precambrian metamorphic conditions and crustal evolution northeastern alberta canada abstract a complex of precambrian polymetamorphic gneisses and granitoids of the churchill structural province northeastern alberta canada has been examined structurally petrographically chemically and geochronologically an archean basement gneiss complex is indicated by rb sr dating of pegmatites which cut both gneisses and granitoids 2470 26 ma with an initial 87sr 86sr ratio of 0 7030 0 0008 a high pressure granulite facies m1 mineral assemblage and older structures d1 are assigned to the archean a moderate pressure granulite facies m2 1 a low pressure amphibolite facies m2 2 a greenschist facies m2 3 and younger structures d2 are of aphebian age formation of granitoids by anatexis of the pre existing archean basement complex during m2 1 is indicated by their aphebian ages ca 1900 ma and high initial 87sr 86sr ratios 0 7100 0 0018 the path of retrograde metamorphism is linked with relatively slow rates of uplift and cooling late aphebian sediments attained low grade greenschist facies metamorphism only and are younger than the other metamorphic rocks the tectonic evolution of this precambrian mobile belt during the aphebian contrasts with the stable archean cratonic block in the slave province to the north

continental transform tectonics great slave lake shear zone ca 1 9 ga northwest canada great slave lake shear zone gslsz is a northeast trending dextral mylonite zone across which the coeval north trending thelon and taltson magmatic zones on the west margin of the churchill province are offset 300 700 km it is proposed that the gslsz is a continental transform structure related to oblique collisional indentation of the active margin of the churchill province by the microcontinental slave province this accounts for geologic and geophysical differences between the thelon and taltson zones and explains the localization of a tibetan type queen maud uplift north of the gslsz and the abrupt loss of slip on the gslsz east of the uplift it also provides a new model for the athapuscow aulacogen analogous to the modern bay of bengal indo burman ranges sagaing transform andaman sea situation

geodynamics of the tarim basin and the tian shan in central asia the india asia collision has caused crustal thickening in tibet by at least a factor of 2 in the last 20 30 myr of this collision the tian shan mountain range has also been reactivated the tarim basin however shows little internal deformation we describe a series of numerical experiments which constrain the effective lithospheric strength parameters of the tarim basin and the tian shan in the context of a thin viscous sheet model we use a finite element representation of the thin viscous sheet model to approximate the deformation field in the india asia collision and compare model crustal thickness distributions with that inferred from a local isostatic model of topography the experiments show that a strong tarim basin while undergoing little internal deformation transfers strain to the tian shan producing significant crustal thickening in the tian shan region based only on diagnostic parameters such as the maximum thickening in the tian shan and the minimum in the tarim basin the principal features of the topography can be approximately reproduced using models in which either the tarim basin is strong or the tian shan is weak or both for a rheological model in which the stress versus strain rate exponent is n 3 the strength coefficient for the strong tarim basin model vtarim is between about 1 7 and 2 for the weak tian shan model the relative strength coefficient vtian is between about 0 65 and 0 75 if both strong tarim and weak tian shan are included there is a trade off between the required values of vtarim and vtian and the shape of the predicted crustal thickness profiles better matches the observed profiles the steep topographic slope on the southern margin of the tarim basin requires that it is anomalously strong while the rapid decrease of topographic height to the north of the tian shan requires that it is anomalously weak similar conclusions are obtained with a rheological model based on n 10 simplified rheological models of the lithosphere show that the variations in lithospheric strength may be explained by changes to the moho temperature of the order of 10 to 30 c

late cenozoic tectonic evolution of the southern chinese tian shan structural sedimentological magnetostratigraphic and 40ar 39ar thermochronological investigations were conducted in the southern chinese tian shan on the basis of our own mapping and earlier investigations in the area the late cenozoic southern tian shan thrust belt may be divided into four segments based on their style of deformation from west to east they are 1 kashi aksu imbricate thrust system 2 the baicheng kuche fold and thrust system 3 the korla right slip transfer system and 4 the lop nor thrust system the westernmost kashi aksu system is characterized by the occurrence of evenly spaced 12 15 km imbricate thrusts the baicheng kuche and korla systems are expressed by a major north dipping thrust the kuche thrust that changes its strike eastward to become a nw striking oblique thrust ramp the korla transfer zone the lop nor system in the eastern most part of the southern chinese tian shan consists of widely spaced thrusts all involved with basement rocks geologic mapping and cross section construction suggest that at least 20 40 km of crustal shortening with a horizontal shortening strain of 20 30 has occurred in the southern chinese tian shan during the late cenozoic these estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections the timing of initial thrusting is best constrained in the kuche basin where crustal shortening may have occurred at 21 24 ma the time of a major facies transition between lacustrine and braided fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy this estimate represents only a minimum age as development of thrusts in the southern chinese tian shan may have propagated southward toward the foreland thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting if our estimate of timing for the thrust initiation 21 24 ma is correct using the estimated magnitude of shortening 20 40 km and shortening strain 20 30 the averaged rates of late cenozoic horizontal slip and shortening strain are 1 1 9 mm yr 1 and 2 9 4 5 10 16 s 1 respectively our reconnaissance 40ar 39ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the chinese tian shan suggests that the magnitude of cenozoic denudation is no more than 10 km most likely less than 5 km we demonstrate via a simple airy isostasy model that when the thermal effect on changes in surface elevation is negligible determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the tian shan using this simple model the loosely constrained magnitude of crustal shortening strain and denudation in the southern chinese tian shan implies that it may have been elevated 1 0 2 0 km since the onset of cenozoic thrusting

deep electrical structure of northern alberta canada implications for diamond exploration geophysical studies of upper mantle structure can provide constraints on diamond formation teleseismic and magnetotelluric data can be used in diamond exploration by mapping the depth of the lithosphere asthenosphere boundary studies in the central slave craton and at fort a la corne have detected conductors in the lithospheric mantle close to or beneath diamondiferous kimberlites graphite can potentially explain the enhanced conductivity and may imply the presence of diamonds at greater depth petrologic arguments suggest that the shallow lithospheric mantle may be too oxidized to contain graphite other diamond bearing regions show no upper mantle conductor suggesting that the correlation with diamondiferous kimberlites is not universal the buffalo head hills in alberta host diamondiferous kimberlites in a proterozoic terrane and may have formed in a subduction zone setting long period magnetotelluric data were used to investigate the upper mantle resistivity structure of this region magnetotell

