crustal reworking at nanga parbat pakistan metamorphic consequences of thermal mechanical coupling facilitated by erosion within the syntaxial bends of the india asia collision the himalaya terminate abruptly in a pair of metamorphic massifs nanga parbat in the west and namche barwa in the east are actively deforming antiformal domes which expose quaternary metamorphic rocks and granites the massifs are transected by major himalayan rivers indus and tsangpo and are loci of deep and rapid exhumation on the basis of velocity and attenuation tomography and microseismic magnetotelluric geochronological petrological structural and geomorphic data we have collected at nanga parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the indus and tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes the localization of this phenomenon at the terminations of the himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics

apatite feldspar u pb thermochronometer a reliable mid range 450 c diffusion controlled system the u pb systematics of apatite are commonly dominated by simple volume diffusion without secondary growth and recrystallization that can affect u pb systematics of sphene monazite and zircon although apatite is a relatively low u mineral both u and pb are lattice bound and thus the u pb systematics are not controlled by micro inclusions as appears to be the case for other low u minerals such as garnet and staurolite closure temperature estimates from both experimental and empirical studies range from 425 c to as high as 500 c for typical diffusion radii 25 m and cooling rates 2 100 c ma respectively this range can be exploited to constrain cooling and exhumation histories of both igneous and metamorphic rocks in addition the closure temperature for pb diffusion in apatite fills a gap between that for pb diffusion in sphene 650 c and diffusion of ar in hornblende 500 c muscovite 350 c and biotite 300 325 c apatite is a common accessory phase in many rock types and u pb apatite dating can be a reliable and powerful thermochronometer as it combines mid range cooling ages with the strength of the u pb system in testing for isotopic closure precisions of u pb apatite dates can range from 1 2 to as low as 0 3 for proterozoic or archean dates the main limitation to accuracy of u pb apatite dates is a moderate to low ratio of u to pb and corresponding dependence of concordia coordinates on the choice of initial pb isotopic composition for proterozoic aged samples with measured 206pb 204pb of 100 the range of geologically reasonable initial pb isotopic compositions can shift the calculated u pb age by 60 ma fortunately even though the diffusion rate of pb in alkali feldspar is less than in apatite it appears that the isotopic composition of pb in coexisting feldspar is a good approximation of the initial isotopic composition in apatite for many geologic settings examples are given from an archean terrane in se wyoming that was unroofed and cooled during proterozoic uplift and from proterozoic terranes in sw us that have contrasting thermal histories

new constraints on the timing of early proterozoic tectonism in the black hills south dakota with implications for docking of the wyoming province with laurentia a question regarding the 1900 1600 ma assembly of laurentia is whether the wyoming province west central united states was part of the hearne province west central canada prior to the early proterozoic trans hudson orogeny or a separate entity welded later to a hearne superior continent central canada new 40 ar 39 ar mica dates help to address this question by extending a middle proterozoic geochronologic front long established along the southern wyoming province into the black hills of south dakota this suggests that previously unexplained north directed fold nappes f 1 in the black hills resulted from island arc accretion to the south ca 1780 ma north northwest trending upright f 2 folds which formed during east west collision of the wyoming and superior provinces must therefore be younger new 40 ar 39 ar hornblende dates recently published age data and crustal heat flow considerations further suggest that this collision began at or before ca 1770 ma and culminated with posttectonic magmatism beginning ca 1715 ma the harney peak granite this tectonic magmatic interval is 50 60 m y younger than that reported for the hearne superior collision trans hudson orogeny in canada comparably young metamorphic dates 1810 1710 ma also typify the eastern and northern wyoming province periphery western dakotas and southwestern montana collectively these data suggest that the hearne and wyoming provinces were once separate continents that were ultimately welded to the superior province and to each other during distinct early proterozoic orogenies regional relationships further suggest that final docking of the eastern wyoming province with laurentia began during the ca 1780 1740 ma interval of island arc accretion along the southern margin of the growing craton

radar stereomapping techniques and application to sir b images of mt shasta one of the goals of the sir b experiment was the definition of optimum radar incidence and intersection angles for radargrammetric stereoscopy by comparing the results from three separate data reduction methods to this end three overlapping images of the prime radargrammetric site mt shasta in northern california were obtained fewer than anticipated this paper describes the mathematical basis for radar stereomapping along with preliminary results from one of the methods using a so called analytical stereoplotter for the mt shasta site height coordinate errors range from 60 to 170 m depending on the density number per ground area of ground control points used

the history of danube loess research the loess paleosol sequences of the danube basin contain the longest and most complete terrestrial records of middle and late pleistocene paleoclimates in europe consequently it is no surprise that scientific research into european loess began in this region and has a long and distinguished history european loess research was initiated by count marsigli in the serbian vojvodina in the late 17th century formed the focus of the international loess commission which straddled both sides of the iron curtain from 1961 and culminated with george kukla s seminal studies in the 1970s correlating the loess archives with deep sea sediments the rise in popularity of the longer chinese loess sequences in the early 1980s shifted the international research focus away from europe and it was not until the change of century that scientific investigation intensified in the region this time with much more sophisticated tools for analysis despite its potential as one of europe s most valuable quaternary archives scientific research into danube loess has until recently been hindered both by landscape heterogeneity and the large number of countries it covers these have presented a major limiting factor in developing a unified research approach for decades regional national loess stratigraphic models were defined separately difficulties in correlating such schemes persist especially since these often change significantly with advances in dating methodologies a cooperative effort is necessary to synthesize the available data which requires not only the insights from but also the perspective provided by an appreciation of the history of the research in this study we present a critical historical overview of loess research in the danube region we classify the major research phases throughout the long tradition of danube loess investigations into five categories 1 the early period late 17th century until 1920 extending from marsigli to penck 2 the classic period 1920 1961 encompassing the contributions of milankovic and the development of pedostratigraphy 3 the modern period 1961 present which oversaw czech innovations and developments in geochronologic methods in particular amino acid racemisation aar and luminescence dating and 4 contemporary research from bronger s unified pedostratigraphy to orbitally tuned timescales finally 5 we present a vision of future danube loess research

structure of a paleoproterozoic continent continent collision zone a lithoprobe seismic reflection profile across the trans hudson orogen canada abstract an 800 km reflection seismic profile across the trans hudson orogen northern saskatchewan and manitoba images crustal scale tectonic imbrication in an unprecedented picture of paleoproterozoic crustal accretion and continent continent collisional tectonism the profile is crudely symmetric about a crustal scale culmination in the western part of an accreted juvenile collage reindeer zone geologic and isotopic data suggest that this culmination is cored by microcontinental archean basement west of the culmination highly reflective juvenile crustal elements dip westward into the lower crust beneath the wathaman batholith and archean continental crust of hearne craton to the east strong reflections in the juvenile reindeer zone crust and reworked archean foreland of the thompson belt have eastward dips persisting into the middle crust and extending beneath the superior craton a continuous reflection moho well defined for 500 km in the western part of the profile shows marked relief 12 15 s including a prominent root below the crustal culmination these imaged structures give evidence of substantial crustal shortening and thickening via large scale imbrication consistent with collisional orogeny w dipping structures below the wathaman batholith and reworked hearne craton may reflect subduction polarity in this part of the orogen however geological evidence suggests that e dipping structures below superior craton are largely related to late post collisional deformation rather than to prior oceanic subduction polarity

tectonic denudation of the darwin metamorphic core complex in the andes of tierra del fuego southernmost chile implications for cordilleran orogenesis cordillera darwin in the southernmost chilean andes is a tectonically denuded metamorphic core complex similar to those widely developed in the north american cordillera detailed comparison can be drawn with the shuswap metamorphic core complex in british columbia and washington analysis of the geotectonic history of the darwin complex reinforces the view that the presence of tectonically thickened continental crust is critical for development of most if not all known core complexes and also suggests that inversion of a marginal basin during a compressional regime is one way of initiating such a crustal welt the cordillera darwin complex is unique in the andes and antarctandes this indicates that although low angle normal faulting occurs at high levels in convergent plate settings such as the central andes and the himalayas extensional denudation of deep crustal levels in active andean type orogens may require special tectonic circumstances uplift of the darwin complex began at about 70 ma in a localized extensional setting within the developing transform zone between the south american and antarctic plates this strike slip setting is analogous to some metamorphic core complexes in north america and possibly to a recently identified core complex in new zealand it is likely to be a common one for core complexes in convergent orogens

