the harney peak granite and associated pegmatites at 1715 ma occurred at the waning stages of regional deformation and staurolite grade regional metamorphism published consortium for continental reflection profiling cocorp results indicate that proterozoic sedimentary rocks were thrust over the archean wyoming province during the trans hudson collision isotopic compositions of the harney peak granite suggest that the exposed proterozoic and archean metasedimentary rocks in the black hills represent source rocks of the granites numerical simulations of the regional metamorphism and harney peak granite generation assuming crustal thickening by thrusting coupled with erosion show the following 1 doubling of the crust with normal distribution of radioactive elements does not yield sufficiently high temperatures to cause anatexis anywhere in the crust or growth of garnet in the now exposed part of the crust 2 a 35 km drop off length for internal heat production can yield sufficient temperature for garnet growth at the current erosion level it is however insufficient to produce staurolite and melting can occur only in the deepest parts of the crust 3 temperatures in crust with stable 70 km thickness for 40 ma and 35 km drop off length for heat production could become sufficient to produce staurolite at the current erosion level and subsequent rapid denudation of the crust could potentially trigger decompression melting of lower crustal rocks although this model could potentially explain the observed temporal relationship between regional metamorphism and leucogranite generation it is inconsistent with melting of upper crustal proterozoic source rocks that is indicated by isotopic compositions of the granites with lack of evidence for rapid denudation of the trans hudson orogen and with confinement of the leucogranites to the deformed proterozoic metapelitic rocks production of the harney peak granite and its relationship to regional metamorphism of the country rocks are best explained by shear heating at the interface between the wyoming province and overthrusted sedimentary rocks we suggest that with reasonable rheologic properties of metapelites and rates of plate convergence shear heating sufficiently perturbs locally the geotherms to cause anatexis in a deep shear zone system and growth of staurolite in the overlying crust modeling rheology of the lithologically stratified thickened crust with granitic basement and metapelitic upper plate shows that the currently exposed part of the crust and the granite source region were ductile through much of the orogeny which explains regional folding of the schists and predicts ductile shear zones in the granite source region because of the lithologic stratification the granitic basement is likely to become significantly weaker during crustal thickening than the upper crust dominated by schists a weak basement under a folded upper crust is likely to contribute to the observed relatively flat topography of high plateaus over thickened orogens

thermo tectonic history of the tethyan himalayas deduced from the palaeomagnetic record of metacarbonates from shiar khola central nepal abstract palaeomagnetic measurements were carried out on low grade metamorphic carbonates of mesozoic age from the shiar area 85 1 e 28 6 n of the tethyan himalaya th in north central nepal two characteristic remanence components carried by pyrrhotite chrm 1 and magnetite chrm 2 could be identified by their unblocking temperature spectra of 270 340 and 430 580 c respectively fold tests are not significant due to the uniform bedding of all sites however according to results from other areas of the th the pyrrhotite component has been probably acquired as a secondary p trm during exhumation and cooling thus the age of remanence acquisition can be related to the last cooling event 25 17 ma in the surrounding areas the inclination of the magnetite component matches the value expected from the indian apwp this may the primary origin of the chrm 2 pyrrhotite site mean directions show a small circle distribution with a best fit parallel to the n s direction backtilting to the expected inclination i exp by intersection of the remanence small circle with the small circle of constant i exp yields a clockwise block rotation of 30 35 with respect to the indian plate characteristics of the pyrrhotite component small circle distribution of site means secondary origin p trm with unblocking temperatures below about 300 c allow the interpretation of the chronologic order of the thermo tectonic history i an earlier main folding phase at elevated temperatures ii a later event of cooling through about 300 c coinciding with the acquisition of chrm 1 iii clockwise block rotations with respect to the indian plate and iv long wave folding as the youngest tectonic event

determination of rapid deccan eruptions across the cretaceous tertiary boundary using paleomagnetic secular variation 2 constraints from analysis of eight new sections and synthesis for a 3500 m thick composite section 1 the present paper completes a restudy of the main lava pile in the deccan flood basalt province trap of india chenet et al 2008 reported results from the upper third and this paper reports the lower two thirds of the 3500 m thick composite section the methods employed are the same i e combined use of petrology volcanology chemostratigraphy morphology k ar absolute dating study of sedimentary alteration horizons and as the main correlation tool analysis of detailed paleomagnetic remanence directions the thickness and volume of the flood basalt province studied in this way are therefore tripled a total of 169 sites from eight new sections are reported in this paper together with the results of chenet et al 2008 these data represent in total 70 of the 3500 m combined section of the main deccan traps province this lava pile was erupted in some 30 major eruptive periods or single eruptive events see each with volumes ranging from 1000 to 20 000 km3 and 41 individual lava units with a typical volume of 1300 km3 paleomagnetic analysis shows that some sees with thicknesses attaining 200 m were emplaced over distances in excess of 100 km both likely underestimates due to outcrop conditions and up to 800 km the total time of emission of all combined sees could have been much less than 10 ka with most of the time recorded in a very small number of intervening alteration levels marking periods of volcanic quiescence so called big red boles the number of boles thickness of the pulses and morphology of the traps suggest that eruptive fluxes and volumes were larger in the older formations and slowed down with more and longer quiescence periods in the end on the basis of geochronologic results published by chenet et al 2007 and paleontological results from keller et al 2008 we propose that volcanism occurred in three rather short discrete phases or megapulses an early one at 67 5 1 ma near the c30r c30n transition and the two largest around 65 1 ma one entirely within c29r just before the k t boundary the other shortly afterward spanning the c29r c29n reversal we next estimate sulfur dioxide likely a major agent of environmental stress amounts and fluxes released by sees they would have ranged from 5 to 100 gt and 0 1 to 1 gt a respectively over durations possibly as short as 100 years for each see the chemical input of the chicxulub impact would have been on the same order as that of a very large single pulse the impact therefore appears as important but incremental neither the sole nor main cause of the cretaceous tertiary mass extinctions

provenance of lower cretaceous wolong volcaniclastics in the tibetan tethyan himalaya implications for the final breakup of eastern gondwana abstract lower cretaceous volcanic lithic arenites widely distributed in the tethyan himalaya provide insights into the continental breakup of eastern gondwana in southern tibet they are represented by the wolong volcaniclastics the volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts whereas rhyolitic dacitic volcanism becomes the predominant source of the upper strata geochemical analyses of basaltic grains and of detrital cr spinels from the wolong volcaniclastics demonstrate the alkaline character of the volcanism and suggest within plate tectonic setting for the volcanism zircon u pb ages confirm that this volcanism continued from 140 ma to 119 ma hf isotope data on these early cretaceous zircons indicate that their parental magmas were mantle derived but in the later stage of magmatic activity mantle derived magmas were mixed with partial melts derived from the continental crust the lower cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of greater india the onset of volcaniclastic deposition in the himalayas appears to become progressively younger toward the west but it ended synchronously during the late albian 102 ma the low volume of volcanic rocks and their intra plate tectonic setting suggest that they are the result of decompressional melting along extensional deep seated fractures cross cutting the continental crust and reflect changes in regional intra plate tectonic stresses when greater india began to separate from the australia antarctica supercontinent

coupling of late orogenic tectonics and secondary pyrrhotite remanences towards a separation of different rotation processes and quantification of rotational underthrusting in the western himalaya northern india abstract the total amount of crustal shortening within the indian plate includes straight forward and rotational shortening the complex tectonic history of the western himalaya impedes a direct observation of rotational shortening between india and its northern margin a combination of palaeomagnetic results and tectonic structures along a 300 km nw se section from nw zanskar 33 9 n 76 5 e to malari 30 0 n 79 6 e in northern india enable a separation of rotations on different scales correction of the observed rotations and quantification of uniform rotational shortening new palaeomagnetic results from metacarbonates of the tethyan himalaya in sarchu losar and spiti and earlier studies from nw zanskar and malari reveal a characteristic remanent magnetisation carried by pyrrhotite unblocking spectrum 250 330 c negative fold tests 99 level indicated remanence acquisition after the main folding of the himalaya the formation of pyrrhotite during thermal events t 200 c suggests a remanence acquisition related to exhumation and cooling temperatures of about 200 300 c indicate a thermochemical remanence acquisition with an eohimalayan age dated in two areas by 40k 39ar techniques on illites the resulting rotation angles in respect to stable india show an anomalous counterclockwise rotation in nw zanskar and a trend of decreasing clockwise rotations to the se these variations may be attributed to meso scale 100 km tectonic features and oroclinal bending for the first time block rotations with respect to stable india are separated into local meso scale and regional significant ones after elimination of local and meso scale effects using structural analysis of late orogenic features modelled rotations due to oroclinal bending were subtracted regionally significant rotations of 0 20 clockwise are interpreted to be caused only by uniform rotational underthrusting

palaeomagnetism and 40ar 39ar geochronology of upper palaeogene volcanic rocks from central tibet implications for the central asia inclination anomaly the palaeolatitude of tibet and post 50 ma shortening within asia summary palaeomagnetic results from upper palaeogene volcanics from three localities in the qiangtang terrane of central tibet record a maximum likelihood palaeolatitude of 28 7 3 7 n these are the first volcanic based palaeomagnetic results from the plateau interior that appear to average secular variation well enough to yield a reliable palaeolatitude estimate this palaeolatitude estimate is significantly lower than the 36 3 n predicted for central tibet by late palaeogene eurasia reference poles consistent with the so called central asia inclination anomaly but it is consistent with values predicted from upper palaeogene reference poles for mongolia the mean inclination of coeval sedimentary rocks from central tibet is 15 5 shallower than our volcanic results similar to the magnitude of sedimentary inclination shallowing estimated from elongation inclination analysis of remanent directions in clastic rocks from the same region we conclude that most of the inclination anomaly observed in volcanic rocks and that which remains after removal of sedimentary flattening in clastic rocks probably is due to the combined effect of reference poles that are not representative of stable asia and by cenozoic intracontinental shortening within central asia on the order of several hundred not more than 1000 kilometers our compilation of palaeomagnetic data from the qiangtang terrane further suggest that the southern margin of the lhasa terrane the southern margin of pre india asia was located as far south as 20 n throughout eocene time these palaeolatitudes in conjunction with the upper crustal shortening record for asia indicate that only approximately one third of the post 50 ma india asia convergence is partitioned in asian lithosphere low latitudes also imply that much of the proto tibetan plateau occupied southerly tropical to subtropical latitudes which has important implications for the weathering and erosion of the tibetan plateau palaeoaltimetry proxies and early cenozoic climate dynamics

40ar 39ar dating of mineral separates and whole rocks from the western ghats lava pile further constraints on duration and age of the deccan traps abstract there has been an ongoing debate for almost two decades on the age and duration of deccan trap volcanism some of the best available ages have been determined using the 40 ar 39 ar method but differences in sample location laboratory procedure type of analyzed material and ages of monitors may have been responsible for much of the apparent spread in the data we have collected and analyzed samples from close to the bottom and top of the main lava pile in the western ghats near bombay we confirm previous findings that alteration and ar recoil strongly affect these rocks particularly measurements made on whole rocks when mineral separates mostly plagioclase are analyzed the spread is much reduced we find a mean age of 65 4 0 7 ma for five flows near the base of the section jahwar and igatpuri formations and 65 2 0 4 ma for a dyke cross cutting the poladpur formation below the topmost mahabaleshwar formation this implies that at least 1800 m out of the 2500 m composite trap section were erupted in a short interval less than 1 ma long and close to 65 5 ma i e close to the cretaceous tertiary boundary we find that older ages near 67 ma found by several authors are essentially based on whole rock samples which display disturbed spectra when this can be checked i e when the spectra are published