regional o sr and pb isotope relationships in late cenozoic calc alkaline lavas of the andean cordillera quaternary recent volcanism in the andes has occurred in three regions 45 33 s 28 16 s and 2 s 5 n each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics isotope ratios of o and sr are lowest and those of pb least variable in the southern volcanic zone svz where medium k lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent the svz lavas were probably derived from an asthenospheric mantle source above a shallow benioff zone parental basaltic magmas rose largely unmodified through thin continental crust where differentiation occurred by low pressure crystal fractionation without concurrent modification of isotopic composition the slight enrichment of 206 pb in the northern volcanic zone nvz where the crust is thin and the benioff zone deep suggests a greater subduction zone component for parental medium k magmas the slight 18 o enrichment suggests a small amount of lower crustal interaction isotope ratios of o and sr are highest and those of pb most variable in the central volcanic zone cvz parental magmas in the cvz were probably generated within the same mantle source region as those in the svz and nvz subsequently during transit through the exceptionally thick continental crust of the central andes high k magmas were produced by a combination of bulk contamination in the lower crust and later low pressure fractional crystallization assimilation processes in the upper crust which altered both chemical and isotopic compositions

crustal and upper mantle structure under the tien shan from surface wave dispersion long period seismograms of nuclear explosions and natural earthquakes in central asia recorded at three digital stations of the laboratoire de detection geophysique ldg network in france attest to a significant propagation effect in the tien shan area this allows for an investigation of the deep structures under the range which has been the site of particularly intense intracontinental deformation since about the middle miocene group velocity curves have been obtained by regionalization for pure paths across either the central or eastern tien shan they indicate a crust of approximately 50 55 km in thickness and particularly low shear wave velocities under the central tien shan our study suggests lateral variation of the deep structures under the range which could be related to a westward increase along the range of cumulative amount and rate of shortening

nd isotopic evidence for crustal recycling in the ca 2 0 ga subsurface of western canada sm nd isotopic data are presented for 23 drill core samples from five aeromagnetically and geochronologically u pb zircon distinct domains of the precambrian basement of northern alberta the domains in question are the taltson 1 96 1 94 ga buffalo head 2 32 1 99 ga chinchaga 2 19 2 09 ga ksituan 1 99 1 90 ga and nova 2 81 ga these domains are truncated to the north and south by the great slave lake shear zone and the snowbird tectonic zone respectively initial end values are 5 0 to 9 7 for the taltson 0 2 to 6 3 for the buffalo head 0 6 to 1 8 for the chinchaga 1 8 to 2 1 for the ksituan and 5 6 for the nova crustal residence model ages fall in the 2 5 2 8 ga range the nd isotopic signatures may be viewed in terms of mixing a minimum of 10 archean continental crust with a depleted mantle component speculations on the tectonic history of the basement domains in question involve the assembly of archean crustal nuclei to form the buffalo head chinchaga composite domain a

a combined o sr nd and pb isotopic and trace element study of crustal contamination in central andean lavas i local geochemical variations systematic variations in o sr nd and pb isotopic ratios and in sr nd pb and rb trace element abundances in central andean andesitic lavas are analyzed in terms of combined crustal assimilation and fractional crystallization the results of integrated isotopic model calculations show that observed local isotopic and trace element variations within the arequipa and barroso volcanic groups of southern peru are best fit by a simple model involving shallow or intermediate depth crustal assimilation of charcani gneiss or an equivalent rock accompanied by dominantly plagioclase fractional crystallization a consequence of this result is that the rbsr pseudoisochrons reported for the arequipa and barroso volcanics are wholly artifacts of the combined processes of assimilation and plagioclase fractionation as are all of the pb isotopic variations measured in the same lavas already interpreted by tilton and barreiro 14 to be the result of crustal contamination isotopic and trace element compositions of the primary magmas for the arequipa and barroso volcanic series are obtained by correcting for assimilation and fractional crystallization the starting magmas are characterized over the entire range of permissible primary compositions by high87sr 86sr ratios low143nd 144nd ratios and high sr concentrations relative to normal island arc andesites these results appear to require prior contamination of the magmas but not involving plagioclase fractionation the uncertainty in pb and o isotope ratios computed for possible primary magmatic compositions is sufficiently large however that it is not possible to preclude a subcrustal origin for at least some regional variation in sr and nd isotopic ratios

oxygen isotopic compositions of central andean plutonic and volcanic rocks latitudes 26 29 south abstract oxygen isotope data are reported for 27 igneous rocks of mesozoic to quaternary age from the central andes 26 29 s the plutonic rocks and most of the volcanics have 18 o values between 6 2 and 8 3 the whole rock 18 o values show a weak correlation with initial 87 sr 86 sr data this o sr array differs from documented trends for calc alkaline plutonic suites from california scotland and northern italy but overlaps with data for volcanic and plutonic rocks from ecuador northern chile and southern peru the oxygen isotope results indicate that the magmas evolved without significant contamination from supracrustal rocks e g rocks that experienced 18 o enrichment during surficial weathering the available o sr and pb isotopic data for these rocks are best explained by magma generation in the upper mantle or lower crust from the late mesozoic on the 87 sr 86 sr values were modified at depth by isotopic exchange between the magma and a continually thickening crust of plutonic rocks of late precambrian to early mesozoic age

a petrographic and chemical section through the northern idaho batholith petrographic and chemical study of a well exposed section through the northern idaho batholith documents a high degree of mineralogical and chemical homogeneity and suggests that all of the rocks belong to a single comagmatic series or superunit felsic biotite granodiorite to granite with minor muscovite forms a main phase unit 37 km across flanked on the southwest by 10 km of near vertical sheets of granodiorite with intervening high grade country rocks widespread metasedimentary schlieren appear to be restite left by voluminous partial melting the magma not rising far above its level of generation the lack of mineralogical and chemical variation across the batholith suggests little differentiation except for separation of late volatile rich fluids to form widespread pegmatites abundant mafic dikes emplaced into fractures in the granitic rocks just before crystallization of the last pegmatitic fluids may represent the later stages of rising basalts which aided the initial crustal melting to for

the four plutonic belts of the transhimalaya himalaya a chemical mineralogical isotopic and chronological synthesis along a tibet nepal section