39ar recoil artifacts in chloritized biotite biotite separates from the metamorphosed granitic rocks of eastern taiwan yield disturbed 40ar39ar age spectra their disturbed profiles become more pronounced with increasing degree of chloritization and are attributed to mixed release of argon from sub micron scale biotite and chlorite intercalations 39ar internal recoil best explains the discordance of the age spectra chlorite is randomly intercalated with biotite at scales ranging from 10 a to several tens of microns both 40ar and k were evidently lost during the chloritization reaction during the neutron irradiation 39ar atoms recoil into the chlorite from the adjacent biotite intercalations the recoil and chloritization combine to produce a variable 40ar39ar spatial distribution in the intercalated biotite chlorite during 40ar39ar step heating analyses argon in biotite is released in two pulses centered at 650 c and 950 1050 c which represent two stages of dehydroxylation of the biotite chlorite out gasses in two pulses centered at 550 c and 700 900 c during dehydroxylation and then decomposes this differential thermal release of argon from each mineral phase with variable 40ar39ar ratios yields discordant incremental heating age spectra

introduction to special issue of canadian journal of earth sciences the trans hudson orogen transect of lithoprobe dedication dr john f lewry 1939 1999 see saskatchewan geological survey 1999 dedicated his career to investigations of the saskatchewan manitoba segment of the trans hudson orogen tho one of the principal paleoproterozoic orogens associated with the assembly of laurentia indeed one can make a strong case that lithoprobe s trans hudson orogen transect thot was designed to test the tectonic models proposed by john lewry he delineated the distinct tectonic provinces in the western part of the tho predicted the presence of an archean craton trapped within the tho and recognized and interpreted the significance of the pelican thrust between the juvenile paleoproterozoic volcanic arc complex of the western flin flon domain and the archean craton now called the sask craton the research published in lewry and stauffer 1990 and many of his ideas provided the framework for the design of the thot geophysical and geological studies john lewry was co leader of the thot until he passed away in 19

the wathaman batholith an early proterozoic continental arc in the trans hudson orogenic belt canada the wathaman batholith a major element of the trans hudson orogenic belt in northern saskatchewan and manitoba has an exposed strike length of 900 km this immense body separates dominantly archean rae hearne provinces to the northwest from early proterozoic arc related terranes of the orogen to the southeast major and trace element data for 42 samples from the saskatchewan part of the batholith are similar to those of phanerozoic arc batboliths for example sierra nevada and andean batholiths data plotted on trace element discrimination diagrams show that the batholith has continental arc affinities u pb zircon geochronology of five main phase granite samples suggests that most of the batholith formed between 1865 and 1850 ma and is coeval with smaller plutons of the la rouge arc the main deformation of the batholith is limited by the 1830 ma age of a late granite sheet that crosscuts folds and foliation of main phase granites the 15 to 25 m y crystallization span of the batholith is similar to the histories of many more recent continental magmatic arcs

early proterozoic evolution of the saskatchewan craton and its allochthonous cover trans hudson orogen the composition chronology and structural relations of the saskatchewan craton and enveloping mylonitic rocks exposed in basement windows of the glennie domain trans hudson orogen have been determined by geochemical geochronologic and structural studies accompanying detailed field mapping basement windows lie along the hinge zone of a regional crustal culmination and consist mostly of 2 4 2 5 ga felsic plutonic rocks enveloped by the nistowiak thrust the nistowiak thrust is a folded 1 2 km thick upper amphibolite facies mylonite zone formed during emplacement of the flin flon glennie complex across the saskatchewan craton it is likely correlative to the pelican thrust which envelops basement windows in the hanson lake block 100 km to the east an internal high strain zone within the overlying nappe pile the guncoat thrust is composed primarily of mylonitized porphyroclastic pelitic and psammitic migmatites u pb geochronological results suggest calc alkaline plutonism from 1889 1837 ma thr

tectonics of nanga parbat western himalaya synkinematic plutonism within the doubly vergent shear zones of a crustal scale pop up structure detailed mapping and geochronologic investigations from the eastern southern and western nanga parbat haramosh massif reveal two thrust displacement shear zones that have a spatial and temporal link with granite plutonism from ca 10 to 1 ma the shear zones define a crustal scale antiformal pop up structure with dominant west northwest vergent and subordinate east southeast vergent thrusting this is substantially different than the surrounding area where the main exposed himalayan structures are oriented parallel to the orogenic trend and are early to middle miocene or older structures mapped throughout nanga parbat demonstrate that its rapid and young exhumation is not due to orogen scale structural unroofing and that sustained high erosion rates are required the observed west northwest directed shortening is proposed to be a result of differential arc parallel motion accommodated at the syntaxial bend of the northwest himalaya

crustal history of the rae and hearne provinces southwestern canadian shield saskatchewan constraints from geochronologic and isotopic data abstract the cree lake zone hearne province and western granulite domain rae province in northern saskatchewan are contrasting parts of the variably reworked upper plate western hinterland of the paleoproterozoic trans hudson orogen their junction is defined by the virgin river shear zone a part of the more extensive snowbird tectonic zone subsurface extension of the 2 0 ga thelon orogen lies west of the western granulite domain we present u pb zircon ages sm nd and rb sr isotopic data from the cree lake zone and western granulite domain and from the junction granite which intrudes the virgin river shear zone data from the western granulite domain indicate that prevalent granulite facies metamorphism occurred between 2 3 and 2 0 ga but is imposed on archean protoliths 2 8 3 0 ga old low amphibolite facies retrogression which increases toward the virgin river shear zone probably postdates 2 1 ga data from the cree lake zone also yield archean protolith ages as old as 3 0 ga and some rocks yield interpreted high grade metamorphic overprint ages of 2 3 2 0 ga however two samples yield data indicating hudsonian high grade metamorphic overprint 1 87 to 1 81 ga the junction granite has a u pb zircon age of 1 82 0 30 ga but relict zircons and sm nd data show that it was derived by melting of archean crust high grade mid crustal thermal resetting in the western granulite domain may be coeval with the 1 9 2 0 ga thelon orogen although the data do not preclude an earlier event or events similar 2 3 2 1 ga reworking extends into the cree lake zone but high grade hudsonian 1 87 1 84 ga reworking is also evident in parts of this zone well to the west of the wollaston domain and may extend entirely across the zone to the virgin river shear zone relative importance of the thelon and hudsonian overprints in cree lake zone is unknown data from the junction granite and western granulite domain suggest that all recorded high strain events in the virgin river shear zone postdate 2 3 ga and are thus unrelated to deep crustal archean granulite facies mylonites in the tantato black lake segment of the snowbird tectonic zone north of the athabasca basin

timing constraints of orogeny to cratonization thermochronology of the paleoproterozoic trans hudson orogen manitoba and saskatchewan canada abstract a comprehensive thermochronometric investigation across a 600 km transect of the paleoproterozoic trans hudson orogen tho in canada has resolved two first order thermal events 1 widespread peak metamorphism at 1 82 1 79 ga associated with terrane amalgamation and 2 a late collisional metamorphic episode at ca 1 77 ga that is characterized by significant strike slip deformation and rapid cooling geophysical observations of the lithosphere beneath the tho reveal crust that is markedly different on either side of the crustal scale tabbernor sturgeon weir tsw strike slip fault system which offsets lower crustal reflectors the tho east of the fault system records u pb titanite and total pb monazite domain ages that are relatively indistinguishable from 40ar 39ar hornblende and mica cooling ages indicating rapid cooling and perhaps exhumation following 1 77 ga metamorphism seismic imaging indicates the eastern orogen is an irregular package of stacked crustal blocks and our thermochronometric data demonstrate it behaved as a single coherent element shortly 20 30 m y following cessation of convergence in contrast rocks located west of the tsw fault system cooled distinctly later preserving 40ar 39ar muscovite and biotite ages between 1 74 and 1 72 ga this crustal architecture delineated from seismic surveys illustrates a more homogeneous slightly 5 km thinner crust in the case of this orogen apparent stabilization and cratonization occurred ca 100 m y after peak terrane amalgamation and notably the orogen was likely cratonized as two blocks east and west of the tsw fault system