young eclogite from the greater himalayan sequence arun valley eastern nepal p t t path and tectonic implications abstract garnet geochronology was used to provide the first direct measurement of the timing of eclogitization in the central himalaya lu hf dates from garnet separates in one relict eclogite from the arun river valley in eastern nepal indicate an age of 20 7 0 4 ma significantly younger than ultra high pressure eclogites from the western himalaya reflecting either different origins or substantial time lags in tectonics along strike four proximal garnet amphibolites from structurally lower horizons are 14 15 ma similar to post eclogitization ages published for rocks along strike in southern tibet p t calculations indicate three metamorphic episodes for the eclogite i eclogite facies metamorphism at 670 c and 15 kbar at 23 16 ma ii a peak t granulite event at 780 c and 12 kbar and iii late stage amphibolite facies metamorphism at 675 c and 6 kbar at 14 ma the garnet amphibolites were metamorphosed at 660 c three models are considered to explain the observed p t t evolution the first assumes that the main himalayan thrust basal thrust of the himalayan thrust system cuts deeper at arun than elsewhere while conceptually the simplest this model has difficulty explaining both the granulite facies overprint and the pulse of exhumation between 25 and 14 ma a second model assumes that aborted subduction slab breakoff and ascent of india s leading edge occurred diachronously 50 ma in the western himalaya 25 ma in the central himalaya of nepal and presumably later in the eastern himalaya this model explains the p t t path particularly heating during initial exhumation but implies significant along strike diachroneity which is generally lacking in other features of the himalaya a third model assumes repeated loss of mantle lithosphere first by slab breakoff at 50 ma and again by delamination at 25 ma this model explains the p t t path but requires geographically restricted tectonic behavior at arun the p t t history of the arun eclogites may imply a change in the physical state of the himalayan metamorphic wedge at 16 25 ma ultimately giving rise to the main central thrust by 15 16 ma

tourmaline as a petrogenetic monitor of the origin and evolution of the berry havey pegmatite maine u s a the berry havey pegmatite oxford pegmatite field androscoggin county maine u s a enriched in li f b be and p is intruded in hornblende rich amphibolite with minor biotite or diopside the pegmatite has a complex internal structure with four texturally and compositionally different zones that show an increasing degree of evolution inward wall zone intermediate zone core margin and core zone the main minerals are quartz feldspars al micas tourmaline with minor fe micas garnet beryl amblygonite montebrasite fe mn phosphates and apatite tourmaline is present in all zones of the pegmatite showing different textures black anhedral crystals in the wall and intermediate zones black prisms of up to 40 cm in length in the intermediate zone black tapered prisms surrounded by a pseudo graphic intergrowth of quartz or albite with black green bluish tourmaline and constituting a continuous layer under the core zone multicolored and watermelon zoned crystals in the core zone and gemmy deep green and color zoned watermelon tourmaline prisms up to 15 cm length inside the pockets a complete chemical evolution from mg rich schorl in the wall zone to elbaite with an important deprotonation in the pockets inside the core zone is observed the most plausible exchange vectors for this chemical evolution are femg 1 y al w o y r 2 w oh 1 and al x x r 2 na 1 where r 2 fe 2 mg 2 mn 2 zn 2 for the tourmalines from the wall and intermediate zones in the core margin tourmaline composition evolves from schorl toward li rich species through the substitution y al y li y r 2 2 later during the crystallization of the core zone this exchange vector combined with the substitution x y al 0 5 x na 1 y li 0 5 finally the gemmy tourmalines from the pockets show a deprotonation related to the exchange vector y al w o 2 y li 1 w oh 2 and may be classified as darrellhenryite these substitutions may reflect an increase in oxygen fugacity and a decrease in li and f related to the crystallization of lepidolite and amblygonite montebrasite in the core zone adjacent to or within the pockets the crystallization of these minerals would reduce the availability of li and f for the very latest tourmaline crystals growing inside the pockets where the deprotonation becomes important chemical and textural variation in tourmaline is consistent with a fractional crystallization process for the internal evolution of the berry havey pegmatite crystallization of the tourmaline layer under the core zone may be related to the exsolution of the fluid phase implied in the formation of pockets

new paleomagnetic results of paleocene volcanic rocks from the lhasa block tectonic implications for the collision of india and asia abstract a combined geochronological and paleomagnetic investigation has been performed on paleocene volcanic sequences in the lhasa block near the localities of mendui 30 1 n 90 9 e a total of 15 sites have been sampled from rhyolitic tuffs stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions the tilt corrected mean direction is d i 359 0 26 1 with 95 9 2 and n 14 sites corresponding to a paleopole at 73 6 n 274 3 e with a 95 7 3 positive fold tests suggest a primary origin for the characteristic remanence in order to provide a more accurate pole we propose to combine site mean directions from this study and achache et al s 1984 the combined average palaeomagnetic direction from early paleocene volcanic rocks is d 356 6 i 25 9 21 7 95 6 8 after tilt correction n 22 sites corresponding to a pole at 73 2 n 282 4 e with a 95 5 4 the paleomagnetic results yield a paleolatitude of 13 6 5 4 n for the southern margin of eurasia at 55 ma compared with expected paleomagnetic directions from the stable india and eurasia blocks significant crustal shortening of 1400 600 km and 2000 550 km respectively may have occurred between the southern margin of eurasia and the stable india and within eurasia since the collision of india and eurasia

palaeomagnetism of cretaceous to tertiary sediments from southern tibet evidence for the extent of the northern margin of india prior to the collision with eurasia abstract palaeomagnetic studies have been carried out on mid cretaceous to latest palaeocene sediments from the tethyan himalaya on the northern margin of the indian plate near the localities of gamba 28 3 n 88 5 e and duela 28 0 n 89 2 e in southern tibet a total of 61 sites more than 700 cores have been sampled mainly from limestones zongshan formation jidula formation member ii zongpu formation sandstones jidula formation member iii and graywackes gamba group member ii with ages well defined from detailed lithological and biostratigraphic analyses carriers of characteristic remanences are magnetite in limestones and graywackes and hematite in sandstones reliable palaeodirections were obtained from 35 sites 356 specimens positive fold tests antiparallel normal and reverse polarities a northward trend of the palaeolatitudes with decreasing age and absence of any metamorphic overprint indicate a primary origin for the characteristic remanences the results have been compared with expected palaeodirections according to available data for the indian subcontinent based on its present day geometry the late maastrichtian sites fourteen sites 156 specimens and the middle late palaeocene sites fourteen sites 113 specimens yield palaeolatitudes of 5 7 s and 4 0 n for the sampling localities in southern tibet respectively these palaeolatitudes are much further north than expected and imply that crustal shortening between the tethyan himalaya and the indian shield since the time of india eurasia collision has occurred on the order of 1500 km at the longitude of gamba duela comparison of our observed palaeolatitudes with palaeolatitudes for the lhasa block during cretaceous and palaeocene times indicate that initial contact of the northern indian margin with the lhasa block was already established at 65 60 ma with suturing completed at about 55 50 ma

pb and nd isotopic composition of the jigongshan granite constraints on crustal structure of tongbaishan in the middle part of the qinling tongbai dabie orogenic belt central china abstract the qinling dabie sulu belt is the world s largest ultrahigh pressure uhp metamorphic belt the uhp metamorphism is well dated at 220 245 ma in the dabie sulu belt but at 507 ma in the qinling belt the tongbaishan is located between the qinling orogenic belt to the west and the dabie sulu uhp metamorphic belt to the east it is the key area for studying the tectonic relation between the qinling and dabie sulu belts and the diachronous uhp metamorphism the jigongshan granitic pluton t 128 ma with a total area of 1200 km 2 composed of monzogranite was mostly emplaced into the tongbai complex an exposed basement in the tongbaishan the jigongshan granites have sio 2 69 85 72 35 k 2 o na 2 o 0 87 1 13 a cnk 0 91 1 03 rb sr 0 14 0 25 and th u 3 3 12 their ree compositions show strongly fractionated patterns with la yb n 14 58 and eu eu 0 79 1 05 the granites are characterized by low radiogenic pb isotopic composition the present day whole rock pb isotopic ratios are 20 6 pb 204 pb 16 707 17 055 207 pb 204 pb 15 239 15 326 and 208 pb 204 pb 37 587 37 853 which are similar to that of the continental lower crust their e nd t values range from 16 to 20 and depleted mantle nd model ages t dm from 1 8 to 2 2 ga the above evidence indicates that the magma of the jigongshan granites was derived from the partial melting of the continental crust the pb and nd isotopic compositions of the jigongshan granites resemble those of the dabie core complex in the dabieshan but are distinct from those of the tongbai complex in the tongbaishan thus the dabie core complex would be the magma source of the jigongshan granites the result implies that the dabie core complex is extended to the west and constitutes the unexposed basement underlaying the tongbai complex in the tongbaishan

persistently low asian paleolatitudes implications for the india asia collision history 1 paleomagnetism provides independent paleolatitude constraints on the india asia convergence however implied cenozoic latitudinal convergence within asia thousands of km largely exceeds geologic estimates of tectonic shortening hundreds of km this discrepancy may result from a notoriously low bias in paleomagnetically determined cenozoic paleolatitudes in asia we provide here new paleomagnetic data from cenozoic mongolian volcanic rocks and from chinese paleogene sediments corrected from the depositional bias of inclination shallowing these results combined with similar asian data sets confirm that paleolatitudes are still 5 10 lower than predicted by the paleomagnetic apparent polar wander path apwp for asia between 50 and 20 ma inclination shallowing being excluded from the selected data sets we investigate the likeliness of other proposed mechanisms for this discrepancy 1 more southerly positions of asia than expected by the apwp due to apwp inaccuracies rather than eurasian non rigidity or 2 non dipolar geomagnetic field contributions fully explaining this discrepancy by only one of these mechanisms would imply either unrealistically large 10 apwp inaccuracies or unrealistically large octupolar field contributions up to 16 a combination of these mechanisms is found more likely to have produced the observed latitudinal discrepancy but their respective contributions cannot be quantified given the still relatively low amount and poor quality of cenozoic paleomagnetic data from stable cratons of asia india and europe by allowing for reasonable time dependant non dipolar contributions and a slight 5 apwp bias the latitudinal discrepancy can be resolved and the excessive amounts of intra asian shortening decrease to values in line with tectonic shortening from structural studies

towards absolute plate motions constrained by lower mantle slab remnants the movement of tectonic plates before the cretaceous period is poorly understood a global mantle tomography model suggests that the longitude of oceanic subduction zones up to 300 million years ago was offset by up to 18 compared with tectonic reconstructions for the same period

position of the lhasa block south tibet during the late cretaceous the latitude of the lhasa block xizang south tibet during the late cretaceous early tertiary is of prime importance for understanding the collision of india and eurasia and the mechanism of the formation of the qinghai xizang plateau during the first field expedition of the french chinese collaboration several geological formations have been sampled in southern tibet between lhasa 29 40 n 91 09 e and dingri 28 35 n 86 38 e we present here the first results obtained for middle cretaceous to palaeocene red beds from the lhasa block north of the yarlung zang bo suture zone recognized as the main suture between the indian and the eurasian plates they indicate a palaeolatitude of the lhasa block of 20 n during the late cretaceous while india was situated 30 s

chemical and isotopic pb sr zonation in a peraluminous granite pluton role of fluid fractionation the davis lake pluton dlp 800 km2 of southwestern nova scotia canada part of the large peraluminous south mountain batholith of ca 380 ma u pb zircon ar ar mica consists of granite and subordinate topaz muscovite leucogranite that hosts greisen tin base metal mineralization a new pb pb isochron age for leucogranite from the most evolved part of the dlp indicates a crystallization age of 378 3 6 ma coincident with other radiometric ages of the dlp rb sr re os pb pb the intrusion displays a compositional zonation defined by lead and strontium isotopic ratios as well as some major elements e g si f incompatible trace elements e g li rb ta u sn and elemental ratios e g k rb and nb ta the greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for pb and sr and their chemical isotopic trends are extensions of the trends displayed by the less evolved granites the covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the pb pb and rb sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma the granitoid magma underwent extensive fractional crystallization of feldspars minor biotite and accessory minerals monazite apatite and zircon in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation during which time the internally derived fluorine rich fluids modified the rb sr u pb and th pb ratios leading to distinct variations of 87sr 86sr 206pb 204pb 238u 204pb and 232th 204pb isotopic ratios these data therefore document the evolution of a granitic magma through magmatic i e crystal fractionation orthomagmatic i e crystal fluid fractionation and hydrothermal i e fluid fractionation stages that culminated in the formation of a tin base metal deposit the pb isotope data also constrain the source region for the dlp as being avalonian basement that by inference must underlie much of the meguma terrane

indian continental subduction and slab break off during tertiary collision high wavespeed seismic anomalies in the transition zone and uppermost lower mantle beneath the india asia collision zone imaged by body wave seismic tomography have been interpreted as subducted fragments of continental material in this study we focus on the prominent anomaly located beneath india between depths of about 450 and 900 km by combining the location of this anomaly with palaeogeographical positions of india we constrain the timing of the subduction event probably related to this anomaly we infer that a large portion of the north western margin of india initiated subduction at 35 5 ma along a 1500 km long wnw ese striking zone and ended with a progressive slab break off process this break off started most probably around 25 ma at the western end of the slab and propagated eastwards until complete break off around 15 ma this study helps to constrain better the amount of convergence between india and asia absorbed by continental subduction