nd sr and pb isotopic characteristics of cretaceous intrusive rocks from deep levels of the sierra nevada batholith tehachapi mountains california the cretaceous gabbroic to granitic intrusive rocks of the tehachapi mountains were emplaced at depths of 25 30 km and thus afford a view of deep processes in the sierra nevada batholith they consist of the 115 ma tehachapi suite and the 100 ma bear valley suite new zircon u pb age data reveal the presence of the latter as far west as grapevine canyon the nd sr pb and o isotopic whole rock data and zircon pb inheritance patterns for the bulk of the suites suggest an origin by mixing between depleted mantle derived magmas and metasedimentary material with a substantial component of old continental material however this mixing is not evident in variations between isotopic ratios and chemical and lithologic parameters this implies that isotopic hybridization of magmas took place deeper than 30 km and that fractionation processes are likely responsible for the bulk of the chemical variation in this part of the sierra nevada batholith consideration of the isotopic data in the context of the sierra nevada batholith as a whole suggests that the well known east to west isotopic gradients in the batholith may reflect a change in the average isotopic character of the preintrusive framework rather than a change in amount of crustal component on the other hand the lack of areal gradients in sr and nd isotopic ratios in the main study area may indicate a lack of pronounced gradation at deep levels at least within the western batholith

strontium and oxygen isotopic variations in mesozoic and tertiary plutons of central idaho regional variations in initial 87sr 86sr ratios r i of mesozoic plutons in central idaho locate the edge of precambrian continental crust at the boundary between the late paleozoic mesozoic accreted terranes and precambrian sialic crust in western idaho the r i values increase abruptly but continuously from less than 0 704 in the accreted terranes to greater than 0 708 across a narrow 5 to 15 km zone characterized by elongate lens shaped highly deformed plutons and schistose metasedimentary and metavolcanic units the chemical and petrologic character of the plutons changes concomitantly from ocean arc type diorite tonalite trondhjemite units to a weakly peraluminous calcic to calcalkalic tonalite granodiorite granite suite the idaho batholith plutons in both suites yield late cretaceous ages but permian through early cretaceous bodies are confined to the accreted terranes and early tertiary intrusions are restricted to areas underlain by precambrian crust the two major terranes were juxtaposed between 75 and 130 m y ago probably between 80 and 95 m y oxygen and strontium isotopic ratios and rb and sr concentrations of the plutonic rocks document a significant upper crustal contribution to the magmas that intrude precambrian crust magmas intruding the arc terranes were derived from the upper mantle subducted oceanic lithosphere and may have been modified by anatexis of earlier island arc volcanic and sedimentary units plutons near the edge of precambrian sialic crust represent simple mixtures of the precambrian wall rocks with melts derived from the upper mantle or subducted oceanic lithosphere with r i of 0 7035 rb sr varies linearly with r i producing pseudoisochrons with apparent ages close to the age of the wall rocks measured 18o values of the wall rocks are less than those required for the assimilated end member by sr o covariation in the plutons however indicating that wall rock 18o was reduced significantly by exchange with circulating fluids metasedimentary rocks of the belt supergroup are similarly affected near the batholith documenting a systematic depletion in 18o as much as 50 km from the margin of the batholith plutons of the bitterroot lobe of the idaho batholith are remote from the accreted terranes and represent mixtures of precambrian wall rocks with melts dominated by continental lower crust r i 0 708 rather than mantle pseudoisochrons resulting from these data are actually mixing lines that yield apparent ages less than the true age of the wall rocks and meaningless ri assimilation fractional crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region

new depositional age constraints for the murmac bay group of the southern rae craton canada abstract the murmac bay group is one of several early supracrustal successions that were deposited on the southern rae craton churchill province western canadian shield and subsequently affected by paleoproterozoic tectono metamorphism where best understood the succession comprises basal quartzite and or psammite with intercalated dolostone patchy iron formation mafic volcanic flows and overlying psammopelite pelite previous attempts to date the murmac bay group have resulted in both archean and paleoproterozoic estimates re interpretation of critical field relationships together with acquisition of new geochronological data have greatly improved estimates for its age of deposition which in turn yield new constraints on the tectonic evolution of the region circa 2 33 2 32 ga detrital zircon from three of four dated samples including a rare basal conglomerate establishes a maximum age for the initiation of deposition a 2322 5 ma quartz feldspar porphyry dyke intruding mafic volcanic rocks near the base of the succession indicates that deposition of the murmac bay group was initiated ca 2 33 ga coeval with the earliest members of a 2 33 2 29 ga granitic suite these post collisional granites were emplaced following a 2 37 ga thermal culmination attributed to the arrowsmith orogeny deposition of the murmac bay group continued until at least 2 17 ga the youngest detrital zircon age analyzed from the upper part of the succession deposition was over by ca 1 93 ga when the murmac bay group underwent regional metamorphism and was intruded by the donaldson lake pluton for which a revised ca 1 93 ga crystallization age has been obtained according to our new age constraints the upper murmac bay group may be temporally correlative with the waugh lake group exposed about 100 km to the west northwest at the eastern margin of the taltson magmatic zone and with rocks of the rutledge river basin located 150 km to the northwest a stronger case can be made for lithostratigraphic correlation of the murmac bay group with the lower three of four assemblages in the amer and ketyet river groups 750 km to the northeast the abundance of ca 2 17 ga detrital zircon in the murmac bay group together with the presence of 2 2 2 07 ga detritus in the amer and ketyet river groups rutledge river basin and in younger sedimentary successions in the region are thought to record an early accretionary event on the western margin of the rae craton

petrogenesis of the cretaceous cassiar batholith yukon british columbia canada implications for magmatism in the north american cordilleran interior the ca 100 ma cassiar batholith is the single largest plutonic body in the hinterland of the canadian cordillera and is part of widespread middle cretaceous to eocene magmatism that occurred in the omineca crystalline belt the cassiar batholith is dominated by muscovite biotite granite and biotite muscovite granodiorite along with subordinate biotite hornblende granodiorite quartz monzodiorite and quartz monzonite the batholith has a wide range of strontium neodymium and oxygen isotope compositions 87 sr 86 sr i 0 706 to 0 734 nd 2 7 to 17 1 t dm 2000 to 980 ma 18 o 7 6 to 11 6 but the batholith9s lead isotope compositions are limited 206 pb 204 pb 19 14 to 19 24 207 pb 204 pb 15 70 to 15 72 208 pb 204 pb 39 14 to 39 26 these data require that the cassiar magmas have been generated from at least two distinct sources an isotopically evolved felsic source and an isotopically more primitive source of mafic to intermediate composition we propose that the felsic source comprised mesoproterozoic to neoproterozoic metasedimentary rocks and paleoproterozoic basement rocks of the north american cratonic margin determining the mafic source is more problematic but most likely it was proterozoic basement rocks of mafic to intermediate composition such as have been reported from various parts of the omineca crystalline belt the cassiar batholith is lithologically geochemically and isotopically similar to other major batholiths in the cordilleran interior including the batholiths of southeastern british columbia and the idaho batholith we interpret these similarities to reflect melting of common crustal source materials namely proterozoic basement and cover rocks of western laurentia the cassiar batholith and other cordilleran interior granitoids are distinct from granitoids generated in known subduction zone environments in terms of their mineralogical geochemical and isotopic characteristics these differences reflect a predominantly or exclusively crustal source region for the cordilleran interior granitoids in contrast subduction zone magmatism contains a significant mantle input even in cases where subduction takes place beneath thick continental crust these results strongly suggest that cordilleran interior granitoids did not form directly from a subduction setting rather an intracontinental collisional setting seems more appropriate in which magma generation occurred in response to crustal thickening followed in some cases by extension