the significance of u pb zircon dates in lower crustal xenoliths from the southwestern margin of the kaapvaal craton southern africa the combination of high precision id tims u pb geochronology with cathodoluminescence cl imaging and spatially resolved microsampling of zircon provides a high fidelity record of the temporal evolution of samples of the middle and lower crust a suite of xenoliths exhumed in cretaceous kimberlites at the southwestern margin of the kaapvaal craton including eclogite granulite amphibolite and greenschist facies plutonic rocks are the subject of these detailed zircon studies cl imaging demonstrates the complexity of chemical and growth structure in single zircons and highlights the importance of sampling on an intragrain scale to extract end member ages for zircon growth events for these diverse metamorphic rocks linking the zircon chronology to geologic events and processes requires an understanding of the kinetics and thermodynamics of zircon stability during metamorphism using an empirical approach we attempt to relate zircon crystallization dissolution and regrowth to metamorphic conditions through a detailed analysis of zircon chemical zonation patterns mineral inclusions and petrography these lines of evidence indicate the crystallization of zircon under p t conditions ranging from eclogite through greenschist facies we discuss four examples of this approach 1 at the lowest grade of metamorphism examined primary igneous zircon 1900 ma in a syenite xenolith from the klipfontein 08 kimberlite underwent two episodes 1015 and 964 ma of probable fluid mediated resorption and reprecipitation in the greenschist facies 2 an upper amphibolite facies xenolith from the markt kimberlite contains two distinct populations of blocky and fir tree sector zoned zircon which are suggested to date prograde hornblende breakdown reactions 1107 ma and melt infiltration under retrograde metamorphic conditions 1092 ma respectively 3 at granulite facies metamorphic zircon crystallized at 1114 ma during prograde reactions involving hornblende breakdown in a granulite xenolith also from the markt kimberlite later 960 ma zircon resorption followed by minor secondary crystallization occurred in both the amphibolite and granulite xenoliths during retrograde greenschist facies reactions possibly enhanced by fluids 4 in a much older episode of eclogite facies metamorphism recorded in a xenolith from the lovedale kimberlite zircon crystallized with the peak metamorphic mineral assemblage or possibly early on the cooling path in the late archean 2630 ma each of these examples provides unique information regarding the evolution of the kaapvaal craton while the links to metamorphic reactions and p t conditions clarify the interpretation of zircon dates obtained from these samples of the middle to lower crust

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

crustal evolution and age of thermotectonic reworking in the western hinterland of the trans hudson orogen northern saskatchewan abstract the hearne province in northern saskatchewan forms the western hinterland of the trans hudson orogen it is a fault bounded terrane lying structurally and geographically between the rae province to its west and the wathaman batholith and orogenic internides to its east the hearne province is comprised primarily of high grade metamorphic archean rocks structurally overlain by archean or proterozoic metasedimentary rocks and intruded by paleoproterozoic granites whereas the rae terrane on its west boundary is comprised primarily of still higher grade archean rocks the calc alkaline wathaman batholith was intruded at ca 1 855 ga and the orogenic internides to the east of it include mostly high grade mafic and intermediate volcanic rocks crystallized between 1 92 and 1 86 ga terminal collision and orogenesis in the internides have been constrained to be later than 1 82 ga and earlier than 1 78 ga mineral u pb data presented here unequivocally demonstrate that high grade metamorphism and large volume anatexis across the southern hearne province were approximately synchronous with terminal collision and high grade metamorphism across the trans hudson orogen zircon data for most samples form complex arrays in concordia space but demonstrate a high grade metamorphic overprint at 1810 1800 ma zircons typically have inherited components of archean age suggesting the presence of extremely old material in the hearne and a potential archean connection with the rae province sm nd data indicating ca 3500 ma model ages for similar parageneses in both terranes further support the proposed archean connection between hearne and rae province and the great antiquity of hearne crust sm nd data also elucidate the similarity of the ca 1 80 ga anatectic granites of the hearne to the 1 82 ga junction granite which intruded the virgin river shear zone and constrains the timing of final motion on that structure monazite 207 pb 206 pb ages are not older than 1812 ma or younger than 1769 ma and rutile 207 pb 206 pb ages and u pb ages are tightly constrained at about 1750 ma these data in combination with results of standard thermobarometric analysis permit determination of post orogenic cooling and denudation rates high cooling 7 c ma and denudation 240 m ma rates suggest fault accomodated uplift during post orogenic collapse

origin of the archean sask craton and its extent within the trans hudson orogen evidence from pb and nd isotopic compositions of basement rocks and post orogenic intrusions u pb zircon ages from the exposed sask craton are 2450 3100 ma from the peter lake domain 2575 2640 ma and from rocks of the trans hudson orogen 1840 1880 ma u pb monazite and zircon ages of post orogenic pegmatites and aplites are 1770 1800 ma common pb and sm nd isotopic compositions of post orogenic intrusions as probes of crust beneath the orogen were compared to sask craton rocks and ca 1850 ma orogenic rocks to infer the origin and subsurface distribution of the sask craton within the internides of the trans hudson orogen results show that post orogenic intrusions within most of the glennie domain and hanson lake block were derived at least in part from archean source materials demonstrating that the sask craton lies beneath paleoproterozoic orogenic rocks present at the surface in contrast common pb and sm nd isotopic compositions from pegmatites and aplites of the la ronge domain are essentially identical with those of the paleoproterozoic orogenic rocks into which they are intruded indicating derivation by partial melting of similar rocks thus if the sask craton extended to the west beneath the la ronge domain it was beneath the zone of melting that produced the post orogenic intrusions making it unlikely that the sask craton is a detached part of the hearne craton many samples from the sask craton have elevated pb 208 pb 204 ratios unlike superior craton or hearne craton rocks suggesting that the sask craton was derived from an exotic source such as the wyoming craton which shares similar elevated pb 208 pb 204 ratios

age and origin of the jan lake complex a glimpse at the buried archean craton of the trans hudson orogen the largely buried sask craton is a continental fragment or microcontinent that collided with and underthrust the paleoproterozoic flin flon glennie protocontinent along the pelican decollement zone at 1840 1830 ma rocks of the sask craton are exposed in three tectonic windows those of the pelican window have been named the jan lake complex and comprise 2960 ma arc derived leucocratic orthogneisses migmatitic paragneisses and a tholeiitic within plate igneous suite comprising 2488 ma dioritic to gabbroic rocks and 2450 ma enderbitic and charnockitic rocks including the sahli granite ages of ca 2450 ma are also common from the other two tectonic windows to the sask craton suggesting that emplacement of the igneous suite was widespread and perhaps part of the coeval matachewan igneous event the absence of rocks in the 2960 and ca 2450 ma age ranges on both the adjacent superior and rae hearne cratonic margins makes it improbable that the sask craton was derived by simple fragmentation without large scale tectonic transport the overthrust paleoproterozoic rocks represent a high grade northwestern extension of the flin flon volcanic belt and include 1856 ma leucotonalite and 1843 ma quartz monzodiorite an 1830 ma suite of homogeneous calc alkaline enderbitic rocks which intrude burntwood semipelitic migmatites throughout the kisseynew domain has also been emplaced in mylonites of the pelican decollement zone zircon and monazite ages in the 1812 1803 ma range record a high grade metamorphic event that resulted from collision between the sask craton and flin flon glennie complex

on the origin of early proterozoic gneiss domes and metamorphic nodes northern michigan biotite 40 r 39 r cooling ages from medium pressure 500 600 mpa rocks in the watersmeet district northern michigan suggest significant cooling uplift and concomitant deformation during gneiss dome formation at 1755 ma well after the close of the 1870 1830 ma penokean orogeny however an 1822 ma hornblende plateau date indicates that the isograds surrounding the dome are penokean in age we attribute gneiss dome formation and doming of penokean aged isograds to an episode of orogenic collapse superimposed on an earlier history of crustal shortening this contrasts with the compressional origin for gneiss domes preserved in the low pressure 200 300 mpa republic district the different origins may reflect the fact that collapse was localized along the overthickened region of the orogenic belt in contrast to the watersmeet area hornblende and biotite 40ar 39ar ages obtained from the republic area are 1720 1680 ma given the relatively shallow depth of this region it is unlikely that temperatures re