new paleomagnetic results from the lhasa block revised estimation of latitudinal shortening across tibet and implications for dating the india asia collision knowing how and when the tibetan plateau formed is of global concern as high topography modifies climate circulation patterns and alters species migration paths previous paleomagnetic work suggested that intracontinental shortening from deformation related to the india asia collision played a major role in building the tibetan plateau however these studies were derived from red bed facies which are often afflicted by inclination shallowing leading to overestimates of continental shortening we report new paleomagnetic data from the lhasa block the southern continental margin of asia before the india asia collision a high temperature characteristic remanent magnetization chrm was isolated from 43 sites of late cretaceous red beds 32 sites of late cretaceous lava flows and nine sites of eocene tuffs all the chrm directions are significantly different from the present day geomagnetic field and the mean directions of red beds lava flows and tuffs pass the fold test the distribution patterns of chrm directions derived from volcanic rocks are consistent with a secular variation model suggesting that secular variation was likely averaged out in contrast the red bed chrm directions exhibit an east west elongated distribution pattern indicative of inclination flattening the mean chrm inclination of the red beds 23 5 degrees 2 5 degrees is consistent with previous results but significantly shallower than that of 41 9 degrees 4 4 degrees carried by the coeval lava flows an elongation inclination analysis of the red bed chrm directions yielded an unflattened mean inclination of 39 7 degrees with 95 confidence limits between 37 7 degrees and 41 9 degrees the paleolatitudes of the late cretaceous volcanic rocks and inclination corrected sediments suggest about 10 degrees less shortening occurred between lhasa and siberia than previous estimates with total shortening lying between 3 3 degrees 2 6 degrees and 7 3 degrees 5 4 degrees since the late cretaceous the mean chrm inclination derived from the eocene tuffs 51 8 degrees 5 2 degrees is indistinguishable from the expected reference direction our new results imply that india asia convergence was accommodated mainly through consumption of the india plate and yield a much younger age ca 43 ma for the initial india asia collision than previously estimated by paleomagnetic data c 2010 elsevier b v all rights reserved

paleomagnetic and plate tectonic constraints on the movement of tibet abstract the paleomagnetic results from tibet north of the yarlung zang bo suture zone show that tibet was at about 15 20 n in middle cretaceous time it then moved south down to 7 10 n in the late cretaceous paleogene the oceanic crust of the xigaze ophiolites was magnetized at 13 n but thereafter migrated further south this movement is compared with the relative movement of india and asia as deduced from magnetic anomalies and paleomagnetism experimental models on deformation help us to explain how tibet moved during the late cretaceous under the constraint of the africa arabia indenter and during the upper tertiary under the constraint of the indian indenter

supplemental material zircon and monazite geochronology in the palmer zone of transpression south central new england usa constraints on timing of deformation high grade metamorphism and lithospheric foundering during late paleozoic oblique collision in the northern appalachian orogen analytical methods for zircon u pb and monazite th pb sims geochronology zircon u pb la icp ms geochronology data tables and supporting previous work

india asia convergence driven by the subduction of the greater indian continent the most spectacular example of plate convergence on earth was the motion of the indian plate towards eurasia and the subsequent collision density estimates of the greater indian continent after its upper crust is scraped off at the himalayan front suggest that this continental plate is readily subductable potentially explaining why the convergence did not halt on collision the most spectacular example of a plate convergence event on earth is the motion of the indian plate towards eurasia at speeds in excess of 18 cm yr 1 ref 1 and the subsequent collision continental buoyancy usually stalls subduction shortly after collision as is seen in most sections of the alpine himalayan chain however in the indian section of this chain plate velocities were merely reduced by a factor of about three when the indian continental margin impinged on the eurasian trench about 50 million years ago plate convergence accompanied by eurasian indentation persisted throughout the cenozoic era1 2 3 suggesting that the driving forces of convergence did not vanish on continental collision here we estimate the density of the greater indian continent after its upper crust is scraped off at the himalayan front and find that the continental plate is readily subductable using numerical models we show that subduction of such a dense continent reduces convergence by a factor similar to that observed in addition an imbalance between ridge push and slab pull can develop and cause trench advance and indentation we conclude that the subduction of the dense indian continental slab provides a significant driving force for the current india asia convergence and explains the documented evolution of plate velocities following continental collision

the tethys himalayan passive margin from late triassic to early cretaceous south tibet the mesozoic succession of south tibet both in lithologies and stratigraphic thicknesses compares much more closely with that of central nepal than has been reported in the literature facies distribution from relatively proximal environments in the south to more distal settings in the north reflects the paleogeography of the tethys himalaya passive margin representing the southern edge of neotethys as in central nepal and nw himalaya accumulation rates increased greatly in the latest triassic when very thick shelfal siliciclastics and locally volcaniclastics tarap formation were followed by coastal sandstones zhamure formation contrary to reports in the literature this latter unit reaches well into the liassic at least in proximal southern sectors the base of the overlying kioto group yielding rich faunal associations characterized by benthic foraminifers and lithiotis similar to those of western tethys carbonate platforms is dated as middle liassic most probably late pliensbachian the monotonous growth of carbonate ramps during the middle liassic to early dogger was interrupted by two siliciclastic episodes related to major paleogeographic changes in the toarcian middle part of the kioto group and in the bajocian bathonian laptal formation an early callovian flooding event ferruginous oolite formation widespread all along the tethys himalaya was locally followed by deposition of a markedly lenticular pelagic unit previously reported only from the thakkhola graben dangar formation the overlying spiti shale is reduced to only 20 m in southern sectors where it is not younger than the oxfordian whereas in northern sectors the unit is several tens of m thick and mainly ascribed to the tithonian volcaniclastic deposition wolong formation reported herein for the first time probably began as early as the kimmeridgian early tithonian and lasted until aptian times when it was replaced by sedimentation of relatively deep water marlstones this magmatic episode related to tectonic extension associated with the detachment of india from gondwanaland took place earlier in south tibet and progressively later in central nepal and northern india as in nepal volcanic products changed with time from predominantly basaltic composition to intermediate and bimodal character the early cretaceous sedimentary evolution of south tibet matches that of offshore nw australia

the boundary between the indian and asian tectonic plates below tibet the fate of the colliding indian and asian tectonic plates below the tibetan high plateau may be visualized by in addition to seismic tomography mapping the deep seismic discontinuities like the crust mantle boundary moho the lithosphere asthenosphere boundary lab or the discontinuities at 410 and 660 km depth we herein present observations of seismic discontinuities with the p and s receiver function techniques beneath central and western tibet along two new profiles and discuss the results in connection with results from earlier profiles which did observe the lab the lab of the indian and asian plates is well imaged by several profiles and suggests a changing mode of india asia collision in the east west direction from eastern himalayan syntaxis to the western edge of the tarim basin the indian lithosphere is underthrusting tibet at an increasingly shallower angle and reaching progressively further to the north a particular lithospheric region was formed in northern and eastern tibet as a crush zone between the two colliding plates the existence of which is marked by high temperature low mantle seismic wavespeed correlating with late arriving signals from the 410 discontinuity poor sn propagation east and southeast oriented global positioning system displacements and strikingly larger seismic sks anisotropy

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

provenance ages and alteration histories of shales from the middle archean buhwa greenstone belt zimbabwe nd and pb isotopic evidence abstract shales of the ca 3 0 ga buhwa greenstone belt zimbabwe were derived from a compositionally diverse provenance whose ages determined by ion probe analyses of detrital zircons in interbedded sandstones range from 3 8 to 3 1 ga geochemical data for the shales were previously interpreted to indicate that sediments had been derived from an intensely weathered source ree concentrations in the shales were interpreted to suggest that the provenance was compositionally mixed with components of felsic tonalite and alkalic granitoid and mafic rocks sm nd and nd isotopic compositions of these rocks can be used to model initial nd isotopic ratios at the time of sedimentation endsed as well as model crustal formation ages tdm the former at the age of sedimentation range from 0 6 to 10 8 consistent with a range of provenance ages the latter range from 4 46 ga to 2 99 ga the oldest crustal formation ages up to 0 7 ga older than known detrital components are interpreted here to indicate that the sm nd system of the sediments experienced open system behavior the implied alteration would have included an increase in sm nd by about 20 25 percent probably in the form of preferential loss of nd with respect to sm the pb isotopic compositions of whole rock samples are quite radiogenic with a range of 206pb 204pb from 25 5 to 154 an array of ten samples lies scattered about a line with a 207pb 204pb 206pb 204pb slope age of about 2 73 ga five individual samples were sequentially leached to further test the timing and characteristics of this u th pb alteration event these arrays of a whole rock three leach steps and a residue also form linear pb pb arrays one is more scattered with ages ranging from 2260 360 ma to 2824 170 ma suggesting that all samples experienced a latest archean to earliest proterozoic enrichment in u pb this age range also may be the approximate age of sm nd enrichment for the shales all samples both whole rocks and leached samples lie grouped on a 208pb 204pb 206pb 204pb diagram around a line with 232th 238u 3 5 2 9 to 3 9 because of the lack of large differences in the th u of the samples through large ranges of u pb we interpret this consistency in th u to mean that the shales of the buhwa belt experienced pb loss rather than u and th gain circumstances that may be responsible for pb loss in a sedimentary basin include loss of saline fluids during basin dewatering such an event would likely have been related to folding associated with the thrusting and magmatic intrusion of the adjacent limpopo belt suggesting that uplift dewatering and geochemical and isotopic alteration can be genetically related

neotethys and the india asia collision insights from a palaeomagnetic study of the dazhuqu ophiolite southern tibet since 1999 independently derived geophysical and geological models have been published arguing for an intra oceanic subduction system along essentially the entire width of the india eurasia collision belt this idea conflicts with earlier proposals where in the eastern part of the convergence zone neotethyan mid ocean ridge generated lithosphere directly north of the indian craton was consumed beneath eurasia in tibet in an andean type plate configuration new palaeomagnetic data are reported from three barremian aptian 120 ma sequences of chert siliceous mudstones and volcaniclastic rocks these rocks form the uppermost part of the dazhuqu supra subduction zone ophiolite terrane which crop out along substantial portions of the india eurasia yarlung tsangpo suture zone in southern tibet the declination data provide little regionally useful tectonic information they are dominated by local rotations presumably related to the dazhuqu terrane s initial obduction onto the india plate in the palaeocene and subsequent movement s as india later collided and indented into eurasia the inclination data are however useful because they yield consistent sub equatorial formation sites 2 7 s 3 0 7 9 n 2 7 1 4 n 5 7 which correspond with the location of the neotethyan intra oceanic subduction system inferred from the seismic tomographic data at the time these dazhuqu terrane rocks formed the intra tethyan subduction zone would have been about 2500 km south of eurasia

reconstruction of the deformed collision zone between india and asia by backward motion of lithospheric blocks 1 on the basis of a synthesis of tectonic data available on the india asia collision we present a first attempt to reconstruct the evolution of the collision zone assuming that the deformation of the lithosphere is localized along narrow shear zones and that the interiors of mantle blocks in between remain relatively undeformed we define block contours from the fault pattern and move back the blocks along their boundary faults along convergent or extensional boundaries the crust is assumed to shorten or stretch coherently step by step we go backward in time to finally reach the collision onset for each time step we find a solution compatible with the data set available and the position of the adjacent blocks for each block the search for compatibility at the scale of the entire collision zone allows for solving the kinematics of regions with fewer data and suggests plausible scenarios for regions where data is lacking for each step we calculate large scale displacement maps and determine euler poles for each block for the most recent time step the map proposed is compared to gps motions the deformation budget implies that extrusion absorbed 30 of the convergence between india and siberia during the entire collision span but varied with time accounting for as little as 3 or as much as 60 of this convergence at different epochs