increase in melt fraction along a south north traverse below the tibetan plateau evidence from seismology abstract a mid crustal low velocity zone and crustal melt fraction contrast the lhasa block in the maxiang yangbajain zone with the tethyan himalayas in the south evidence from wide angle reflections is extended by vertical reflection and differential s to p wave teleseismic delays the temperature and rheology implied allow phase transitions which may change crustal buoyancy and elevation le pichon et al 1997 and may allow lateral mass flow in the uppermost mantle particular path geometries to a tight temporary teleseismic array allow detection of a decrease in velocity northward through the lhasa block a larger relative variation in late teleseismic s with respect to p indicates an increase in poisson s ratio hence of partial melt fraction this may be seen as evidence of a higher position of the mantle asthenosphere level tibet hence appears to be underlain by variation in the structure within the crust across the indus tsangpo suture and in the structure within the mantle further northward of the middle lhasa block these variations are most readily attributable to spatial variation in partial melt fraction and hence temperature which also induces phase transformations mode and amount of deformation and control of buoyancy on elevation may vary accordingly the dynamics of the system is evidenced by seismic anisotropy which we relate to ductile flow

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

petrological and geochemical characteristics of a paleoproterozoic magmatic arc narsajuaq terrane ungava orogen canada and comparisons to superior province granitoids abstract the narsajuaq terrane is a major component of the paleoproterozoic ungava trans hudson orogen of northern quebec and is a well exposed mid crustal block interpreted to be the plutonic roots of a magmatic arc the narsajuaq terrane is comprised of a series of hornblende pyroxene bearing plutons and supracrustal rocks which were accreted to the archean superior province basement during the paleoproterozoic on the basis of geological field relationships and geochronological data the plutonic rocks of the narsajuaq terrane are divided into four main groups 1 the cape smith suite 1898 1839 ma consists of various deformed and undeformed plutons that are found intrusive into the cape smith belt lithologies 2 the gneissic suite which ranges in age from 1863 1844 ma and consists of a well layered sequence of diorite and tonalite intruded by and interlayered with variably deformed and metamorphosed granitic veins 3 the younger suite consisting of discrete kilometre size bodies of metamorphosed plutons diorite monzodiorite tonalite granite intrusive into the gneissic suite ranging in age from 1836 1821 ma and 4 granitic sheets of the anatectic suite which intruded the narsajuaq terrane ca 1800 ma a series of metamorphosed sedimentary rocks predominantly semi pelites of the sugluk group are also found in the narsajuaq terrane and contain zircons of both archean and paleoproterozoic age plutons of the gneissic and younger suites are predominantly calc alkaline and metaluminous to mildly peraluminous and contain variable but generally moderate to high concentrations of the large ion lithophile elements and light rare earth elements rees pronounced negative nb and ti anomalies and fractionated rees comparable to modern arc granites a range in isotopic compositions is noted for the suites with the gneissic and younger suites having initial end and 87sr 86sr i values ranging from juvenile 4 0 and 0 7020 respectively to enriched 10 7 and 0 7048 the anatectic suite granites range to extremely enriched isotopic compositions with initial end up to 18 5 and 87sr 86sr i 0 7064 the cape smith suite has a much more restricted isotopic composition which is predominantly juvenile initial end 3 2to 1 5 and 87sr 86sr i 0 7020 to 0 7024 and suggests a minor role for crustal contamination primarily via sediment subduction the isotopic signature of the narsajuaq terrane plutons indicate that at least three sources were involved in their petrogenesis depleted mantle average 2 8 ga superior province crust and older 3 1 ga superior province crust when compared to plutonic suites from the ne and se superior province minto block abitibi pontiac and opatica belts the narsajuaq terrane suites define a similar chronology and evolution indicating a commonality and dominance of arc accretionary processes for crustal formation in the precambrian shield of canada higher trace element abundances in the narsajuaq terrane plutons compared to the archean samples may be related to different subduction zone fluxes which may reflect different bulk compositions of subducted oceanic sediments during the proterozoic

geology and geochemistry of jasperoids from the gold bar district nevada gold bar is one of several carlin type gold mining districts located in the battle mountain eureka trend nevada it is composed of one main deposit gold bar five satellite deposits and four resources that contain 1 6 moz 50 t of gold all of the deposits and resources occur at the intersection of north northwest and northeast trending high angle faults in slope facies limestones of the devonian nevada group exposed in windows through ordovician basin facies siliciclastic rocks of the roberts mountains allochthon igneous intrusions and magnetic anomalies are notably absent the gold bar district contains a variety of discordant and stratabound jasperoid bodies especially along the wall fault zone that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models four types of jasperoids j0 j1 j2 and j3 were distinguished on the basis of their geologic and structural settings and appearance field relations suggest that j0 formed during an early event petrographic observations geochemistry and 18o values of quartz suggest it was overprinted by the hydrothermal event that produced ore related j1 j2 and j3 jasperoids and associated gold deposits the greater amount of siliciclastic detritus present in j0 jasperoids caused them to have higher 18o values than j1 2 3 jasperoids hosted in underlying limestones ore related jasperoids are composed of main ore stage replacements and late ore stage open space filling quartz with variable geochemistry and an enormous range of 18o values 24 5 and 3 7 jasperoids hosted in limestones with the most anomalous au ag hg as sb tl concentrations and the highest 18o values are associated with the largest deposits the 28 range of jasperoid 18o values is best explained by mixing between an 18o enriched fluid and an 18o depleted fluid the positive correlation between the sizes of gold deposits and the 18o composition of jasperoids indicates that gold was introduced by the 18o enriched fluid the lowest calculated 18o value for water in equilibrium with late ore stage quartz at 200 c 15 and the measured d value of fluid inclusion water extracted from late ore stage orpiment and realgar 116 indicate that the 18o depleted fluid was composed of relatively unexchanged meteoric water the source of the 18o enriched ore fluid is not constrained the 34s values of late ore stage realgar orpiment and stibnite 5 7 15 5 and barite 31 5 40 9 suggest that h2s and sulfate were derived from sedimentary sources likewise the 13c and 18o values of late stage calcite 4 8 to 1 5 and 11 5 to 17 4 respectively suggest that co2 was derived from marine limestones based on these data and the apparent absence of any eocene intrusions in the district gold bar may be the product of a nonmagmatic hydrothermal system