paleoproterozoic 1 90 1 86 ga arc volcanism in the flin flon belt trans hudson orogen canada geochemical and isotopic nd sr data are reported on paleoproterozoic 1904 1864 ma maficintermediate 1870 ma collision and subsequent intra arc deformation

growth stabilization and reactivation of proterozoic lithosphere in the southwestern united states growth of proterozoic continental lithosphere in the southwestern united states involved assembly of tectonostratigraphic terranes during several pulses of convergent tectonism ca 1 74 1 70 and 1 65 1 60 ga prograde metamorphism accompanied orogenic assembly and peak metamorphic conditions outlasted deformation regions now characterized by the highest metamorphic grades underwent slow isobaric cooling and were not uplifted until more than 200 m y after assembly regions of low metamorphic grade were not uplifted substantially after assembly we suggest that 1 relatively thin lithospheric fragments were assembled into isostatically stable normal thickness continental lithosphere 2 assembly did not erase lithospheric scale heterogeneities 3 the present juxtaposition of different crustal levels reflects differential uplift related to 1 4 1 1 ga tectonomagmatic activity and 4 the boundaries between different lithospheric blocks were repeatedly reactivated from precambrian through tertiary time

exhumation of a collisional orogen a perspective from the north american grenville province combined structural and geochronologic research in the southernmost portion of the contiguous grenville province of north america ontario and new york state show protracted periods of extension after the last episode of contraction the grenville province in this area is characterized by synorogenic extension at ca 1040 ma supported by u pb data on titanites and 40 ar 39 ar data on hornblendes followed by regional extension occurring along crustal scale shear zones between 945 and 780 ma as recorded by 40 ar 39 ar analysis of hornblende biotite and k feldspar by ca 780 ma the southern portion of the grenville province from ontario to the adirondack highlands underwent uplift as a uniform block tectonic hypotheses have invoked various driving mechanisms to explain the transition from compression to extension however such explanations are thus far geodynamically unconstrained numerical models indicate that mechanisms such as gravitational collapse and mantle delamination act over timescales that cannot explain a protracted 300 m y extensional history that is contemporaneous with ongoing uplift of the grenville province rather the presence of a plume upwelling underneath the laurentian margin combined with changes in regional stress directions permitted the observed uplift and extension in the grenville province during this time the uplift history while on a slightly different timescale from those of most plume models is similar to that seen in models of uplift and extension caused by the interaction of a plume with the base of the lithosphere some of the protracted extension likely reflects the contribution of farfield effects possibly caused by tectonic activity in other cratons within the rodinian supercontinent effectively changing the stress distributions in the grenville province of northeastern north america

slow post orogenic cooling in a deeply eroded orogen reindeer zone trans hudson orogen saskatchewan we present 40ar 39ar hornblende and muscovite data from a 200 km transect across the northern flank of the paleoproterozoic trans hudson orogen tho exposed along reindeer lake in saskatchewan canada the transect crosses a series of distinct lithotectonic domains that experienced pressure temperature p t conditions of 600 730 c at 4 5 6 0 kbar 1 kbar 100 mpa during d2 peak metamorphism at 1820 1790 ma the distribution of 40ar 39ar hornblende ages shows that most of the transect cooled through approximately 500 c by ca 1765 ma with two significant exceptions at the north end of the transect older ages in the peter lake domain up to 2200 ma suggest that this region did not experience high grade metamorphism that affected the rest of the transect and may indicate a structural break between these archean rocks and the cree lake zone to the north in the south central part of the transect u pb monazite titanite and 40ar 39ar hornblende muscovite ages indicate that rocks in the vicinity

geophysical transect across a paleoproterozoic continent continent collision zone the trans hudson orogen a summary and comparison of geophysical data and models for the trans hudson orogen in northern manitoba and saskatchewan are presented magnetic total field and bouguer gravity maps are used to define the along strike extension of geological domains of the orogen exposed on the canadian shield and a two dimensional density model is produced which accounts for the observed variations of the bouguer gravity field across the orogen an 800 km long crustal section across the entire continent continent collision zone including the edges of the bounding cratonic blocks is presented it incorporates seismic reflectivity seismic velocities resistivity and density models key results include 1 evidence for west vergent crustal stacking and exhumation in the eastern trans hudson orogen in the form of preserved moho topography and the presence of higher grade higher velocity rocks in the hanging wall of an east dipping crustal stack 2 definition of the eastward extent of the archean sask craton in the

comportement du systeme u pb dans la monazite et chronologie de la deformation et du metamorphisme des metasediments du domaine de kisseynew orogene trans hudsonien manitoba canada the kisseynew domain is the central unit of the reindeer zone of the paleoproterozoic trans hudson orogen in manitoba and saskatchewan the southern flank of the domain is a transition zone betwee

an improved u th pb age calculation for electron microprobe dating of monazite abstract determination of u th pb ages using the electron probe microanalyser epma is an inexpensive alternative method for dating monazite the method is rapid and reliable both for simple monogenenetic monazite and for complex polygenetic monazite having undergone metamorphic events involving fluid interaction and recrystallization the main limitation of the method is its rather poor precision i e 45 to 120 ma for ages ranging from 300 to 3000 ma calculated on each spot the precision is limited by pb content and by counting statistics which cannot give precision better than 2 for individual determinations on u th and pb even at high levels a procedure that uses the new th pb vs u pb diagram to improve the calculated precision on u th pb ages gives results to within 5 to 15 ma for ages ranging from 300 to 3000 ma with complex polygenetic monazite in which either the points show large scattering indicated by a large mswd or the regression line exhibits a slope very different from neighbour theoretical isochrons the procedure must be applied separately on homogeneous domains only this makes it possible to distinguish between events separated by a gap of 20 to 60 ma according to the range of ages concerned i e 300 to 3000 ma several examples are given to illustrate these systematics

lithospheric structure of the trans hudson orogen from seismic refraction wide angle reflection studies the trans hudson orogen tho is the world s largest paleoproterozoic orogenic belt data from three refraction profiles are used to investigate its lithospheric structure in saskatchewan and manitoba r1 crosses the orogen from the hearne craton on the west to the superior craton on the east r2 and r3 are along the orogen p wave velocity structural models are generated using a ray based technique on line r1 higher crustal velocities in its eastern part coincide with rocks of the flin flon namew gneiss complex depth to moho is in the 40 45 km range and equates to that from the reflection data including a small crustal root below the sask minicontinent along lines r2 and r3 depth to moho varies from about 40 km up to 55 km at the north end of r2 and south end of r3 in general variations in crustal velocity and depth to moho do not correlate with the location and extent of geological domains they appear to reflect the complex deformation and metamorphic history of the crustal rocks mantle velo

geodynamic evolution and thermal history of the central flin flon domain trans hudson orogen constraints from structural development 40ar 39ar and stable isotope geothermometry shear zones in the flin flon area record five episodes of development these have been studied using structural analysis and 40ar 39ar thermochronology deformation occurred as a result of collision between the archean rae hearne and archean superior plates during the trans hudson orogeny 1900 to 1700 ma the geodynamic evolution encompasses a transition from island arc supracrustal development 1900 ma to tectonic mountain building under brittleductile conditions 1860 ma two episodes of ductile lineation schistosity l s shear zones on the prograde metamorphic path 1860 1810 ma and two episodes of retrograde displacement between 1790 and 1690 ma during final suturing of the archean cratons with paleoproterozoic assemblages metamorphism evolved in three stages contact aureoles to granitoid bodies formed at 1860 1840 ma hornblende homfels facies 40ar 39ar ages indicate cooling through 500 c at 1840 ma this was followed by regional heating burial which produced regional peak thermal recrystallization at 1820 1790 ma under lower greenschist to lower amphibolite conditions lastly retrograde metamorphism took place during unroofing between 1790 and 1690 ma with a regional cooling rate of 2 c m y