geologic correlation of the himalayan orogen and indian craton part 2 structural geology geochronology and tectonic evolution of the eastern himalaya despite being the largest active collisional orogen on earth the growth mechanism of the himalaya remains uncertain current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the himalayan and tibetan crust during cenozoic india asia collision a major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the himalayas from 91 e to 97 e which makes up about one quarter of the mountain belt to address this issue we conducted detailed field mapping u pb zircon age dating and 40ar 39ar thermochronology along two geologic traverses at longitudes of 92 e and 94 e across the eastern himalaya our dating indicates the region experienced magmatic events at 1745 1760 ma 825 878 ma 480 520 ma and 28 20 ma the first three events also occurred in the northeastern indian craton while the last is unique to the himalaya correlation of magmatic events and age equivalent lithologic units suggests that the eastern segment of the himalaya was constructed in situ by basement involved thrusting which is inconsistent with the hypothesis of high grade himalaya rocks derived from tibet via channel flow the main central thrust in the eastern himalaya forms the roof of a major thrust duplex its northern part was initiated at ca 13 ma while the southern part was initiated at ca 10 ma as indicated by 40ar 39ar thermochronometry crustal thickening of the main central thrust hanging wall was expressed by discrete ductile thrusting between 12 ma and 7 ma overlapping in time with motion on the main central thrust below restoration of two possible geologic cross sections from one of our geologic traverses where one assumes the existence of pre cenozoic deformation below the himalaya and the other assumes flat lying strata prior to the india asia collision leads to estimated shortening of 775 km 76 strain and 515 km 70 strain respectively we favor the presence of significant basement topography below the eastern himalaya based on projections of early paleozoic structures from the shillong plateau i e the central shillong thrust located 50 km south of our study area since northeastern india and possibly the eastern himalaya both experienced early paleozoic contraction the estimated shortening from this study may have resulted from a combined effect of early paleozoic and cenozoic deformation

a palaeomagnetic reconnaissance of kashmir northwestern himalaya india abstract thermal demagnetization studies of 800 palaeozoic and lower mesozoic samples mainly carbonates from 14 localities in kashmir showed four magnetic components a a recent field component of normal polarity and large intensity b a middle to late tertiary secondary component of large intensity and of reversed and normal polarity which indicates clockwise rotation of central and eastern kashmir over approximately 45 with respect to peninsular indo pakistan c primary magnetic components of triassic late permian early carboniferous siluro ordovician and middle to early ordovician age the triassic and late permian components show a 20 30 cumulative counterclockwise offset with respect to the indo pakistan apparent polar wander path apwp this indicates a post middle to late triassic counterclockwise rotational movement of kashmir over 65 75 with respect to peninsular indo pakistan d a component of low inclination and predominantly east west declination which is observed only in rocks of pre panjal traps age a secondary origin associated with extrusion of the panjal traps flows is surmised but could not be established beyond doubt alternatively this component may be related to the regional foliation the triassic and late permian primary component directions are predominantly of normal polarity this suggests a pre punjabian late permian upper age limit for the permo carboniferous kiaman reversed polarity interval which is earlier than the early to late tatarian limit observed in the u s s r

40k 40ar dating of the main deccan large igneous province further evidence of ktb age and short duration most mass extinctions coincide in time with outpourings of continental flood basalts cfb some 20 years ago it was shown courtillot v besse j vandamme d montigny r jaeger j j cappetta h 1986 deccan flood basalts at the cretaceous tertiary boundary earth planet sci lett 80 361 374 courtillot v feraud g maluski h vandamme d moreau m g besse j 1988 deccan flood basalts and the cretaceous tertiary boundary nature 333 843 846 duncan r a pyle d g 1988 rapid eruption of the deccan flood basalts at the cretaceous tertiary boundary nature 333 841 843 that the age of the deccan traps was close to the cretaceous tertiary kt boundary and its duration under 1 myr we have undertaken a new geochronological study using the unconventional 40k 40ar cassignol gillot technique which is particularly well suited to the potassium poor deccan lavas the mean of 4 determinations from the topmost ambenali and mahabaleshwar formations is 64 5 0 6 ma they straddle the c29r c29n reversal boundary for which they provide a new constraint the mean age of 3 determinations from the oldest jawhar formation is 64 8 0 6 ma the difference in age between top and bottom of a 3500 m composite section probably comprising 80 of the total deccan volume is statistically insignificant with the overall mean age being 64 7 0 6 ma n 7 our results are consistent with the most recent 40ar 39ar determinations knight k b renne p r halkett a white n 2003 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps earth planet sci lett 208 85 99 knight k b renne p r baker j waight t white n 2005 reply to 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps discussion by a k baksi earth planet sci lett 239 374 382 confirming that there should be no systematic difference between the two methods when they are used in an optimal way an earlier smaller but significant pulse of volcanism between 68 and 67 ma extending over at least 500 km in latitude in the northern part of the deccan cfb has also been identified after 2 to 3 ma of quiescence the second major phase of volcanism occurred near 65 ma expanding over most of the area covered by the first pulse and another 500 km to the south consistent with drift of india by 300 to 450 km at not vert similar 150 mm yr during the quiescence period new paleontological data from the remote rajahmundry section keller g adatte t gardin s bartolini a bajpai s humler e in prep the cretaceous tertiary boundary in deccan traps of the krishna godavari basin of southeastern india epsl to be submitted suggest that this second pulse can itself be divided into two major pulses one starting in c29r and ending at the kt boundary the second starting in the upper part of c29r and ending within c29n

reassembling the paleogene eocene north atlantic igneous province new paleomagnetic constraints from the isle of mull scotland abstract the paleomagnetic data sets from the british tertiary igneous province btip have recently been criticized as being unreliable and discordant with data from elsewhere in the north atlantic igneous province naip riisager et al earth planet sci lett 201 2002 261 276 riisager et al earth planet sci lett 214 2003 409 425 we offer new paleomagnetic data for the extensive lava flow sequence on the isle of mull scotland and can confirm the paleomagnetic pole positions emanating from important earlier studies our new north paleomagnetic pole position for eurasia at 59 0 2 ma has latitude 73 3 n longitude 166 2 e dp dm 5 2 7 0 a re evaluation and an inter comparison of the paleomagnetic database emanating from the naip were carried out to test for sub province consistency we find a general agreement between the eurasian part of naip btip and faeroes and east greenland data however a compilation of west greenland data displays a large and unexplained dispersion we speculate on if this is related to different sense of block rotation of the tertiary west greenland constituents combining all data from the naip constituents give a pole position at 75 0 n 169 9 e n 25 k 84 3 a95 3 2 in eurasian reference frame

cretaceous paleomagnetic results from western tibet and tectonic implications we present paleomagnetic results obtained from samples collected in the summer of 1989 along a 1000 km traverse between yecheng xinjiang and shiquanhe tibet in the western qinghai xizang plateau along 80 e longitude during this field trip 480 paleomagnetic cores in jurassic to tertiary limestones and sandstones were drilled at 49 sites 400 were measured isothermal remanent magnetization analysis indicates the existence of high coercivity minerals and thermal rather than alternating field demagnetization was found to be more efficient in isolating paleomagnetic directions the characteristic magnetization has rather low unblocking temperatures 250 560 c in most limestone samples and higher unblocking temperatures 400 c 680 c in the sandstone samples about half of the measured samples show the present geomagnetic field or aberrant directions the remainder display stable multicomponent magnetizations the stable characteristic magnetizations indicate 1 a probable jurassic remagnetization 2 a cretaceous 120 80 ma magnetization which passes a fold test yielding a mean pole at 66 2 n 245 0 e a95 5 1 n 14 sites and 3 a large scatter of tertiary directions which may be due to tectonic problems and or remagnetization in the sampling zone the cretaceous paleodirections from aksaichin and longmuco 11 sites on the qiangtang north tibet block are very similar to those from shiquanhe three sites on the lhasa south tibet block separated by 300 km this suggests that the north and south tibet blocks formed a single unit at least as early as the cretaceous comparing our results with previous paleomagnetic work on the tibetan plateau it is found that the paleolatitude of 10 6 5 1 obtained from this study for a reference at domar 33 75 n 80 4 e is close to that 9 6 2 8 obtained by achache et al 1984 and lin and watts 1988 from the takena formation 110 100 ma in the lhasa region 30 n 91 e in addition it appears that the southern margin of the tibetan blocks i e eurasia occupied an e w trending position at about 7 6 n latitude paleodeclination differences between the western and eastern tibetan blocks suggest internal deformations of the plateau at different scales from 1 to 1000 km according to differences in paleomagnetic declinations the arcuate shape of the blocks would have been partly acquired after the collision

geomagnetic secular variation in the cretaceous normal superchron and in the jurassic abstract it is now widely thought that geomagnetic polarity reversals occur spontaneously as a result of normal dynamo action rather than being externally triggered if this is the case then it may well be that periods of time in which the geomagnetic reversal frequency was dramatically different were characterised by different styles of secular variation two such periods were the cretaceous normal superchron cns 84 125 ma when the field was dominantly of a single polarity for 40 myr and the jurassic period 145 200 ma when reversals occurred at an average rate of as much as 4 6 myr 1 here we analyse a database of new and published palaeomagnetic directions from lavas emplaced during these periods in order to obtain first order descriptions of the palaeosecular variation psv during these times we then compare these records with one another and with that produced for the period 0 5 ma with average reversal frequency of 4 0 myr 1 our results are more equivocal than those obtained in a previous similar study mcfadden p l merrill r t mcelhinny m w lee s h 1991 reversals of the earths magnetic field and temporal variations of the dynamo families journal of geophysical research solid earth and planets 96 3923 3933 we demonstrate that this is probably a result of the previous study being affected by an artefact of their correction for within site scatter the usefulness of our jurassic record is severely limited by the restricted palaeolatitudinal span of the available data however our record for the cns is sufficient to allow us to conclude that it was likely that secular variation then was different from that in the 0 5 ma period this supports the hypothesis of a link between psv and reversal frequency and therefore endorses psv analysis as a first order tool for determining geomagnetic stability in the past

a long lived metamorphic history in the contact aureole of the mooselookmeguntic pluton revealed by in situ dating of monazite grains preserved as inclusions in staurolite porphyroblasts emplacement of the mooselookmeguntic pluton located in the western maine region of the northern appalachians was thought to have occurred towards the end of the acadian deformation at around 370 ma crystallization ages from different parts of the pluton suggest a more sequential emplacement history over a period of c 20 myr foliation inflection intersection axes fias within porphyroblasts from its aureole reveal at least five periods of garnet and staurolite growth the orientation of fias in both garnet and staurolite porphyroblasts trend successively from ese wnw nnw sse e w ene wsw to ne sw electron probe microanalysis dating of monazite grains included in staurolite porphyroblasts containing one of these five periods of fia development reveals a succession of apparent ages from 410 ma to 345 ma a similar spread of crystallization ages can be observed for plutons from maine and adjacent regions this succession indicates that deformation and metamorphism began well before and continued long after what is classically regarded as the acadian orogeny the thermal structure of the orogen progressively evolved to enable pluton emplacement and it continued to develop afterwards with magmatic fluids still forming at depth

nature of the crust in maine usa evidence from the sebago batholith neodymium and lead isotope and elemental data are presented for the sebago batholith 293 2 ma the largest exposed granite in new england the batholith is lithologically homogeneous yet internally heterogeneous with respect to rare earth elements ree and nd isotopic composition two mica granites in the southern central portion of the batholith group 1 are characterized by ree patterns with uniform shapes cen ybn chondrite normalized 9 4 19 and eu eu eu anomaly 0 27 0 42 and nd t 3 1 to 2 1 peripheral two mica granites group 2 spatially associated with stromatic and schlieric migmatites have a wider range of total ree contents and patterns with variable shapes cen ybn 6 1 67 eu eu 0 20 0 46 and nd t 5 6 to 2 8 the heterogeneous ree character of the group 2 granites records the effects of magmatic differentiation that involved monazite coarse grained leucogranites and aplites have kinked ree patterns and low total ree but have nd isotope systematics similar to group 2 granites with nd t 5 5 to 4 7 rare biotite granites have steep ree patterns cen ybn 51 61 eu eu 0 32 0 84 and nd t 4 6 to 3 8 the two mica granites have a restricted range in initial pb isotopic composition 206pb 204pb 18 41 18 75 207pb 204pb 15 60 15 68 208pb 204pb 38 21 38 55 requiring and old high u pb but not th u source component the nd isotope data are consistent with magma derivation from two sources avalon like crust nd 3 and central maine belt metasedimentary rocks nd 4 without material input from the mantle the variations in isotope systematics and ree patterns are inconsistent with models of disequilibrium melting which involved monazite