three dimensional elastic wave velocity structure of the western and central tien shan arrival times of compressional p and shear s waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central tien shan are used to determine one and three dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt the best fit one dimensional structures suggest that the average depth of the mohorovicic discontinuity in this area is 50 km the three dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region a 7 km thick wedge of sediment beneath the chu depression is outlined at depth by a south dipping plane of seismic activity suggesting the presence of an active decollemont these low velocities extend continuously to the southeast toward issyk kul suggesting a structural relationship between the two however rather than being consumed it appears that issyk kul is overthrusting the surrounding ranges the low velocity sediments in the fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity velocities at midcrustal depths generally are lower beneath the central tien shan than beneath the western tien shan this pattern becomes more evident in the uppermost mantle with p velocity contrasts of as much as 10 across a boundary that corresponds roughly to the geographical position of the talasso fergana fault the low velocities beneath the central tien shan exceed 150 km depth but do not appear to be deeper than 300 km depth there is no evidence for a lithospheric root beneath this part of the range rather the low velocities imply the presence of a positive buoyancy force uplifting the mountains evidence that this low velocity region existed before the collision suggests that the tien shan may not owe its rejuvenation simply to its location at the northern edge of a strong tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere

nd isotopic evolution of the taltson magmatic zone northwest territories canada insights into early proterozoic accretion along the western margin of the churchill province the taltson magmatic zone is a north trending belt of early proterozoic granitoids exposed over an area exceeding 18 000 km 2 in the northwest territories of canada three petrologically and geochronologically discrete intrusive suites comprise the taltson zone these are the 1986 ma deskenatlata suite the 1955 to 1925 ma slave suite and the 1935 ma konth suite the deskenatlata suite is composed of biotite hornblende diorite to granite and displays a more mafic geochemical character relative to the other two intrusive suites initial epsilon nd values for the deskenatlata suite range from 2 7 to 3 6 the slave and konth suites are compositionally heterogeneous to homogeneous leucocratic granitoids that contain numerous xenoliths of dominantly pelitic high grade paragneiss and display a peraluminous granite mineralogy and geochemistry initial epsilon nd values for the konth suite cluster within 4 7 and 5 5 and are completely overlapped by the 4 1 to 6 0 range of the pelitic xe

geochemical and nd pb o isotope systematics of granites from the taltson magmatic zone ne alberta implications for early proterozoic tectonics in western laurentia the early proterozoic taltson magmatic zone tmz comprises the southern part of the taltson thelon orogenic belt of northwestern canada the tmz is dominated by 1 96 1 99 ga i type and 1 93 1 95 ga s type suites of granitoids previous workers attributed the formation of the i type granitoids to subduction of oceanic crust beneath the churchill craton in an andean type setting and generation of the s type granitoids to subsequent collision between the churchill craton and the buffalo head terrane to test this hypothesis we carried out a geochemical and isotopic nd pb and o study of granitoids and associated metasedimentary rocks and basement gneisses from the southern part of the tmz both i and s type suites of granitoids are characterized by relatively high sio2 content with the great majority of samples containing more than 64 wt sio2 the two suites also have strongly crustal end t values of 3 8 to 9 8 and an average tdm of 2 8 ga the initial pb isotope composition of the granitoids as determined from leached feldspar mineral separates forms a linear array that is bracketed by the isotopic compositions of basement orthogneisses and metasediments similarly 18o values of the granitoids overlap those of the orthogneisses and metasediments these isotopic data are consistent with derivation of the granitoids from a mixture of metaigneous and metasedimentary source rocks similar to those presently exposed in the tmz importantly the data suggest that both i and s type granitoids of the southern tmz had an exclusively intra crustal origin unlike phanerozoic examples of subduction related magmatism where a significant mantle contribution is easily recognized therefore we argue that the existing andean type model for early tmz magmatism is not viable the tmz granitoids show geochemical and isotopic similarities to the cordilleran interior granitoids of western north america which formed in response to crustal thickening in the distant hinterland of a convergent plate margin we propose that these hinterland granitoids are a more appropriate analogue for tmz magmatism than are the granitoids of plate margins the tectonic implication of this proposal is that the taltson thelon orogenic belt does not mark the location of the plate boundary in western laurentia at 1 9 2 0 ga rather the tectonic setting of the tmz at that time may have been more akin to the present day intra continental mountain belts of central asia e g tian shan mountains where compressional tectonism and crustal thickening are occurring well inboard from a convergent plate margin