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

a late miocene pliocene origin for the central himalayan inverted metamorphism abstract perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the himalayan main central thrust mct generally thought to have been active during the early miocene however in situ 208 pb 232 th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the mct footwall occurred in this portion of the central himalaya at only ca 6 ma the apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the mct which tectonically telescoped the young metamorphic sequence this explanation may resolve some outstanding problems in himalayan tectonics such why the mct and not the more recently initiated thrusts marks the break in slope of the present day mountain range it also renders unnecessary the need for exceptional physical conditions e g high shear stress to explain the generation of the himalayan leucogranites

core complex development in central anatolia turkey tectonic models for the central anatolian segment of the alpine himalayan orogen have not considered this region as part of the aegean extensional province we have found evidence however for oligocene miocene extension and exhumation of midcrustal rocks in a metamorphic dome nigde massif that is structurally and petrologically similar to miocene core complexes 500 km to the west strong correspondence in the timing and kinematic development of the menderes western turkey and nigde core complexes suggests that extensional tectonics and exhumation of mid to lower crustal rocks affected much of the eastern mediterranean region in the nigde massif supracrustal rocks were buried to depths of 16 20 km at high temperatures 700 c during contraction associated with closure of neo tethyan seaways in late mesozoic early cenozoic time following cooling and decompression to 600 c and 10 km metasedimentary rocks underwent a second heating event at low pressures during miocene magmatism that postdated much of the unroofing of the massif development of the nigde core complex was related to exhumation of thickened and thermally weakened continental crust in the upper plate of a north dipping subduction zone the partial subduction of the tauride carbonate platform in the eocene resulted in choking of the subduction zone followed by isostatic rebound and exhumation of the buoyant platform this in turn caused a rapid emergence of the upper plate resulting in erosion of upper crustal rocks and exhumation of midcrustal rocks along the northern edge of the inner tauride suture zone although northern regions of central anatolia contain the same protoliths as the nigde massif and underwent extensive tertiary magmatism they did not undergo similar extension suggesting that core complex development in south central turkey was controlled by the location of thickened and thermally weakened crust adjacent to a suture zone

early proterozoic deformation along the southern margin of the kisseynew gneiss belt trans hudson orogen a 30 ma progressive deformation cycle fold axes and lineations across the southern margin of the kisseynew gneiss belt were progressively reoriented from northwesterly to northeasterly during an early proterozoic southwest directed compressive deformation cycle partitioning of deformation resulted in a concentration of non coaxial strain along the boundary between the kisseynew gneiss belt and flin flon snow lake belt the structural evolution was complicated by streamlining of the movement vector around the comparatively rigid flin flon snow lake belt the relationship of metamorphic assemblages to kinematic fabrics indicates that deformation was coeval with a prograde retrograde metamorphic cycle progressive deformation occurred during a period of 30 ma which is consistent with phanerozoic orogenic time scales

a unified view of craton evolution motivated by recent deep seismic reflection and refraction results summary an attempt is made to integrate recent continental seismic reflection refraction and geologic observations into a unified scheme of craton evolution 1 nascent continental crust of basaltic bulk composition is produced in island arcs as with oceanic crust moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below the petrologic crust mantle boundary lying 25 per cent deeper in the lithosphere has no seismic expression and corresponds to a depositional magmatic contact between cumulate ultramafic rocks above and residual mantle ultramafic tectonite below both contacts are presumably extensively disrupted by magmatic injection 2 island arcs are amalgamated into continents at collision zones delamination of the lower mafic ultramafic portion of the crust along with the mantle lithosphere during hard collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition 3 subsequent extensional collapse of the overthickened crust together with thermal re equilibration of the lithosphere results in youthful stable continental crust which is 30 km thick with its top surface awash at sea level and has the bulk composition of andesite this crust characteristically exhibits extensional features half grabens in the upper portion and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination the moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high strain zone or intrusive contact separating mafic intermediate composition material above from fertile mantle lherzolite below 4 subsequent cratonization is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of myr as manifest by the ubiquitous overlapping dike swarms of the precambrian shields this results in net long term thickening of the crust by underplating shifts the bulk crust back toward mafic composition periodically produces a new deeper moho and petrologic crust mantle boundary that do not coincide and through repeated dike injection disrupts pre existing laminated reflectors the first magmatic injection event sweats out the light melting fraction in the pre existing andesitic crust producing voluminous granitic magmatism typified by the great proterozoic anorogenic granite rhyolite provinces subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences the cumulative result is shield type crust in which intermediate crustal levels are exposed over wide areas crustal thickness commonly exceeds 40 km average crustal velocity is somewhat higher than in younger stable crust and the crust is commonly complexly reflective diffractive throughout

microprobe monazite geochronology putting absolute time into microstructural analysis abstract high resolution compositional mapping and dating of monazite on the electron microprobe is a powerful addition to microstructural analysis and an increasingly important tool for tectonic analysis microprobe monazite geochronology can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains but more importantly its in situ nature and high spatial resolution offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on p t d paths constrain the rates of metamorphic and deformational processes and provide new links between metamorphism and deformation microprobe geochronology is particularly applicable to three persistent microstructural microtextural problem areas 1 constraining the chronology of metamorphic assemblages 2 constraining the timing of deformational fabrics and 3 interpreting other geochronological results although some monazite generations can be directly tied to metamorphism or deformation at present the most common constraints rely on monazite inclusion relations in porphyroblasts that in turn can be tied to the deformation and or metamorphic history microprobe mapping and dating allow geochronology to be incorporated into the routine microstructural analytical process resulting in a new level of integration of time t into p t d histories

evolution of the snow lake portion of the palaeoproterozoic flin flon and kisseynew belts trans hudson orogen manitoba canada abstract understanding growth of early precambrian crust requires knowledge of the evolution of granite greenstone and metasedimentary belts two of the fundamental building blocks of early precambrian cratons u pb geochronologic data from the snow lake area which straddles the flin flon granite greenstone and kisseynew metasedimentary belts of the trans hudson orogen documents a 60 70 ma period of palaeoproterozoic crust formation from inception of these belts in ocean basins through their eventual structural telescoping during ocean basin consumption and collision with bounding archaean cratons earliest volcanic and plutonic rocks of the flin flon granite greenstone terrane at snow lake formed 1 89 ga mainly in a juvenile oceanic arc tectonic setting sedimentary rocks of the kisseynew belt previously thought to be contemporaneous with snow lake area arc volcanism are up to 30 50 ma younger 1 86 1 84 ga and postdate deformation associated with 1 88 1 87 ga arc arc collisions and 1 87 1 84 ga calc alkaline plutonism that affected much of the western flin flon belt deformation at snow lake occurred during two widely spaced episodes both of which effect the post 1 86 1 84 ga turbidite sequence and thus postdate 1 88 1 87 ga intraoceanic accretion in the western flin flon belt the older deformational episode involved isoclinal folding formed during 1 84 ga convergence of the kisseynew and flin flon belts and thrusting of kisseynew sedimentary lithologies southwards onto the flin flon volcanic rocks the younger deformational episode occurred 15 25 ma later and involved further folding and southwest thrusting accompanied by a 1 81 ga high grade thermal metamorphic episode centred in the kisseynew belt only minor increase of crustal material in the form of late intrusions were added following this episode

geology and thermotectonic evolution of the western margin of the trans hudson orogen evidence from the eastern sub athabasca basement saskatchewan in the cree lake zone of northern saskatchewan reworked archean orthogneisses are overlain by a highly deformed supracrustal sequence the paleoproterozoic wollaston group this package of rocks was deformed and metamorphosed during the ca 1 8 ga continent continent collision of the trans hudson orogen tho forming the wollaston fold thrust belt that underlies the eastern athabasca basin the hudsonian structural metamorphic and magmatic evolution of the wollaston fold thrust belt in the eastern athabasca area involved six major stages 1 early collisional stage dp1 at 1860 1835 ma involved burial of wollaston group metasediments from surface to depths equivalent to 3 5vkbar 1 kbar 100 mpa by thrust pile stacking or imbrication tectonics prograde metamorphism with garnet growth and development of early leucosomes and emplacement of ca 1840 ma grey granite suite 2 collisional stage dp2a at 1835 1820 ma involved continued deeper burial of wollaston group metasediments along a prograde