subduction history of the tethyan region derived from seismic tomography and tectonic reconstructions 1 in the mantle underneath the tethyan suture zone large volumes of positive velocity anomalies have been imaged by seismic tomography and interpreted as the present day signature of subducted tethyan lithosphere we investigate the mesozoic cenozoic subduction history of the region by integrating independent information from mantle tomography and tectonic reconstructions three different subduction scenarios for the tethyan oceanic lithosphere representative for the available tectonic reconstructions are used to predict the present thermally anomalous volumes associated with the lithospheric surface subducted since the late mesozoic next these predicted thermal volumes and their expected positions are compared to the relevant anomalous volumes derived from seismic tomographic images in this analysis we include among others the possible effects of ridge subduction and slab detachment after the cenozoic continental collisions absolute plate motion and slab thickening in the mantle our preferred subduction model comprises the opening of large back arc oceanic basins within the eurasian margin the model points to slab thickening by a factor of 3 in the mantle in which case the estimated volumes allow for active oceanic spreading 1 2 5 cm yr in the tethyan lithosphere during convergence our results further indicate the occurrence of early oligocene slab detachment underneath the northern zagros suture zone followed by both westward and eastward propagation of the slab tear and diachronous eocene to miocene slab detachment below the eastern to western himalayas free sinking rates of the detached material of 2 cm yr in the lower mantle provide the best fit between the tomographic mantle structure and our tethyan subduction model

two event lode ore deposition at butte usa 40ar 39ar and u pb documentation of ag au polymetallic lodes overprinted by younger stockwork cu mo ores and penecontemporaneous cu lodes abstract the ore genesis model for world class deposits of the butte mining district montana usa is deep pre main stage porphyry cu mo and overlying main stage ag zn cu zoned lode deposits both of which formed from hydrothermal fluids driven by minor volumes of rhyolitic magma the lode specific model is that hydrothermal processes diminished in intensity outward from district center along lode veins synchronously forming metal zones the accepted models are controverted by new geologic and multi method geochronologic studies the new data reveal the following sequence of events 1 thermal study of country rock indicates that the 76 9 ma butte granite cooled to 350 400 c by 4 m y after emplacement 2 five quartz porphyry rhyolite dikes were emplaced at 67 65 ma and another at 60 ma shrimp u pb into the cooled butte granite without resetting 40ar 39ar ages in country rock 3 fifty eight white mica and k feldspar samples from alteration envelopes adjacent to ag au polymetallic lodes in outer parts of the district zn rich lodes in intermediate parts and cu rich lodes in the district center yield 40ar 39ar ages of 73 70 ma for ag rich lodes 65 64 ma for cu rich lodes and complex age spectra of 69 65 ma for zn rich lodes the data show that ag au polymetallic lodes occupied cross district fractures by about 73 ma forming the greater butte mining district at 67 65 ma minor quartz porphyry dikes were emplaced into central and eastern parts of the rejuvenated fracture system but without evidence of related cupola or volcanic rocks or of thermal disturbance in the country rock at 64 5 ma overlapping hydrothermal cells formed two stockwork cu mo domes in deep parts of the fracture system at 65 64 ma and closely related to late stage stockwork cu mo activity a penecontemporaneous hydrothermal pulse formed a high sulfidation hydrothermal plume that 1 utilized the large re opened fractures to cannibalize and remobilize cu from autologous stockwork and older ag au polymetallic lodes 2 deposited the rich high sulfidation cu lodes and 3 mobilized metals from early ag au polymetallic veins in middle parts of the district transported the metals outward and redeposited them enriching early veins especially in the intermediate zn plus cu areas metals zones in lodes of the butte district are the result of an intensely focused cu rich hydrothermal plume that variably reworked the center of significantly larger 10 m y older ag au polymetallic lodes

palaeomagnetic results from the tibetan sedimentary series of the manang area north central nepal summary the remanent magnetization of palaeozoic and mesozoic carbonates from the tibetan sedimentary series tss of the manang area north of the annapurna range north central nepal has been investigated the tss forms part of the tectonic unit between the indian eurasian plate boundary in the north and the main central thrust mct in the south apart from a present field direction three remanence components have been separated a normal polarity post folding remanence negative fold test is carried by pyrrhotite the core corrected mean direction for five sites with 47 specimens from silurian devonian rocks is decl 312 9 incl 58 3 95 17 6 k 19 8 the origin of this component is not understood a more precisely determined reversed polarity post folding remanence 99 per cent negative fold test is also carried by pyrrhotite the core corrected mean direction for 12 sites with 229 specimens from carboniferous triassic rocks is decl 196 4 incl 65 9 95 3 2 k 183 one site of lower carinan age shows in addition to the pyrrhotite component a second stable remanence direction carried by magnetite the fold test is 99 per cent positive and there is a high probability that the remanence is of detrital origin the bedding corrected direction for one site with 40 specimens is decl 334 3 incl 54 1 95 4 2 k 30 2 palaeolatitude 34 6 s the direction of the magnetite component coincides with other triassic palaeodirection data from the literature and the inclination suggests a palaeoposition of the tss at the northern margin of india at this time the northern extent of india in the triassic cannot be reconstructed from the data because of the strong anticlockwise rotation 48 of stable india since this time the inclination of the reverse polarity pyrrhotite component is 19 steeper than expected from the maximum northern position of the area this suggests a regional northward tilt along the mct which is supported by steep inclinations found in other areas declination data for the reverse polarity pyrrhotite component and for the magnetite component deviate clockwise from expected values for the area by 23 34 calculated from the indian apwp utilizing klootwijk conaghan powell s 1985 rotational underthrusting model the minimum magnitude of continental underthrusting between the tss and stable india at the longitude of central nepal 84 e has been calculated from the declination of our reverse polarity pyrrhotite component a possible range of 520 1100 km results from consideration of different tectonic models

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

paleogeographic and tectonic evolution of southern tibet since middle cretaceous time new paleomagnetic data and synthesis during the second french chinese paleomagnetic campaign in tibet in the summer of 1981 we sampled the middle cretaceous red beds of the takena formation and the cenozoic volcanics of the lingzizong formation a detailed analysis of about 400 specimens of red beds reveals a stable component of normal magnetization n in the north of the lhasa block north of the nyainqentanglha range and two stable components in the south lt ht where the red beds are more strongly folded and are unconformably overlain by the lingzizong volcanics all three components are carried by hematite blocking temperatures range from 400 to 600 c for lt and up to the neel temperature of hematite for both n and ht lt is shown to postdate part of the folding of the takena whereas n and ht both indicate a positive fold test at the 99 probability level in addition a small less than 10 of the natural remanent magnetization antiparallel component is superimposed in most samples and may be consistent with a model of long term acquisition of a chemical remanent magnetization for the red beds analysis of the volcanics reveals a single stable component with mixed polarities carried either by hematite or magnetite the paleomagnetic directions indicate that 1 the lhasa block stood at 12 5 3 n by upper cretaceous and at 13 5 6 5 n in the paleocene eocene thus confirming the limited displacement of the margin of eurasia between albian aptian deposition of the red beds and eocene onset of the india asia collision these values imply crustal shortening by as much as 1900 850 km north of the block since the upper cretaceous and by 1400 1200 km south of the block since the onset of the collision 2 southern tibet formed a single tectonic unit by middle cretaceous time and 3 differential rotations are observed within the block and are related to the collision these rotations are discussed in the frame of a recent propagating extrusion tectonics model this study confirms the close paleogeographic relationship between indochina khorat plateau and southern tibet prior to the india asia collision

oxygen isotope zoning profiles in hydrothermally altered feldspars estimating the duration of water rock interaction the characterization of intragrain mineralogical and isotopic zoning patterns provides the basis for estimating the duration of fluid rock interaction associated with mineral replacement reactions in the rico colorado hydrothermal system oxygen isotope ratios in reaction rims on partially reacted plagioclase feldspar exhibit some of the largest gradients yet reported for individual grains to 15 o the extent of rim formation and accompanying isotopic exchange vary across the system as a function of temperature fluid isotope composition and the local fluid rock ratio distal feldspars show narrow rims with 1 8 o enrichments relative to pristine feldspars feldspars intermediate or proximal to the system s center have wide reaction rims or are completely exchanged and show 180 depletions formation times of reaction rims and associated isotopic patterns have been estimated with a coupled reaction diffusion model that suggests that hotter 250 350 c hydrothermal circulation was active for 100 300 k y in the center part of the system perhaps only while the igneous heat engine was still magma cooler 150 200 c circulation was widespread lasting for 1000 k y

hydrothermal alteration and mass exchange in the hornblende latite porphyry rico colorado the rico paleothermal anomaly southwestern colorado records the effects of a large hydrothermal system that was active at 4 ma this hydrothermal system produced the deep silver creek stockwork mo deposit which formed above the anomaly s heat source and shallower base and precious metal vein and replacement deposits a 65 ma hornblende latite porphyry is present as widespread sills throughout the area and provided a homogenous material that recorded the effects of the hydrothermal system up to 8 km from the center hydrothermal alteration in the latite can be divided into a proximal facies which consists of two assemblages quartz illite calcite and chlorite epidote and a distal facies which consists of a distinct propylitic assemblage temperatures were gradational vertically and laterally in the anomaly and decreased away from the centra heat source a convective hydrothermal plume 3 km wide and at least 2 km high was present above the stock work molybdenum deposit and consisted of upwelling high temperature fluids that produced the proximal alteration facies distal facies alteration was produced by shallower cooler fluids the most important shallow base and precious metal vein deposits in the rico district are at or close to the boundary of the thermal plume latite within the plume had a large loss of na2o large addition of cao and variable sio2 exchante distal propylitized latite samples lost small amounts of na2o and cao and exchanged minor variable amounts of sio2 the edge of the plume is marked by steep na2o exchange gradients na2o exchange throughout the paleothermal anomaly was controlled by the reaction of the albite components in primary plagioclase and alkali feldspars initial feldspar alteration in the distal facies was dominated by reaction of the plagioclase and the initial molar ratio of reactants alkali feldspar albite component to plagioclase albite component was 0 35 this ratio of the moles of plagioclase to alkali feldspar albite components that reacted evolved to 0 92 as the reaction progressed much of the alkali feldspar albite component in the proximal facies reacted while the primary plagioclase was still unreacted but the ratio for these assemblages increased to 1 51 when the plagioclase entered the reaction paragenesis plagioclase reaction during distal propylitic alteration resulted in pseudomorphic albite mixed with illite and a loss of na2o cao is lost in the distal facies as hornblende reacts to chlorite although some calcium may be fixed in calcite cao is added to the proximal facies as the quantity of chlorite replacing hornblende increases and epidote and calcite are produced

leucogranites in the black hills of south dakota the consequence of shear heating during continental collision leucogranites are common features of convergent orogens however the heat necessary for producing leucogranite magma from pelitic sources in convergent orogens is a major unresolved issue we suggest that the generation of the harney peak granite black hills south dakota during the proterozoic trans hudson orogeny is best explained by shear heating during thrusting of a proterozoic sedimentary sequence over the archean wyoming basement our thermotectonic model successfully reproduces pressure temperature time constraints on early regional metamorphism of schists and subsequent generation of the late orogenic harney peak granite the model is consistent with isotopic data for the granite which require both proterozoic and archean source rocks and its emplacement as small batches of melt the similarity of geologic relationships in the black hills to those of other collisional orogens suggests that shear heating may be a viable mechanism for leucogranite generation in thickened crusts

late oligocene early miocene initiation of shortening in the southwestern chinese tian shan implications for neogene shortening rate variations abstract the cenozoic tian shan is one of the preeminent examples of an intracontinental orogen however there remains a significant controversy over when deformation related to the india asia collision commenced and therefore how shortening within the mountains has been partitioned over time one approach has been to look at the modern shortening rate as measured by geodetic studies combined with estimates of the total shortening across the range and extrapolate backwards this approach suggests that the onset of range construction was ca 10 ma k y abdrakhmatov s a aldazhanov b h hager m w hamburger t a herring k b kalabaev k b kalabayev v i makarov p molnar s v panasyuk m t prilepin r e reilinger i s sadybakasov b j souter y a trapeznikov v y tsurkov a v zubovich relatively recent construction of the tien shan inferred from gps measurements of present day crustal deformation rates nature 384 6608 1996 450 453 an alternate method is to determine the age of the onset of exhumation using thermochronology we present 19 new apatite fission track aft results from the southwestern chinese portion of the belt this region represents the first area exhumed during the late tertiary along a transect at ca 76 e exhumation commenced at the oligocene miocene boundary 24 ma along the maidan and muziduke thrusts which bound the southern side of the kokshaal range subsequently deformation propagated ca 20 km south to the kashi basin bounding thrust kbt which was exhumed by no earlier than 18 9 3 3 ma three detrital aft samples from plio pleistocene strata deposited ca 20 km farther south contain fission track grain age peaks that young monotonically upwards from 20 9 7 0 5 3 ma to 15 9 5 4 4 0 ma with a fairly constant lag time of 16 to 18 ma these ages combined with structural data suggest that both the hanging wall and the footwall of the kbt experienced a renewed episode of exhumation during the latest cenozoic the discrepancy between the late oligocene miocene initiation of significant exhumation shown herein and the 10 ma initiation estimate from geodesy suggests that the tian shan has undergone a complex late cenozoic shortening history assuming that the present shortening rate could account for the total amount of cenozoic shortening in 10 ma and realizing that shortening initiated at least 15 myr earlier we conclude that the shortening rate must have varied over time possibly in pulsed southward migrating events and that the present rate may not reflect the average rate since initiation of range uplift