chemical and isotopic evolution of the coastal batholith of southern peru southeast of arequipa the coastal batholith of southern peru is composed of two segments arequipa and toquepala including five superunits which were emplaced in discrete magmatic pulses from the jurassic to the paleocene eras 190 61 ma most superunits intruded a precambrian basement dominated by granulitic and amphibolitic rocks showing a strong enrichment in large ion lithophile elements low end p 21 to 29 and 206pb 204pbp 16 11 17 03 tilton and barreiro 1980 and high esr p 396 to 999 values major and trace element analyses reveal that each superunit is formed by distinct suites of calc alkaline plutons i e i type that range in composition from quartz gabbro to monzogranite for the whole plutonic suite located in southern peru the evolution toward negative end i and positive esr i values is followed by a significant decrease in 206pb 204pbi ratios but is also related to the density of precambrian outcrops this led us to classify the intrusives into three groups group 1 consists of intrusives carrying positive end i 2 4 to 0 4 and generally negative esr i values 7 4 to 0 7 they are located in the ilo moquegua transect 17 22 17 80 s an area where precambrian exposure is scarce group 2 consists of plutons with intermediate e values i e end i 0 5 to 2 2 and esr i 7 1 to 55 7 which are found in the vicinity of arequipa and tarata where numerous precambrian outcrops are present finally group 3 is composed of intrusives showing negative end i 4 4 to 8 0 and positive esr i values 27 1 to 56 1 including one anomalous granodiorite exposed near tarata and two samples collected in the arequipa quadrangle near the contact with the charcani gneiss there are several petrogenetic models which can explain the trace element isotopic and geographic correlations observed within the coastal batholith of southern peru one simple model advocates that the parental mafic magma s of the plutonic suites of each superunit must be ultimately derived from an isotopically depleted mantle wedge above the subduction zone the gabbro diorite granodiorite monzogranite association is formed via fractional crystallization and crustal assimilation i e afc during ascent in the andean crust if the exposed precambrian crust represents a good compositional average of the upper crustal material assimilated then the isotopic and some of the trace element variations observed in the three groups can be related to various amounts of crustal assimilation i e small for group 1 but substantial for group 3 although surprisingly little assimilation is required when considering all southern peruvian intrusives more complex models advocate different parental mafic magmas formed by various mixtures of lower crustal and mantle derived melts both having heterogenous but depleted nd and sr isotopic compositions the parental magmas would then ascend differentiate and assimilate upper crustal materials of different compositions i e a crust similar to the exposed precambrian basement for groups 2 and 3 and a less enriched crust not exposed in the area for group 1 if the first model is correct then the relatively small degree of crustal assimilation experienced during the formation of the coastal batholith of southern peru is consistent with the hypothesis that the thick andean crust was thinned during the prevailing extensional tectonic regime that led to the formation of a marginal basin in central peru during the late mesozoic this allowed the basic magmas to encounter the base of the precambrian section at shallower depth and spend less time ascending and differentiating in this enriched crust effectively limiting the amount of assimilation

stable isotope systematics in mesozoic granites of central and northern california and southwestern oregon 18o d and h2o contents were measured for whole rock specimens of granitoid rocks from 131 localitics in california and southwestern oregon with 41 new determinations in the klamath mountains and sierra nevada initial strontium isotope ratios are known for 104 of these samples large variations in 18o 5 5 to 12 4 d 130 to 31 water contents 0 14 to 2 23 weight percent and initial strontium isotope ratios 0 7028 to 0 7095 suggest a variety of source materials and identify rocks modified by secondary processes regular patterns of variation in each isotopic ratio exist over large geographical regions but correlations between the ratios are generally absent except in restricted areas for example the regular decrease in d values from west to east in the sierra nevada batholith is not correlative with a quite complex pattern of 18o values implying that different processes were responsible for the isotopic variations in these two elements in marked contrast to a good correlation between 87sr 86sr o and 18o observed in the peninsular ranges batholith to the south such correlations are lacking except in a few areas d values on the other hand correlate well with rock types chemistry and 87sr 86sr o except in the coast ranges where few of the isotopic signatures are primary the uniformly low d values of samples from the mojave desert indicate that meteoric water contributed much of the hydrogen to the rocks in that area even so the 18o values and 18o fractionations between quartz and feldspar are normal in these same rocks

anatexis and metamorphism in tectonically thickened continental crust exemplified by the sevier hinterland western north america the generation of granitoid magmas by partial melting of crustal rocks during continental thickening events is well documented in many geological provinces throughout the world including the late mesozoic sevier belt of western north america we present a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening where the only mantle contribution is the normal conductive supply of heat through the base of the lithosphere i e advection of mass and energy are excluded we distinguish between formation of migmatites and generation of mobile granitoid magmas and examine the temporal and spatial relationships between these two distinct anatectic processes between anatexis and regional deformation and between anatexis and metamorphism a fundamental conclusion is that if protoliths rich in hydrous minerals are present regional anatexis is the end product of classical barrovian metamorphism in thickened continental crust even in the absence of a free water rich fluid phase barrovian metamorphic facies series are predicted with thickening ratios maximum crustal thickness attained initial crustal thickness as low as 1 3 but mobile granitoid magmas are not formed if this ratio is less than approximately 1 5 above these lower bounds barrovian metamorphism and anatectic granitoid magmatism occur independently of the magnitude of thickening and of the way in which thickening is accomplished both processes are sensitive to a diminished heat supply lowering either moho heat flow or crustal radioactive heat production results in blueschist eclogite metamorphism and inhibits the formation of mobile granitoid magmas we model anatexis under fluid absent conditions and show that with such a constraint migmatization is always a syn kinematic process relative to the crustal thickening event whereas generation of mobile granitoid magmas is in most cases post kinematic relative to crustal thickening but can be syn kinematic if thickening takes more than approximately 50 myr the typical time intervals for melting are consistent with geological observations mobile granitoid magmas are predicted by most of our models within approximately 10 myr of the end of the crustal thickening event this incubation period results primarily from the temperature increase required for the dehydration melting reactions capable of producing large melt fractions to occur the energetic requirements of anatexis are relatively minor compared to conductive crustal thermal budgets as shown by the fact that once the necessary p t conditions are attained melting reactions are completed within time intervals on the order of 1 myr i e 1 2 orders of magnitude smaller than the characteristic time scales of the tectonic processes involved in crustal thickening

tectonic setting of the taltson magmatic zone at 1 92 0 ga a granitoid based perspective the paleoproterozoic taltson magmatic zone is one of the key tectonic features of western laurentia the existing tectonic model for the belt envisions its formation by subduction of oceanic crust beneath a continental margin followed by direct collision between formerly separate crustal blocks we tested this model by comparing the large geochemical and isotopic database available for taltson magmatic zone granitoids with similar databases for phanerozoic granitoid suites from different tectonic environments the comparison reveals that the early granitoid suite of the taltson magmatic zone which had been ascribed to the subduction phase of orogenesis lacks the mantle signature apparent in granitoids of phanerozoic continental margin arc settings instead both early and late suites appear to have an intracrustal origin similar to mesozoic and cenozoic granitoids of the cordilleran interior of western north america which formed in the distant hinterland of a convergent plate margin in light of thes