high temperature low pressure metamorphism in the kisseynew domain trans hudson orogen crustal anatexis due to tectonic thickening the kisseynew domain within the central trans hudson orogen constitutes a high temperature t and low to medium pressure p metasedimentary belt characterized by uniform peak metamorphic conditions of 750 50 c and 5 6 kbar 1 kbar 100 mpa to investigate the thermal evolution of the kisseynew domain 1 dimensional thermal modelling was conducted for a range of thermal parameters including the effects of exhumation and incorporating current information about the crustal architecture the results show that tectonic thickening of the sedimentary pile to 40 km consistent with present day thickness estimates and erosional levels is capable of producing enough crustal heating to melt the base of the sedimentary pile advect heat to shallower crustal levels and produce the observed p t conditions deep seated mantle heat sources though not excluded are not required

the pelican thrust zone basal detachment between the archean sask craton and paleoproterozoic flin flon glennie complex western trans hudson orogen the pelican thrust zone is a 3 7 km wide recrystallized mylonite zone along which paleoproterozoic arc volcano plutonic and derived sedimentary rocks of the flin flon glennie complex were thrust over an archean package jan lake complex consisting of ca 3 1 ga calc alkaline orthogneisses pelitic migmatites and a ca 2 45 ga tholeiitic charnockite norite intrusive suite a regional northeast plunging stretching lineation and a variety of kinematic indicators imply southwesterly transport matching that of other coeval shallower crustal level structures observed throughout the northern flin flon domain and southern flank of the kisseynew domain e g sturgeon weir and annabel lake shear zones subsequent east side up displacement on the tabbernor fault together with domal fold interference has exposed the pelican window and mantling mylonite zone where it has been seismically profiled along lithoprobe transects tectonic windows also expose archean rocks in the glennie domain to the west where similar southwest verging recrystallized mylonite zones have been documented together these zones record collision and underplating of the flin flon glennie complex by the archean sask craton during prolonged protocontinent continent collision over a minimum 1826 1805 ma interval resume la zone de chevauchement pelican est une zone de mylonite recristallisee d une largeur de 3 7 km le long de laquelle des roches volcano plutoniques d un arc datant du paleoproterozoique et des roches sedimentaires derivees du complexe de flin flon glennie ont ete chevauchees par dessus un ensemble archeen le complexe de jan lake qui est compose d orthogneiss calco alcalins et de migmatites pelitiques gees d environ 3 1 ga ainsi que d une suite intrusive charnockite norite tholeiitique gee d environ 2 45 ga une lineation regionale d etirement a pendage nord est et divers indicateurs cinematiques correspondent a un transport vers le sud ouest rejoignant celui d autres structures contemporaines de niveaux crustaux moins profonds observees a travers tout le domaine de flin flon nord et le flanc sud du domaine kisseynew p ex les zones de cisaillement de sturgeon weir et d annabel lake un deplacement subsequent le cote est vers le haut le long de la faille de tabbernor avec une interference de plis de dome a fait affleurer la fenetre pelican et la zone mylonitique qui la recouvre elle a ete determinee par des profils sismiques le long des transects lithoprobe des fenetres tectoniques permettent aussi l affleurement de roches archeennes du do maine de glennie vers l ouest la ou ont ete documentees des zones semblables de mylonites recristallisees a pendage sud ouest ensemble ces zones enregistrent la collision et le sous placage du complexe de flin flon glennie par le craton archeen sask au cours d une collision prolongee entre le protocontinent et le continent au cours d un intervalle d au moins 1826 a 1805 ma

incremental heating of hornblende in vacuo implications for 40ar 39ar geochronology and the interpretation of thermal histories increasing evidence of the incompatibility between the geologic thermal histories deduced from the sup 40 ar sup 39 ar incremental heating technique and those derived form other methods have necessitated a careful evaluation of the assumptions inherent in the former approach sup 40 ar sup 39 ar age spectra are frequently used to infer the spatial distribution of ar in hydrous minerals and as such require that the mineral remains stable while heated in vacuo aliquots of mmhb 1 and inter laboratory hornblende standard used for k ar and sup 40 ar sup 39 ar dating were heated in vacuo at temperatures of 500 700 930 and 1 050c for 30 min up to 700c no physical changes in the hornblende were observed with the scanning electron microscope although x ray diffraction data indicate that it had dehydroxylated to oxyhornblende at this temperature the onset of the structural decomposition of the oxyhornblende as revealed by backscattered and secondary electron images occurs at approximately 930c and is directly correlated with the initial release of 6 of its radiogenic ar sup 40 ar the breakdown of the oxyhornblende by the internal nucleation of incongruent melt is almost 93 complete by 1 050c the reaction products being titano magnetite melt a pyribole phase and internal voids because ar is released simultaneously frommore these products and is subsequently homogenized in vacuo this breakdown can yield plateau age spectra regardless of any original ar spatial gradients in the mineral consequently independent evidence may be required to establish whether a plateau date from hornblende is truly a geologically significant age less

archean rocks in the southern rottenstone domain significance for the evolution of the trans hudson orogen an augened strongly flasered and multiply folded monzonitic gneiss occurs in a structural dome in the black bear island lake region of northern saskatchewan within the ca 1850 ma paleoproterozoic southern rottenstone domain of the trans hudson orogen tho a sample of this rock has yielded thermal ionization mass spectrometry tims and sensitive high resolution ion microprobe shrimp zircon data consistent with a formation age of at least 2500 ma zircons also show somewhat younger ca 2380 ma overgrowths indicating a complex history the nd model age tdm of 2726 ma also confirms the late archean age of the rock as does the isotopic composition of common pb from a k feldspar sample u pb analyses of titanites yield ages of ca 1800 ma indicating recrystallization during terminal closure of the tho it is unlikely that the archean rocks are a part of the archean sask craton in the glennie domain for lithoprobe seismic sections indicate that the sask craton dips westward beneath the la ronge a

core complex model proposed for gneiss dome development during collapse of the paleoproterozoic penokean orogen minnesota gneiss domes preserved throughout the paleoproterozoic 1870 1820 ma penokean orogenic belt are commonly viewed as collisional features however 1700 ma mica 40ar 39ar ages from archean rocks of the mcgrath gneiss dome minnesota are in marked contrast with 1750 1760 ma mica ages obtained everywhere outside the dome suggesting the dome may have formed well after compression had ceased we hypothesize that the bimodal age pattern is a result of crustal excision between archean basement and paleoproterozoic cover during extensional collapse of the overthickened orogen removal of structural section and progressive unroofing may have led to the development of a domed basement cover shear zone in this respect the mcgrath gneiss dome would represent one of the oldest analogues of a metamorphic core complex postorogenic collapse of the penokean orogen was apparently long lived with significant pulses occurring at 1755 ma and at 1700 ma the sudden and widespread crustal fusion at 1770 1760 ma that preceded the proposed collapse suggests that rapid subcrustal lithospheric thinning may have triggered collapse as has been proposed recently for several phanerozoic collapsed orogens

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

regional setting and kinematic features of the needle falls shear zone trans hudson orogen needle falls shear zone is the southern part of a major northeast trending ductile shear system within the paleoproterozoic trans hudson orogen in saskatchewan throughout its exposed length of 400 km the shear zone separates reworked archean continental crust and infolded paleoproterozoic supracrustals of the cree lake zone to the northwest from mainly juvenile paleoproterozoic arc terrains and granitoid plutons of the reindeer zone to the southeast it also defines the northwest margin of the ca 1855 ma wathaman batholith which forms the main protolith to shear zone mylonites although not precisely dated available age constraints suggest that the shear zone formed between ca 1855 and 1800 ma toward the end of peak thermotectonism in this part of the orogen in the needle falls study area shear zone mylonites exhibit varied sequentially developed ductile to brittle fabric features including c s fabrics winged porphyroclasts especially delta type small scale compressional and extensional