linking the late cretaceous to paleogene pacific plate and the atlantic bordering continents using plate circuits and paleomagnetic data 1 late cretaceous to paleogene paleomagnetic data from the pacific plate the emperor seamounts can be compared with data from the atlantic bordering continents through the use of plate circuit reconstructions here we summarize the uncertainties in all data sets and present formal tests we report agreement between late cretaceous pacific paleomagnetic data and predictions based on the estimates of non pacific pole positions from synthetic apparent polar wander paths this congruency points to the veracity of the plate circuits and the accuracy of the paleomagnetic estimates in contrast to the agreement seen for the late cretaceous small discrepancies are observed in the comparisons of the pacific paleogene data and predictions from synthetic apparent polar wander paths such a disparity in a younger time interval is unexpected given the agreement of the late cretaceous data the possibility that minor temporally variable nondipole field components contribute to the discrepancy cannot be completely discounted however an alternative and more straightforward explanation is suggested by further comparisons of the mean non pacific paleomagnetic data and the highest quality poles that contribute to the means in particular we note that 1 the pacific paleogene data are in full agreement with coeval poles from north america meeting strict reliability criteria and 2 the non pacific paleogene poles of synthetic apparent polar wander paths are dominated by results from the north atlantic igneous province naip but taken as a whole the naip data fail a paleomagnetic reversal test hence minor discrepancies between paleocene paleomagnetic data from the pacific and atlantic hemispheres may point to limitations of the latter which incorporate a relatively large number of older lower quality data these findings call for renewed data collections utilizing comprehensive rock magnetic and paleomagnetic demagnetization procedures to improve resolution of paleocene non pacific data the uncertainties of the pacific paleolatitude data and the non pacific reference poles are larger than the differences related to the use of the two alternative plate circuits through east to west antarctica and through australia to the lord howe rise that link the atlantic and pacific hemispheres while no preference can thus be given to either plate circuit their overall consistency with paleomagnetic data suggests that they can be used to investigate long term motion of the pacific plate since late cretaceous times

petrogenesis of the ultrapotassic lincoln syenite maine late silurian early devonian melting of a source region modified by subduction driven metasomatism the late silurian early devonian 418 1 ma lincoln syenite exposed discontinuously over a distance of 75 kilometers in south central and mid coastal maine represents one of the most unusual igneous rocks in the appalachian orogen detailed field and petrographic study reveals that most of the exposed syenites have been variably affected by post magmatic recrystallization however a small proportion of the syenites in south central maine preserve original igneous minerals and textures and allow for an unobstructed study of the igneous petrogenesis the syenites are characterized by large dark colored alkali feldspar megacrysts set in a finer grained matrix dominated by alkali feldspar clinopyroxene orthopyroxene biotite oxides the rocks are intermediate in terms of sio2 content 55 59 wt yet are ultrapotassic 6 7 wt k2o and relatively primitive in terms of their mgo mg 67 cr 400 ppm and ni 150 ppm concentrations additionally the rocks exhibit large ion lithophile element enrichment relative to the high field strength elements along with elevated concentrations of light rare earth elements 200 300 chondritic abundances the syenites show limited radiogenic isotopic variability with relatively high initial 87sr 86sr values 0 707 to 0 710 relatively low initial end values 4 8 to 6 7 and initial pb isotopic compositions characteristic of igneous rocks in central maine for example 207pb 204pb 15 577 to 15 639 the geochemical characteristics of these rocks are most consistent with a model involving the partial melting of a mantle wedge previously metasomatized by subduction derived fluids crystallization of this magma was influenced by the presence of significant amounts of water that suppressed plagioclase feldspar crystallization facilitated the crystallization of mg rich biotite and ultimately resulted in the nucleation and growth of the alkali feldspar megacrysts correlation with the geochemically similar parks pond pluton in eastern maine extends the distribution of late silurian early devonian ultrapotassic magmatism in maine to an orogen parallel distance of over 150 km comparisons with rocks of similar composition in known tectonic environments suggest the lincoln syenite and by association the parks pond pluton formed as a result of lithospheric extension superimposed on a sub continental lithospheric mantle source region previously modified by subduction driven metasomatism the timing and structural position of this ultrapotassic magmatism is consistent with a geodynamic model involving slab break off or the convective removal of lithospheric mantle delamination in association with acadian convergence both processes facilitate a brief period of crustal extension and decompression melting of source rocks previously modified by subduction related processes

regional geochemical and isotopic variations of northern new england plutons implications for magma sources and for grenville and avalon basement terrane boundaries examination of 36 devonian to pennsylvanian plutons within and bordering the central maine terrane of north central new england shows that the plutons display regional variations in mineral and bulk rock compositions and in oxygen and sulfur isotopic values these variations principally correspond to the regional structural trends of the central maine terrane plutons located on that terrane9s southeast flank adjacent to the massabesic gneiss complex on the terrane9s northwest flank adjacent to the bronson hill anticlinorium and in the northeastern part of the northeast kingdom batholith of vermont have biotites with relatively low al concentrations 3 0 cations p f u per formula unit feldspars with higher an contents up to an 55 and lower 18 o 7 9 and higher 34 s average of 0 bulk rock values compared to plutons within the central parts of the central maine terrane these characteristics are most compatible with metaigneous or metavolcaniclastic sources as shown by others the plutons of vermont and northwestern maine preserve grenvillian signatures whereas those of southeastern new hampshire specifically the granite at milford new hampshire and maine sebago phillips have avalonian signatures within the interior of the central maine terrane of new hampshire and western maine and in the connecticut valley trough of vermont are distinct regions characterized by peraluminous plutons with high 18 o values 11 low 34 s values 25 al rich biotites 3 3 cations p f u and an poor plagioclases 30 these data are most consistent with metasedimentary sources or at a minimum the plutons were heavily contaminated by central maine terrane or connecticut valley trough metasedimentary rocks these peraluminous plutons do not have the unique signatures of the underlying precambrian basements several metaluminous plutons in the central maine terrane along the new hampshire maine border separate two regions of peraluminous plutons these metaluminous plutons have biotite and isotopic compositions that are similar to those of the plutons along the flanks of the central maine terrane indicative of less paleozoic metasedimentary input compared to the peraluminous plutons in the interior of the central maine terrane the metaluminous plutons also have trace element characteristics identical to those of plutons in vermont and northwestern maine that have a known grenville basement terrane source we suggest that the metaluminous plutons also had sources in the underlying grenville basement and that the grenville avalon boundary cuts diagonally across northern new england because the plutons with grenvillian signatures are adjacent to plutons with avalonian signatures the sebago batholith and phillips pluton we suggest that there is no medial new england basement in this region of new england some peraluminous two mica plutons have trace element and stable isotopic characteristics that are compatible with partial melting of metasedimentary rocks the components of which were deposited in an anoxic basin such as the smalls falls formation of the central maine terrane but many other plutons have compositions that are inconsistent with this model we suggest that the compositions of many new hampshire plutonic suite plutons indicate greater heat input than that available from u enriched metasedimentary rocks and instead require emplacement of mafic magmas from the mantle or lower crust

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

stratigraphy and sedimentary history of the nepal tethys himalaya passive margin abstract the sedimentary history of the nepal tethys himalaya began with deposition of thick carbonates in the cambro ordovician followed by a mixed siliciclastic carbonate epicontinental succession recording two major deepening events in the early silurian and late devonian fossiliferous carbonate ramp deposits in the tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins rift stage break up in the mid early permian locally associated with effusion of tholeiitic lava flows was followed by a transgressive sandy to shaly locally coal bearing or bioclastic unit capped by condensed pelagic carbonates in the middle to late permian juvenile ocean stage subsidence of the cooling stretched crust led close to bathyal water depths in the olenekian but then slowed down in the middle triassic to increase again sharply in the late triassic owing to renewed extensional tectonic activity and sediment loading during up and out building of the indian continental terrace deposition of tropical platform carbonates finally became widespread in the middle liassic mature passive margin stage the initial fragmentation of gondwana in the middle jurassic led to rejuvenation of the indian craton and deposition of quartzo feldspathic hybrid arenites capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late bathonian middle callovian next a discontinuous pelagic grey marly limestone unit was followed by the ammonoid rich offshore spiti shale in the late jurassic the final disintegration of gondwana began in the berriasian when quartzose siliciclastics derived again from the rejuvenated indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the valanginian followed in the hauterivian to albian by more felsic differentiates such as the trachyandesites exposed in the lesser himalaya 120 km to the south a widespread drowning episode fostered by waning volcaniclastic supply during a global eustatic rise is documented by a major glauconitic horizon deposited at middle southern latitudes in the late albian overlain by scaglia like pelagic limestones in the latest albian the final part of sedimentary history during the rapid northward flight of india and its collision with eurasia is not documented anywhere in nepal due to later erosion of upper cretaceous to lower tertiary strata

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

paleomagnetic constraints on the structure of the deccan traps abstract new paleomagnetic results from two widely separated areas of the deccan traps allow one to test the simple normal reversed normal nrn magnetostratigraphic model previously proposed the new data from the mandla region to the northeast of the main outcrop of the deccan traps together with earlier apparently conflicting results are all compatible with the nrn stratigraphy provided the flows are not strictly horizontal but have a synform antiform structure with 500 m amplitude and 150 km wavelength the structure parallels the paleozoic narmada son rift system with slopes never exceeding 0 5 sampling in rajahmundry a remote southeastern outlier of the deccan with well constrained age allowed identification of the rn boundary in an area where only normal polarities had been reported it has also been possible to correlate the detailed geochemical stratigraphy in the western ghats with the nrn magnetic stratigraphy a compilation of 35 magnetic studies together with the above findings confirms the general applicability of the nrn model and reveals the large scale topography of the rn boundary which is an isochron over most of the deccan the rn isochron surface displays a boomerang shape with topography ranging from sea level to above 1600 m this observed topography is consistent with a simple model in which doming and renewed rifting and dyke intrusion resulted from impingement 70 65 ma ago of the reunion plume under the indian lithosphere the narmada son rift was rejuvenated and a continental rift system with minor extension with a triple junction in the cambay graben resulted and was subsequently covered by the deccan flows although in places tectonic faulting and volcanism continued to interfere the rajahmundry outcrops constrain the original extent and southeastward dip away from the triple junction of much of the traps a significant part of which has now been removed by erosion and rifting away of the seychelles

greater india basin hypothesis and a two stage cenozoic collision between india and asia cenozoic convergence between the indian and asian plates pro duced the archetypical continental collision zone comprising the himalaya mountain belt and the tibetan plateau how and where india asia convergence was accommodated after collision at or be fore 52 ma remains a long standing controversy since 52 ma the two plates have converged up to 3 600 35 km yet the upper crustal shortening documented from the geological record of asia and the himalaya is up to approximately 2 350 km less here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic indian lithosphere within the himalaya be tween approximately 50 and 25 ma paleomagnetic data show that this extended continental and oceanic greater india promontory resulted from 2 675 700 km of north south extension between 120 and 70 ma accommodated between the tibetan himalaya and cratonic india we suggest that the approximately 50 ma india asia collision was a collision of a tibetan himalayan microconti nent with asia followed by subduction of the largely oceanic greater india basin along a subduction zone at the location of the greater himalaya the hard india asia collision with thicker and contiguous indian continental lithosphere occurred around 25 20 ma this hard collision is coincident with far field deforma tion in central asia and rapid exhumation of greater himalaya crys talline rocks and may be linked to intensification of the asian monsoon system this two stage collision between india and asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