geochemistry of the trans labrador granitoid belt canada a quantitative comparative study of a proterozoic batholith and possible phanerozoic counterparts abstract the trans labrador granitoid belt tlgb of eastern canada is a 600 km long magmatic province that forms part of a wider belt of lower to middle proterozoic crust along the southern margin of the pre 1800 ma cratonic nucleus of north america the size geometry and tectonic position of the tlgb invite comparison with continental margin batholiths such as those of circum pacific areas quantitative comparative studies using a large statistically representative database suggest that there are significant contrasts between the compositional spectra and differentiation trends of the tlgb and typical continental arc batholiths specifically the tlgb is dominated volumetrically by alkali calcic granites s s showing locally agpaitic tendencies and relatively enhanced levels of f zr and ree such trace element characteristics invite comparison with a type anorogenic or within plate granites and some components of the tlgb fall into this category in terms of bulk compositional spectra the tlgb is similar empirically to post tectonic and anorogenic intrusions of the newfoundland appalachians it is suggested that a continental arc setting is inappropriate for the tlgb and that a more likely environment is a post orogenic setting dominated by crustal thickening uplift and transcurrent faults it is also possible however that the tlgb was generated in an intraplate environment by melting and vertical differentiation of continental crust in response to thermal input from the mantle it is suggested that the use of large representative databases coupled with quantitative comparative studies of compositional spectra and differentiation trends may be a useful approach in the discrimination of batholiths from different tectonic environments at the present time however studies of this type are hampered by the lack of representative data from type areas

granites of western karakorum and northern kohistan pakistan a composite mid cretaceous to upper cenozoic magmatism abstract the petrography the chemical mineralogical typology and the ages of six plutonic units four from the karakorum axial batholith darkot pass ghamu bar batura and hunza units and two of northern kohistan gindai and nomal plutons are defined and compared based on field data and analyses of up to 78 samples major elements ree rb s sbnd sr and k ar isotopes the karakorum axial batholith is a composite body three major intrusive stages occurred 1 around mid cretaceous times ca 110 95 ma with the emplacement of subalkaline i e monzonitic darkot pass and calc alkaline hunza ghamu bar units 2 during palaeogene maybe up to 43 ma batura subalkaline unit a strong tectonommetamorphic event recorded in the gneissification of the cretaceous intrusives occurred between these two stages it may be of palaeocene age p t estimates of the highest metamorphic grade rocks of the hunza unit have yielded values of 580 640 c and 5 0 5 kbar and 3 during upper miocene ca 9 ma baltoro subalkaline unit debon et al 1986c in addition a conspicuous network of aplo pegmatitic dykes emplaced into the hunza area possibly from the eocene up to the upper cenozoic with a maximum during the middle miocene ca 15 ma most of these major magmatic stages are met again among the acidic intrusives and dykes of northern kohistan the first one as blastomylonitic tholeiitic plutons nomal ca 102 ma petterson and windley 1985 the second one as subalkaline plutons gindai ca 59 ma and a third one as leucocratic dykes of oligocene age ca 30 ma petterson and windley 1985 these data may be related to the geodynamic evolution of the nw part of the india eurasia suture zone thus allowing better constraints on the major steps of this evolution the partly synchronous closures by n dipping subduction of the two tethys branches which are assumed to have encircled the kohistan arc in upper mesozoic times may have generated both the karakorum and the kohistan intrusives of cretaceous and palaeogene ages the northern branch very likely closed before the southern one at the time of the second intrusive stage palaeogene a kohistan and karakorum had already collided were welded and had suffered the same major tectonometamorphic event b subduction of the southern tethys floor beneath the welded kohistan karakorum was still active c however collision between india and kohistan karakorum may have already begun particularly at the level of the nanga parbat promontory finally it is emphasized that the intrusive processes continued in the karakorum long after the collision e g baltoro granite

shallow seismic events with combined source mechanism abstract the non shear source component of shallow seismic events such as crustal earthquakes and mining tremors is examined the radiation patterns of a pure shear model as well as a combined shear tensile seismic source model were compared with the isc bulletin first motion signs although the set of events studied is not statistically significant a series of earthquakes was found for which the combined shear tensile solution was more consistent than the pure double couple one in the combined mechanism the shear component with preferably dip slip orientation is dominant while the tensile component only accounts for a small percentage of the shear component along faults with strike slip motion such as the anatolian fault no events with a combined mechanism were detected which points to a possible relation between tensile fracturing and dip slip displacements a set of weak mining tremors which occurred in the mid bohemian coal mining region was treated by the amplitude inversion method and a combined shear implosive source mechanism was found the implosive component was small while the dominant shear component coincided well with the local tectonic fault i e these mining tremors were almost purely of a tectonic nature if this were found to be true generally then even weak mining tremors could be utilized as large scale models of tectonic earthquakes

volcanic rocks of the 1985 tibet geotraverse lhasa to golmud volcanic rocks encountered during the tibet geotraverse have been studied in the field in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment rocks from a total of eleven distinct volcanic provinces were studied in this way they provide evidence for an active continental margin or post collision province of probable devonian early carboniferous age in the northern kunlun mountains an active continental margin of late carboniferous age in the southern lhasa terrane permian continental rifts in the central qiangtang and central kunlun terranes triassic volcanic arcs in the southern lhasa and northern qiangtang terranes a triassic active continental margin dyke swarm in the northern kunlun mountains a jurassic post collision or back arc rifting province in the southern qiangtang terrane a jurassic island arc in the northern lhasa terrane a cretaceous post collision province in the northern lhasa terrane possibly extending into the southern qiangtang terrane and a palaeogene active continental margin in the southern lhasa terrane an oligocene trachyte plug in the northern qiangtang terrane was the only evidence encountered during the geotraverse of volcanism post dating the palaeogene india eurasia collision however the composition of this plug coupled with new and published analyses from miocene volcanics in the southern lhasa terrane and from the pliocene recent volcanic province of northwest tibet places important constraints on models for post collision underplating of tibet by continental lithosphere any underplating is likely to have been a much later than the start of collision b directed beneath tibet from the north as well as the south and c limited in extent