crustal structure and evolution of the trans hudson orogen results from seismic reflection profiling a 400 km long deep seismic reflection transect across northeastern montana and northern north dakota reveals the crustal scale structural fabric of the early proterozoic trans hudson orogen beneath the williston basin comparison with deep seismic reflection data across the canadian portion of the same orogen 700 km to the north reveals first order similarities in crustal architecture but documents significant along strike variation in orogenic evolution both transects display a broad crustal scale antiform axial to the orogen in the north geologic data suggest that this antiform is cored by an archean microcontinent in the south west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the archean wyoming craton within the wyoming craton the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the early proterozoic trans hudson orogen as interpreted from potential field and drill core data on the basis of subsurface geochronologic data from the crystalline basement and by analogy with the glennie domain within the exposed trans hudson orogen in canada we suggest that the southern antiform is cored by an archean crustal fragment that was caught up in the terminal collision of the wyoming and superior cratons during hudsonian orogeny the eastern side of the trans hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections we interpret the easterly dip of these reflections as evidence that the superior province was thrust westward over the interludes of the orogen during terminal collision although juvenile early proterozoic terranes characterize the exposed segment of the trans hudson orogen in canada limited drill core information within the dakota segment of the orogen shows a predominance of granulitic archean age crust this difference in basement lithologies along strike within the orogen may indicate the following either juvenile crust comparable to that exposed in the northern trans hudson was never present in the south or it was removed by progressive over thickening erosion and or faulting postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north

isostatic constraints on the assembly stabilization and preservation of cratonic lithosphere cratonic uplift or subsidence as recorded geologically by erosion or deposition indicates the net change in lithospheric buoyancy density change is primarily controlled by temperature change such that the geologic record reveals the net lithospheric heating or cooling our modeling shows that thermal stabilization of lithosphere can occur in 100 m y largely determined by the time for crustal radioactivity to warm the entire lithosphere longer term thermal evolution 350 400 m y is controlled by the feedback between isostatic and thermal responses crustal heat production is the dominant factor for evolution of crustal geo therms whereas the detailed initial thermal structure is less important the fact that many cratons are eroded to only modest depths indicates that they have a history of warming slightly since their formation implying a rather cool 700 1000 c for initial lithospheric mantle this is consistent with the growth of cratonic lithosphere by progressive thickening during continental assembly and or subduction stacking of oceanic lithosphere relatively warm early lithosphere due to elevated crustal heat production or higher initial lithospheric mantle temperatures may have had a lower preservation potential due to preferential deformation and destruction study of the proterozoic orogen of the southwestern united states indicates that the distinct thermal and isostatic records of adjacent crustal blocks are compatible with observed differences in crustal heat production but also with similar initial lithospheric mantle temperatures

upb geochronology and tectonic development of the southern flank of the kisseynew domain trans hudson orogen canada abstract the southern flank of the kisseynew domain northern manitoba underwent polyphase deformation and metamorphism during the palaeoproterozoic trans hudson orogen and represents a lithological metamorphic and structural transition from greenschist grade volcano plutonic rocks of the flin flon domain to amphibolite grade paragneisses of the central kisseynew domain the timing of geological events has previously been constrained using widely dispersed sample sites but this study has integrated u pb geochronology with detailed lithological and structural mapping in the cleunion lake area pink gneisses interpreted as missi supracrustal rocks were deposited after 1855 ma but prior to 1835 ma and thus are similar in age to fluvial sedimentary rocks in the lower grade flin flon domain all the zircons in the pink gneisses appear to be inherited or resorbed during later amphibolite grade metamorphism and a late archaean component is also indicated corundum bearing gneisses in the missi group and leucosomes in burntwood river group rocks are deformed by f1 and are indicative of an early high t low p metamorphic event m1 that may have been synchronous with the intrusion of 1835 to 1827 ma intrusions and related to regional crustal extension the above rocks are folded during a continuous compressional deformation event d2 related to terminal collision between the palaeoproterozoic rocks of the trans hudson orogen and the superior province three main fold generations f1 f2 f3 have been identified which occurred during d2 a 1818 ma granitic vein is approximately synchronous with f2 and peak metamorphic m2 conditions in the cleunion lake area cooling uplift and intrusion of pegmatites during f3 at 1800 ma provides a time limit on the waning stages of compressional deformation the main phase of compressional deformation d2 in the southern flank of the kisseynew domain thus lasted for about 30 million years

kisseynew metasedimentary gneiss belt trans hudson orogen canada back arc origin and collisional inversion the kisseynew metasedimentary gneiss belt initially developed in a back arc setting the ages of detrital zircons and crosscutting plutons constrain turbidite sedimentation to 1 855 1 841 ga postdating early deformation and metamorphism associated with arc arc and arc continent collisions affecting much of the trans hudson orogen these turbidites are interpreted to have been deposited in a back arc basin behind a retreating subduction boundary the kisseynew domain and its southern flank are marked by 1 84 1 83 ga magmatic rocks that may be related to subduction of back arc basin oceanic crust collapse of the kisseynew basin by ductile fold and thrust deformation began during 1 84 1 83 ga magmatism and continued through peak metamorphism at ca 1 820 1 805 ga from authors

evolution of nd and pb isotopes in central pacific seawater from ferromanganese crusts abstract hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts by analysing nd pb and be isotope profiles within crusts it is possible to reconstruct seawater tracer histories depth profiles of 10be 9be ratios in three pacific ferromanganese crusts have been used to obtain growth rates which are between 1 4 and 3 8 mm ma nd and pb isotopes provide intact records of isotopic variations in pacific seawater over the last 20 ma or more there were only small changes in pb isotope composition in the last 20 ma this indicates a constant pb composition for the erosional sources and suggests further that erosional nd inputs may have been uniform too end values vary considerably with time and most probably reflect changes in ocean circulation the end values of the crusts not only vary as a function of age but also as a function of water depth from 25 to 0 ma crust va13 2 from 4 8 km water depth has a similar pattern of end variation to the two shallower crusts from 1 8 and 2 3 km but about 1 0 to 1 5 units more negative this suggests that end stratification in pacific seawater as demonstrated for the present day has been maintained for at least 20 ma each crust shows a decrease in end from 3 5 ma to the present which is interpreted in terms of an increase in the nadw component present in the pacific from 10 to 3 5 ma ago the crusts show an increase in end this suggests a decreasing role for a deep water source with end less than circum pacific sources in this regard the panamanian gateway restriction from 10 ma with final closure at 3 5 ma may have played an important role in reducing access of atlantic derived nd to the pacific

u pb zircon ages from the lynn lake and rusty lake metavolcanic belts manitoba two ages of proterozoic magmatism two ages of magmatism have been determined from zircon in felsic flows and plutons in the churchill province of manitoba a rhyolite flow from the lynn lake metavolcanic belt has a u pb age of and a rhyolite flow from the adjacent rusty lake metavolcanic belt has an age of tonalite and quartz diorite from two composite plutons emplaced into the volcanic rocks at lynn lake have ages of and indistinguishable from the age of the rusty lake belt rhyolite the arcuate domain of metavolcanic rocks that includes the rusty lake belt in the southeast the lynn lake belt in the north and the la ronge belt saskatchewan in the southwest has previously been considered a single structural sub province with similar ages throughout our results and published u pb ages from saskatchewan indicate that an older magmatism is represented by volcanic rocks in the lynn lake belt a younger magmatism by volcanic rocks in the rusty lake and la ronge belts and plutons in the lynn lake belt at lynn lake the older magmati

tectonic implications of new shrimp and tims u pb geochronology of rocks from the sask craton peter lake domain and hearne margin trans hudson orogen saskatchewan this study reports new sensitive high resolution ion microprobe shrimp and thermal ionization mass spectrometry tims u pb geochronological data for thirteen rocks from the archean to paleoproterozic sask craton the peter lake domain and hearne margin saskatchewan seven samples from the exposed sask craton in both the glennie domain and pelican window record zircon growth between 2425 and 2525 ma and at 1800 ma older precursor material has been identified most notably a refined crystallization age of 3117 ma for a sample of quartzofeldspatic gneiss q gneiss in the pelican window which also experienced ca 2450 ma zircon growth three samples from the peter lake domain and a sample from the linn island inlier of the wollaston domain yield ages of 2575 ma one sample from the peter lake domain is distinctly older yielding an age of 2640 ma but also displays zircon growth at 2575 ma the simplest interpretation of these data is that the peter lake domain is likely a part of the hearne margin