stable isotope evidence for magmatic fluids in the pueblo viejo epithermal acid sulfate au ag deposit dominican republic stable isotope h c o and s systematics of the pueblo viejo acid sulfate system the world s largest bulk mineable acid sulfate gold deposit were investigated to determine the importance of magmatic fluids in hydrothermal systems of this type mineralization at pueblo viejo is hosted by a maar diatreme system that cuts sedimentary rocks and spilites of the late cretaceous los ranchos formation the deposit is characterized by two stages of advanced argillic alteration stage i fluids produced deep alunite quartz and shallower kaolinitc quartz both associated with disseminated pyrite stage ii fluids overprinted stage i and produced deep pyrophyllite diaspore and an overlying silica cap an increase in temperature from stage i 250oc to stage ii 250o 300oc is indicated by mineral stabilities and sulfur isotope thermometry of coexisting sulfide sulfate pairs stage i quartz alunite kaolinite pyrite alteration and associated disseminated au ag mineralization are interpreted to

tectonic setting and regional correlation of ordovician metavolcanic rocks of the casco bay group maine evidence from trace element and isotope geochemistry ordovician metamorphic rocks of the casco bay group are exposed in an approximately 170 km long ne trending belt liberty orrington belt in southern and south central maine geochemical analysis of rocks within the spring point formation 469 3 ma of the casco bay group indicate that it is an assemblage of metamorphosed bimodal volcanic rocks the mafic rocks originally basalts have trace element and nd isotopic characteristics consistent with derivation from a mantle source enriched by a crustal and or subduction component the felsic rocks originally rhyolites and dacites were likely generated through partial melting of continental crust in response to intrusion of the mafic magma relatively low initial e nd values for both the mafic 1 3 to 0 6 and felsic 4 1 to 3 8 rocks suggest interactions with gander zone continental crust and support a correlation between the casco bay group and the bathurst supergroup in the miramichi belt of new brunswick this correlation suggests that elements of the early to middle ordovician tetagouche exploits back arc basin can be traced well into southern maine a possible tectonic model for the evolution of the casco bay group involves the initiation of arc volcanism in early ordovician time along the gander continental margin on the eastern side of the iapetus ocean basin slab rollback and trenchward migration of arc magmatism initiated crustal thinning and rifting of the volcanic arc around 470 ma and resulted in the eruption of the spring point volcanic rocks in a back arc tectonic setting

geometry and crustal shortening of the himalayan fold thrust belt eastern and central bhutan we present a new geologic map of eastern and central bhutan and four balanced cross sections through the himalayan fold thrust belt major structural features from south to north include 1 a single thrust sheet of subhimalayan rocks above the main frontal thrust 2 the upper lesser himalayan duplex system which repeats horses of the neoproterozoic cambrian baxa group below a roof thrust shumar thrust carrying the paleoproterozoic daling shumar group 3 the lower lesser himalayan duplex system which repeats horses of the daling shumar group and neoproterozoic ordovician jaishidanda formation with the main central thrust mct acting as the roof thrust 4 the structurally lower greater himalayan section above the mct with overlying tethyan himalayan rock in stratigraphic contact in central bhutan and structural contact above the south tibetan detachment in eastern bhutan and 5 the structurally higher greater himalayan section above the kakhtang thrust cross sections show 164 267 km shortening in subhimalayan and lesser himalayan rocks 97 156 km structural overlap across the mct and 31 53 km structural overlap across the kakhtang thrust indicating a total of 344 405 km of minimum crustal shortening 70 75 our data show an eastward continuation of lesser himalayan duplexing identified in northwest india nepal and sikkim which passively folded the overlying greater himalayan and tethyan himalayan sections shortening and percent shortening estimates across the orogen although minima do not show an overall eastward increase which may suggest that shortening variations are controlled more by the original width and geometry of the margin than by external parameters such as erosion and convergence rates

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

petrogenesis of migmatites in maine usa possible source of peraluminous leucogranite in plutons high t low p facies series conditions during deformation within a devonian crustal scale shear zone system defined by kilometer scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain at upper amphibolite facies grade metapelites are partially melted the onset of which is introduction

cretaceous tertiary geology of the gangdese arc in the linzhou area southern tibet abstract knowledge of the cretaceous tertiary history of upper crustal shortening and magmatism in tibet is fundamental to placing constraints on when and how the tibetan plateau formed in the lhasa terrane of southern tibet the widely exposed angular unconformity beneath uppermost cretaceous lower tertiary volcanic bearing strata of the linzizong formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs during the indo asian collision in the linzhou area located 30 km north of the city of lhasa a 3 km thick section of the linzizong formation lies unconformably on cretaceous and older rocks that were shortened by both northward and southward verging structures during the late cretaceous the linzizong formation dips northward in the footwall of a north dipping thrust system that involves triassic jurassic strata and a granite intrusion in the hanging wall u pb zircon geochronologic studies show that the linzizong formation ranges in age from 69 ma to at least 47 ma and that the hanging wall granite intrusion crystallized at 52 ma coeval with dike emplacement into footwall cretaceous strata 40ar 39ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 ma followed by an episode of accelerated cooling to upper crustal levels beginning at 42 ma the onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern tibet and is interpreted be a consequence of either 1 tertiary thrusting in this region or 2 regional rock uplift and erosion following removal of overthickened gangdese arc lower crust and upper mantle or break off of the neo tethyan oceanic slab

the takena formation of the lhasa terrane southern tibet the record of a late cretaceous retroarc foreland basin our understanding of the processes involved in the indo asian collision and the construction of the tibetan plateau are in part predicated on our understanding of the tectonic setting and crustal conditions of southern tibet in the time period immediately preceding the indo asian collision late cretaceous several hypotheses have been proposed that describe the middle to late cretaceous tectonic and paleogeographic evolution of southern tibet each with different implications for when and how the tibetan plateau was uplifted we examined the mid upper cretaceous takena formation of the lhasa terrane in southern tibet in order to reconstruct the middle to late cretaceous tectono sedimentary history of this area and test the competing tectonic hypotheses the takena formation consists of 2 km of sedimentary strata that include a lower marine limestone member penbo member and an upper member of clastic red beds lhunzhub member the aptian albian penbo member consists of 250 m of orbitolinid limestone beds that were deposited in a shallow marine seaway during a time when there was little regional subsidence the overlying lhunzhub member 1500 m consists primarily of fl strata that were deposited during a period of increased subsidence overall the lhunzhub member coarsens upward from meandering and anastomosing stream deposits interbedded with numerous paleosols near the base to multistory multilateral braided stream deposits near the top the fl uvial sandstone units are lithic rich and contain abundant plagioclase and volcanic grains and paleocurrent data record northwest directed transport these data indicate that the sediment was derived from the gangdese volcanic arc that developed along the southern margin of the lhasa terrane following deposition but prior to ca 70 ma the beds of the takena formation were folded and partially eroded the sedimentary and stratigraphic characteristics of the takena formation are most consistent with deposition in a retroarc foreland basin setting the limestone beds of the penbo member were deposited in a shallowmarine seaway that was eventually infi lled by clastic sediment derived principally from the gangdese volcanic arc the low subsidence rate recorded in the lowermost strata of the takena formation including the penbo member and the paleosol rich interval of the lhunzhub member is interpreted to be associated with the passage of a fl exural forebulge the overlying upward coarsening fl uvial strata were deposited in progressively more proximal locations within the foredeep of a foreland basin the late cretaceous folding of the takena formation indicates that the foreland basin strata were eventually incorporated into a fold and thrust belt the upper cretaceous sedimentary strata of the lhasa terrane of southern tibet predict that a north verging fold and thrust belt existed along the southern margin of the lhasa terrane during late cretaceous time prior to the indo asian collision collectively the evidence indicates that the southern margin of the lhasa terrane had thickened crust and was likely to have been at high elevations immediately before the indo asian collision

copper isotope ratios in magmatic and hydrothermal ore forming environments multi collector inductively coupled plasma mass spectrometry now provides for precise and accurate measurements of cu isotope ratios copper minerals prepared by direct dissolution with and without chromatographic purification yield identical cu isotope ratios within analytic precision of about 0 04x 1r cu isotope ratios have been measured for copper minerals from worldwide magmatic and hydrothermal copper deposits and for several weathered deposits natural variations in d65cu values relative to nbs976 range over 9x chalcopyrite samples from mafic intrusions lie within a narrow range of about 1 5x and most cluster tightly between 0 10xand 0 20x this range lies within the broader black smoker chalcopyrite and iron meteorite ranges and possibly represents a bulk mantle cu isotope ratio most values for hydrothermal native copper from the michigan native copper district also show a narrow range just larger than 0 1xand suggest a common homogeneous source for cu in this large hydrothermal system later copper sulfide and arsenide minerals from this district range to values more than 2x lower than native copper ratios for chalcopyrite and bornite from moderate to high temperature porphyry skarn and replacement deposits as a group and within individual deposits exhibit a broad range of values variations of nearly 1xare observed over distances on the order of 1 m in some cases these variations may result from multiple mineralization events or copper remobilization during retrograde or later hydrothermal activity fractionations between chalcopyrite and bornite where they occur in the same sample or in related samples cluster near 0 4x suggesting equilibrium cu isotope fractionation between them at moderate temperatures in addition weathering of hydrothermal copper minerals produces a wide range of values in secondary copper phases in the supergene environment cuprite typically has higher values than associated native copper therefore redox states appear to exert a significant control over fractionation at low temperatures significant questions remain to be answered before the source of copper in hydrothermal environments can be fully addressed source reservoir cu ratios need to be determined and chemical and physical factors that control cu isotope fractionation must be quantitatively defined d 2003 elsevier b v all rights reserved

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

the phillips pluton maine usa evidence of heterogeneous crustal sources and implications for granite ascent and emplacement mechanisms in convergent orogens the phillips pluton age of 403 8 1 3 ma was assembled at a crustal level below the contemporary brittle plastic transition during regional dextral reverse transpressive deformation the pluton is composed dominantly of medium to coarse grained leucogranite sensu lato s l but within its bounds includes decametric massive outcrop of fine to medium grained granodiorite s l in places the leucogranite contains centimetric enclaves apparently of the granodiorite granodiorite is host to more biotite than muscovite and more calcic oscillatory zoned plagioclase compared to the leucogranite pegmatitic granite and composite pegmatite aplite occur as metric sheets within the pluton and as larger bodies outside the pluton to the sw magmatic fabrics defined by biotite schlieren occur locally in the leucogranite the attitude of these fabrics and layering within the leucogranite are concordant with the ne striking steeply dipping country rock foliation k2o contents rb sr ratios rb sr and ba covariations and chondrite normalized rare earth element ree patterns of leucogranite are consistent with high to moderate a h2o muscovite dehydration equilibrium eutectic melting of a predominantly pelite source similar to metasedimentary rocks of the surrounding central maine belt cmb the ree patterns and rb sr ratios of granodiorite also suggest derivation from a metasedimentary source but more likely by moderate to low a h2o muscovite biotite dehydration equilibrium eutectic to non eutectic minimum melting of a protolith dominated by greywacke in which garnet and plagioclase were residual phases both granite s l types have heterogeneous initial nd isotope compositions samples of granodiorite define a range in end 404 ma of 1 8 to 0 1 0 3 2 uncertainty and samples of leucogranite define a range in end 404 ma of 8 0 to 5 3 0 3 2 uncertainty this bimodal distribution suggests that melts were derived from a minimum of two sources the data are consistent with these sources being cmb metasedimentary rocks end 404 ma 4 for the granodiorite the range of nd isotope compositions within each granite type most likely reflects isotopic heterogeneity inherited from the source these data imply that the integrity of individual melt batches was maintained during ascent and that extensive mixing of melt batches during emplacement at this level in the pluton did not occur although centimetric enclaves have intermediate nd isotope compositions consistent with small scale interactions between magmas we infer that the phillips pluton represents the root of a larger pluton and that what remains of this larger pluton is the feeder constructed from multiple melt batches arrested during waning flow of granite magma through a crustal scale shear zone system

isotopic evidence for multiple contributions to felsic magma chambers gouldsboro granite coastal maine abstract the gouldsboro granite forms part of the coastal maine magmatic province a region characterized by granitic plutons that are intimately linked temporally and petrogenetically with abundant co existing mafic magmas the pluton is complex and preserves a felsic magma chamber underlain by contemporaneous mafic magmas the transition between the two now preserved as a zone of chilled mafic sheets and pillows in granite mafic components have highly variably isotopic compositions as a result of contamination either at depth or following injection into the magma chamber intermediate dikes with identical isotopic compositions to more mafic dikes suggest that closed system fractionation may be occurring in deeper level chambers prior to injection to shallower levels the granitic portion of the pluton has the highest nd isotopic composition e nd 3 0 of plutons in the region whereas the mafic lithologies have nd isotopic compositions e nd 3 5 that are the lowest in the region and similar to the granite and suggestive of prolonged interactions and homogenization of the two components sr and nd isotopic data for felsic enclaves are inconsistent with previously suggested models of diffusional exchange between the contemporaneous mafic magmas and the host granite to explain highly variable alkali contents the felsic enclaves have relatively low nd isotopic compositions e nd 2 1 indicative of the involvement of a third lower e nd melt during granite petrogenesis perhaps represented by pristine granitic dikes contemporaneous with the nearby pleasant bay layered intrusion the dikes at pleasant bay and the felsic enclaves at gouldsboro likely represent remnants of the silicic magmas that originally fed and replenished the overlying granitic magma chambers the large isotopic and chemical contrasts between the enclaves and granitic dikes and granitic magmas may be in part a consequence of extended interactions between the granitic magmas and co existing mafic magmas by mixing mingling and diffusion alternatively the granitic magmas may represent an additional crustal source using granitic rocks such as these with abundant evidence for interactions with mafic magmas complicate their use in constraining crustal sources and tectonic settings fine grained dike rocks may provide more meaningful information but must be used with caution as these may also have experienced compositional changes during mafic felsic interactions