characterization of the paleoproterozoic hottah terrane wopmay orogen using multi isotopic u pb hf and o detrital zircon analyses an evaluation of linkages to northwest laurentian paleoproterozoic domains abstract the paleoproterozoic wopmay orogen is modeled as a simple arc continent collision involving the archean slave craton and an enigmatic continental arc terrane termed the hottah terrane hottah is a composite terrane that includes a juvenile 2 6 2 1 ga basement and a younger 1 93 1 90 ga continental arc the extent and crustal age structure of the terrane is poorly known which limits interpretations as to its origin and eventual role in the development of the orogen multi isotopic u pb hf o analyses of detrital zircon from rocks of the hottah terrane and its cover sequence are used to indirectly evaluate its age and crustal characteristics the data confirm that ca 2 6 2 1 ga crust is an important component of the terrane but hf isotopic data indicate mostly non juvenile compositions consistent with recycled neoarchean crust sources ages of detrital zircon are distinct from those expected of the slave craton but are similar to adjacent 2 0 1 90 ga paleoproterozoic terranes of northwestern laurentia the similarity to terranes such as the ksituan buffalo head and taltson thelon magmatic zones south and east of the wopmay orogen indicate that hottah could represent a terrane that initially evolved adjacent to the earliest assembled parts of the nuna continent between 2 0 and 1 9 ga and was rifted prior to the collision with the slave margin that produced the calderian orogeny

thermochronological history of the northern canadian shield abstract the northern canadian shield is comprised of multiple archean cratons that were sutured by the late paleoproterozoic to form the canadian component of supercontinent nuna more than 2000 combined k ar and 40ar 39ar cooling ages from across the region reveal a stark contrast in upper and lower plate thermal responses to nuna forming events with the churchill province in particular revealing near complete thermal reworking during the late paleoproterozoic we review the detailed cooling history for five regions that span the churchill province and trans hudson orogen tho thelon tectonic zone south rae reindeer zone south hall peninsula and the cape smith belt the cooling patterns across churchill province are revealed in two 1500 km transects at the plate scale churchill s cooling history is dominated by tho accretionary and collisional events during which it formed the upper plate cooling ages generally young from west to east across both southern and central churchill and latest cooling in the tho is 50 myr older in southernmost churchill reindeer zone compared to eastern churchill hall peninsula indicating diachronous thermal equilibration across 2000 km strike length of the tho churchill exhibits relatively high post terminal tho cooling rates of 4 c myr which support other geological evidence for widespread rapid exhumation of the tho upper plate following terminal collision potentially in response to lithospheric delamination

petrogenesis of the colville igneous complex northeast washington implications for eocene tectonics in the northern u s cordillera eocene igneous complexes of the u s and canadian cordillera have long been thought to represent a broad and somewhat discontinuous subduction related magmatic arc extending from northern british columbia to wyoming this model has become increasingly untenable as knowledge of the structural and tectonic settings of these rocks has expanded new major trace and rare earth element and isotope data presented here virtually preclude the interpretation of a contemporaneous subduction related source for mantle derived magmas in the colville igneous complex of northeast washington state we suggest an alternative model whereby post laramide orogenic collapse resulted in partial melting of a mid crustal source followed by partial melting of a distinctly different lower crustal source the magma from which mixed with magma derived from a lithospheric mantle source the subduction signature and the calc alkalic nature of the magmas was inherited from previous proterozoic subduction events

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

geochemical characterization of mid cretaceous granitoids of the kootenay arc in the southern canadian cordillera within southeast british columbia mid cretaceous granitoid batholiths are exposed in the omineca belt a north south trending metamorphic and plutonic orogenic belt the bugaboo horsethief creek and fry creek batholiths are post kinematic with respect to regional metamorphism that affected their host rocks and are composed of hornblende and biotite granodiorites and granites in all three batholiths and two mica granites in fry creek the biotite granites are weakly peraluminous have initial esr ranging from 36 to 56 and initial end ranging from 4 8 to 7 5 and overlap the range of nd sr isotopic compositions for precambrian basement gneisses and proterozoic metasediments found in southeast british columbia the initial 206pb 204pb versus 208pb 204pb and 207pb 204pb for these granitoids lie in bands between model lower and upper crustal pb isotopic compositions above the northern hemisphere reference line for oceanic basalts these granitoids have high nb and rb and low ba abundances relative t

crustal structure beneath the western canada sedimentary basin constraints from gravity and magnetic modelling regional gravity and magnetic anomalies that originate from crystalline basement rocks extend over many parts of the western canada sedimentary basin although these potential field anomalies provide a basis for tectonic subdivisions of the basement crust most previous interpretations of these features have been largely qualitative in nature this study focuses on numerical simulation and quantitative interpretation of five regional potential field anomalies in alberta canada for which independent constraints on crustal structure are available from lithoprobe seismic and electromagnetic studies the kimiwan high 55 n 116 w is a roughly 250 km long linear magnetic high seismic profiles across this anomaly provide evidence for a crustal scale extensional fault system that offsets the winagami reflection sequence a series of mid crustal sills we find that the magnetic anomaly can be modelled as either a 15 40 km wide zone of moderate positive susceptibility 4 5 10 3 si units in the hanging wall

field relationships and geochemistry of pre collisional india asia granitoid magmatism in the central karakoram northern pakistan in the central karakoram range northern pakistan seven major granitic units which were emplaced prior to the eocene ca 50 ma india asia collision have been mapped and analysed geochemically the mid cretaceous hunza plutonic complex dominates the karakoram batholith in the west and is composed of quartz diorite granodiorite plutons which have been deformed in the south by later collision related thrusting the sost and khunjerab plutons intrude the palaeozoic early mesozoic sediments north of the batholith in the east pre collision magmatic units occur both north of k2 gneiss broad peak quartz diorite muztagh tower unit and south of hushe gneiss the batholith which is dominated by a miocene monzogranite leucogranite pluton baltoro unit the plutonic units to the north of the batholith are mid to late cretaceous with an age range of ca 115 80 ma whereas the hushe gneiss to the south is dominantly jurassic spanning the age range 200 145 ma all the pre collision magmatic units display similar chemical and isotopic characteristics major trace and ree variations in the hunza plutonic complex are controlled by high level fractionation of clinopyroxene hornblende plagioclase biotite ilmenite and allanite high lile hfse and lree hree ratios together with negative nb p and ti anomalies suggest an ultimate source in the mantle wedge above the subducting slab 87sr 86sr initial ratios 0 7055 0 7157 in the hunza plutonic complex can best be explained by contamination of a mantle derived magma by a crustal component with highly radiogenic sr all the units except the hushe gneiss are probably related to subduction during closure of a back arc basin between the karakoram to the north and the kohistan dras arc to the south during the mid to late cretaceous and correlate with widespread plutonism from the same period in kohistan ladakh and southern tibet along the southern margin of asia the hushe gneiss is probably related to an earlier northward directed subduction phase during the jurassic