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

new constraints to the onset of the india asia collision paleomagnetic reconnaissance on the linzizong group in the lhasa block china abstract to better understand the onset of the india asia collision a paleomagnetic study has been conducted on paleogene volcanic rocks and sediments of the linzizong group 64 44 ma from the linzhou and namling basins of the lhasa block at the southern margin of the asian continent following detailed rock magnetic investigations and progressive thermal or hybrid thermal and alternating field demagnetization stable characteristic remanent magnetizations chrms were successfully defined from the dianzhong 64 60 ma nianbo and pana 60 44 ma formations these chrms are of dual polarity and pass a fold test with some of them in volcanic rocks carried by oxidation induced hematite we thus interpret them as primary remanences paleomagnetic results from the bulk of the collection imply that the southern leading edge of asia i e the lhasa block remained essentially stationary in low latitudes at 10 n without paleomagnetically detectable tectonic rotation and latitudinal variation during the formation of the linzizong group consequently a large scale post collisional northward convergence in the tibetan plateau and central asia is very likely to have occurred after formation of the linzizong group and the definitive paleoposition of the lhasa block during the period embracing the interval between 64 and 44 ma provides further constraints on the latitudinal position of the initial contact between india and asia this evidence rules out the model for a younger 35 ma india asia collision and implies that the initial india asia collision occurred between 65 and 50 ma given that greater india comprised the present indian continent together with a 950 km extension beyond its northern margin prior to the rifting of gondwana in the middle jurassic we conclude that the initial india asia collision most probably occurred between 55 and 60 ma

the german bank pluton offshore sw nova scotia age petrology and regional significance for alleghanian plutonism permian plutonism is widespread in the u s appalachians but not in the canadian sector of the appalachians the german bank pluton sampled in two boreholes offshore southwestern nova scotia gave a u pb age of 300 1 ma determined by isotope dilution thermal ionization mass spectrometry id tims on thermally annealed and chemically etched zircon grains electron microprobe dating of monazite yielded an age of 308 13 ma late paleozoic plutonism in the canadian appalachians was dominated by northwestward subduction of the rheic ocean whereas the rheic ocean in the u s appalachians closed by southeasterly subduction a slab tear therefore developed at the southwestern margin of the rheic ocean slab beneath the canadian appalachians the high sr and ba metaluminous magnetite granite of the german bank pluton resulted from melting of underplated lower crust and or lithospheric mantle by a rising asthenospheric diapir related to this slab tear geochemically the german bank pluton is similar to most 300 265 ma plutons of the southern appalachians except in its strongly negative nd isotopic composition that results from the underthrusting of a ca 2 ga lower crustal block of saharan craton

the massabesic gneiss complex new hampshire a study of a portion of the avalon terrane geochemical data from the 625 ma massabesic gneiss complex of southern new hampshire show strong affinities for other avalonian rocks of southern new england and suggest continental rifting in the late proterozoic migmatized paragneiss the dominant rock type in the complex has major and trace element compositions that are compatible with graywackes from continental arcs the para gneiss also has strong lithologic metamorphic and isotopic similarities to the rocks of the hope valley zone of connecticut avalon suggesting a possible hope valley massabesic correlation at 625 ma the paragneiss end values are similar to avalonian crust in other locations of the orogen two types of amphibolite are present in minor amounts in the paragneiss of the massabesic gneiss complex the first type is a paramphibolite and consists of calc silicate layers in the massabesic paragneiss the second type is metaigneous major and trace element abundances reveal that the protoliths of the orthoamphibolites range from continental rift alkaline basalts and tholeiites to n type morbs orthoam phibolite end 625 ma values range from 2 4 to 4 as expected of rift related magmas derived from partial melting of a depleted mantle source and have the same values as iapetus ocean floor rocks of similar age orthoamphibolite major and trace element geochemical characteristics overlap those of the middlesex fells amphibolites of the esmond dedham zone of eastern massachusetts avalon which range from alkaline to transitional basalts erupted in a continental rift setting the compositions of orthoam phibolites define a potential magmatic continuum produced by batch partial melting of the mantle initiated during continental rifting and proceeded to ocean basin forma tion the inferred continuity of mafic magmatism from the esmond dedham mid dlesex fells formation to the massabesic gneiss complex and hope valley zone suggests that these zones are not distinct lithotectonic zones but are parts of a single landmass massachusetts avalon esmond dedham represents the continental section of avalon where the alkaline to transitional magmas of the early rifting stages are preserved according to our tectonic reconstruction the massabesic gneiss complex is the oceanward continental margin represented by volcanoclastic sediments with the morbs representing the initiation of ocean basin development the leading edge of this landmass of which the massabesic gneiss complex is the only observable remnant collided with laurentia during the acadian orogeny the inboard thicker more continental trailing edge that is platform avalon esmond dedham collided later during the alleghanian orogeny

constraints on the early uplift history of the tibetan plateau the surface uplift history of the tibetan plateau and himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during cenozoic time its influence on monsoon intensity and its reflection of the dynamics of continental plateaus models of plateau growth vary in time from pre india asia collision e g 100 ma ago to gradual uplift after the india asia collision e g 55 ma ago and to more recent abrupt uplift 7 ma ago and vary in space from northward stepwise growth of topography to simultaneous surface uplift across the plateau here we improve that understanding by presenting geologic and geophysical data from north central tibet including magnetostratigraphy sedimentology paleocurrent measurements and 40ar 39ar and fission track studies to show that the central plateau was elevated by 40 ma ago regions south and north of the central plateau gained elevation significantly later during eocene time the northern boundary of the protoplateau was in the region of the tanggula shan elevation gain started in pre eocene time in the lhasa and qiangtang terranes and expanded throughout the neogene toward its present southern and northern margins in the himalaya and qilian shan

cretaceous tertiary shortening basin development and volcanism in central tibet the geologic map pattern of the qiangtang terrane in central tibet defines a 600 km long and up to 270 km wide east plunging structural culmination it is characterized by early mesozoic blueschist bearing melange and upper paleozoic strata in the core and mainly triassic jurassic strata along the limbs in the western qiangtang terrane 84 e the culmination is unconformably overlain by weakly deformed mid cretaceous volcanic flows and tuffs along the bangong suture to the south 32 n 84 e mid cretaceous nonmarine red beds and volcanic rocks lie unconformably on jurassic suture zone rocks these relationships demonstrate that west central tibet was above sea level during the mid cretaceous and experienced significant denudation prior to mid cretaceous time growth of the qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early mesozoic melange and upper paleozoic strata during the early cretaceous lhasa qiangtang collision the north south width of the inferred thrust sheet provides a minimum slip estimate of 150 km at 84 e decreasing eastward to 70 km at 87 e paleogene deformation in the qiangtang terrane is characterized by widely distributed mainly north dipping thrust faults that cut eocene oligocene red beds and volcanic rocks in their footwall along the bangong suture the north dipping shiquanhe gaize amdo thrust system cuts 64 and 43 m y old volcanic tuffs in its footwall and accommodated 40 km of post mid cretaceous shortening the tertiary south dipping gaize siling co backthrust bounds the southern margin of the bangong suture and marks the northernmost limit of mid cretaceous marine strata in central tibet cretaceous deformation and denudation in central tibet is attributed to northward underthrusting of the lhasa terrane beneath the qiangtang terrane along the bangong suture this model implies that 1 cretaceous strata along the bangong suture and in the northern lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the qiangtang terrane and 2 the central tibetan crust was thickened substantially prior to the indo asian collision although its magnitude is poorly known tertiary shortening in the qiangtang terrane is more prevalent than in the lhasa terrane this difference may be attributed to the presence of underthrust melange in the deeper central tibetan crust which would have made it weaker than the lhasa terrane during the indo asian collision

stable isotope and fluid inclusion investigations of epithermal vein and porphyry molybdenum mineralization in the rico mining district colorado the rico mining district western san juan mountains colorado contains epithermal vein deposits carbonate replacement deposits and a large zone of porphyry style molybdenum mineralization historically the vein and replacement deposits have produced significant amounts of silver lead and zinc with minor gold and copper all the mineralization formed nearly contemporaneously about 5 m y ago the veins and replacement deposits occur in paleozoic and mesozoic sedimentary rocks that have been uplifted into the rico dome which is cored by a horst of precambrian greenstone and quartzite the porphyry molybdenum mineralization 40 million tons of 0 31 mo is 1 500 m beneath the surface in the east end of the district and consists of stockwork veining in precambrian quartzite and greenstone and in pennsylvanian sedimentary rocks the epithermal veins occur above and peripheral to the porphyry mineralization widespread high silica alaskite porphyry dikes were also emplaced at the same time as the porphyry mineralization and are probably related to the source intrusion for the molybdenum mineralization and the heat source for the hydrothermal system that produced the epithermal and replacement deposits fluid inclusions in quartz from the veins homogenize at temperatures in the range 200 degrees to 300 degrees c and yield salinities less than 5 equiv wt percent nacl the delta d values of quartz inclusion fluids 112 to 121ppm and calculated delta 18 o values of fluids in equilibrium with the quartz 3 to 17ppm show that the epithermal vein fluids were 18 o shifted meteoric waters vein samples with large 18 o shifts also exhibit very saline fluid inclusions indicating that the fluids which experienced the greatest degree of water rock interaction contain the highest concentration of dissolved components the porphyry fluid delta d 90 to 104ppm delta 18 o 2ppm was derived from mixed magmatic and meteoric sources primary inclusions in the porphyry veinlets homogenize to both liquid and vapor in the temperature range 350 degrees to 420 degrees c suggesting that boiling occurred during their formation trains of secondary inclusions are abundant in the veinlets and show that a later lower temperature fluid encroached upon the porphyry system no evidence for mixing of magmatic and meteoric fluids was found in any of the epithermal veins the porphyry mineralization formed early in the history of the rico hydrothermal system epithermal vein formation occurred later than the porphyry mineralization and the meteoric hydrothermal fluid collapsed into the porphyry core of the system during the retrograde stages of the hydrothermal system

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors

mesoproterozoic lamprophyres in the labrieville massif quebec clues to the origin of alkalic anorthosites mesoproterozoic lamprophyre dikes occur at four localities within the 1010 ma labrieville anorthosite in the grenville province of southern quebec all dikes are thin 20 cm wide oriented n57 w and steeply dipping three are biotite lamprophyres but the fourth is dominated by amphibole laser total fusion 40ar 39ar analyses on amphibole or biotite yield minimum ages for the dikes of 945 5 8 ma 907 0 5 9 ma 892 0 6 2 ma and 889 2 9 8 ma a crystallization age of 1 ga for all dikes is consistent with these data and rb sr isotopic systematics all lamprophyres contain high levels of cr 284 393 ppm ni 101 180 ppm ba 1380 5580 ppm sr 2359 3109 ppm and rare earth elements ree lan 191 288 with average lan lun 26 and no eu anomalies at 1 ga all lamprophyres have a similar initial sr isotopic ratio of 0 7034 similar to bulk earth in contrast e nd values vary considerably from 4 to 10 2 indicating sources with variable long term light ree enrichment negative correlations between 143nd 144nd and la nb and k suggest involvement of at least two source components probably asthenosphere and metasomatized subcontinental lithosphere the lamprophyres are only slightly younger than 1010 ma labrieville massif which is also alkalic and enriched in sr and ba relative to most other anorthosites the lamprophyres provide unequivocal evidence for a sr and ba rich component in the lithospheric mantle that may have contributed to the parental magmas of labrieville and other andesine anorthosites in this part of quebec