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

the age of unconformity related uranium mineralization in the athabasca basin northern saskatchewan age data are presented for major athabasca basin uranium deposits at cigar lake cluff lake collins bay dawn lake eagle point mcarthur river midwest and rabbit lake as well as for several minor or undeveloped deposits including hughes lake and nisto the best constrained data indicate that almost all the deposits formed in a restricted time interval between about 1330 and 1380 ma this range of ages is believed to be real and not the result of uncertainties in the calculation of ages based on discordant data the one major exception is the recently discovered nias free deposit at mcarthur river for which a well determined age of 1514 18 ma 2 has been obtained even this deposit yields an age in the1330 1380 ma range for some material periods of reworking redeposition occurred at 1280 1000 575 and 225 ma these may be basin wide affecting to some degree all the deposits that we have studied other times of redeposition are less well determined but may be present as well no ages tha

proterozoic collisional tectonism in the trans hudson orogen saskatchewan isotopic and structural data from the juvenile reindeer zone of the trans hudson orogen northern saskatchewan indicate a pre 1 85 ga thermotectonic event possibly reflecting arc continent collision followed by a more extensive nappe forming ca 1 83 1 80 ga thermotectonism during terminal continent continent collision preliminary data from the adjacent ensialic cree lake zone suggest high grade reworking of archean crust by the pre 1 85 ga event in the rae province to the west high grade metamorphism and reworking of archean crust occurred about 2 0 ga and may be related to the formation of the coeval taltson magmatic zone

seismic reflection images of high angle faults and linked detachments in the trans hudson orogen postcollisional 1 8 1 7 ga intracontinental deformation in the trans hudson orogen canada produced a series of orogen parallel high angle faults and folds in seismic reflection profiles the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4 8 s 12 24 km the folded and faulted upper part of the crust is underlain by laterally coherent shallow dipping reflections that are locally bounded by discrete highly reflective zones these zones are interpreted as detachments shear zones and can be traced from the upper to lower crust where some of them appear to pass into laterally continuous reflections that define the moho two distinct regimes of postcollisional crustal deformation are inferred from the seismic images high angle faulting and lateral block extrusion in the upper crust and low angle ductile shearing in the mid lower crust the surface geology indicates that the faults resulted in southwest orogen parallel extrusion of the orogen s internal zone relative to the bounding archean hearne and superior cratons faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction the seismic images suggest that the high angle fold fault structures are kinematically linked to low angle detachments represented by laterally coherent highly reflective zones the detachment shear zones are inferred to have a top to the southwest sense of shear associated with a subhorizontal northeast southwest extension direction parallel to those observed for 1 83 1 80 ga collisional shear zones exposed in major postcollisional fold culminations long lived orogen parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the superior and hearne margins

constraints on pb closure temperature in titanite based on rocks from the ungava orogen canada implications for u pb geochronology and p t t path determinations on the basis of multiequilibrium metamorphic pressure temperature p t estimates we have determined that 660 700 c is a reasonable minimum estimate of the closure t of pb diffusion in titanite a temperature significantly higher than previously suggested activity corrected reactions that produce titanite define narrow t stability fields 40 c that correspond closely to metamorphic p t determinations derived for coronitic garnet clinopyroxene hornblende samples growth of titanite occurring between 1789 and 1814 ma in the underthrust archean basement of the paleoproterozoic ungava orogen northern quebec is associated with retrograde metamorphism during underthrusting of the archean basement approx 1 b y after formation of the original granulitic assemblages

partial resetting of the upb isotope system in monazite through hydrothermal experiments an sem and upb isotope study abstract natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their u pb isotope systematics measured before and after the runs runs were carried out with monazite grain size fractions ranging from 40 to 125 m in diameter and on 207 pb 206 pb ages close to 380 ma u concentrations remained always unchanged all recrystallized monazites from the powdered starting material define a discordia with intercept ages of 379 3 2 3 and 7 18 ma the latter indicating the hydrothermally induced pb loss at the time of the experiment sem observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces run products from monazite powders showed recrystallization and grain growth induced by a dissolution precipitation mechanism increasing temperature resulted in a distinct enlargement of grain sizes at 650 and 750 c a dissolution precipitation process in presence of a fluid phase rather than pb volume diffusion acts as a very efficient mechanism for the resetting of the u pb isotope system of monazites even at low temperatures this process is most important in shear zones where channelized fluid flow may occur

40ar 39ar evidence for middle proterozoic 1300 1500 ma slow cooling of the southern black hills south dakota midcontinent north america implications for early proterozoic p t evolution and posttectonic magmatism 40ar 39ar total gas and plateau dates from moscovite and biotite in the southern black hills south dakota provide evidence for a period of middle proterozoic slow cooling early proterozoic 1600 1650 ma mica dates were obtained from metasedimentary rocks located in a synformal structure between the harney peak and bear mountain domes and also south of bear mountain metamorphic rocks from the dome areas and undeformed samples of the 1710 ma harney peak granite hpg yield middle proterozoic mica dates 1270 1500 ma two samples collected between the synform and bear mountain dome yield intermediate total gas mica dates of 1550 ma we suggest two end member interpretations to explain the map pattern of cooling ages 1 subhorizontal slow cooling of an area which exhibits variation in mica ar retention intervals or 2 mild folding of a middle proterozoic 1500 ma 300 c isotherm according to the second interpretation the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure and downwarping of relatively cold rocks during a period of approximately east west contraction and slow uplift during the middle proterozoic the mica data together with hornblende data from the black hills published elsewhere indicate that the ambient country rock temperature at the 3 4 kbar depth of emplacement of the hpg was between 350 c and 500 c suggesting that the average upper crustal geothermal gradient was 25 40 c km prior to intrusion the thermochronologic data suggest hpg emplacement was followed by a 200 m y period of stability and tectonic quiescence with little uplift we propose that crust thickened during the early proterozoic was uplifted and erosionally thinned prior to 1710 ma and that the hpg magma was emplaced into isostatically stable crust of relatively normal thickness we speculate that uplift and crustal thinning prior to hpg intrusion was the result of differential thinning of the subcrustal lithosphere beneath the black hills if so this process would have also caused an increase in mantle heat flux across the moho and triggered vapor absent melting of biotite to produce the hpg magma this scenario for posttectonic granite generation is supported in part by the fact that in the whole of the black hills the hpg is spatially associated with the deepest exposed early proterozoic country rock

u pb geochronology in the trans hudson orogen northern saskatchewan canada we have obtained u pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern trans hudson orogen in northern saskatchewan these data constrain the timing of early proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the trans hudson orogen to other early proterozoic orogens in north america with one exception all units studied so far yield zircon ages of 1890 1835 ma most of which are systematically earlier than previously reported rb sr isochron ages on the same or similar units suggesting open system behavior in the rb sr systems five metarhyolites from volcanic sequences in the la ronge domain glennie domain and hanson lake block give ages ranging from 1888 to 1876 ma most of the plutons we dated ranging from gneissic syntectonic tonalites and granodiorites to less deformed late intrusions such as the wathaman batholith and other smaller bodies yield ages of 18

40 ar 39 ar dating and mineral paragenesis for carlin type gold deposits along the getchell trend nevada evidence for cretaceous and tertiary gold mineralization the orebodies at the getchell and twin creeks mines were studied through mineral paragenesis geologic relationships and 40 ar 39 ar dating mineral paragenetic relationships are based on observations made during the logging of 18 000 m of drill cuttings and core and crosscutting relationships recognized in the field and main pit at the getchell mine ages for igneous and mineralizing events were determined through 40 ar 39 ar incremental heating analyses of 15 samples of biotite k feldspar sericite and vein adularia a thermal history for the area was also developed using k feldspar multiple diffusion domain results and age determinations on cogenetic minerals which have different argon closure temperatures mineral paragenetic relationships and 40 ar 39 ar dating at the getchell and twin creeks deposits document that gold mineralization occurred in five stages between 95 and 42 ma stage 1 pyrrhotite quartz arsenopyrite chalcopyrite biotite gold mineralization in skarn is associated with 95 ma dacite dikes and granodiorite plugs stage 2 chalcopyrite sphalerite galena pyrite arsenopyrite or gold and silver mineralization in skarn is related to the 92 ma osgood mountains stock secondary k feldspar and sericite in stage 3 quartz pyrite kaolinite gold mineralization formed at 83 ma and are not related to 95 or 92 ma igneous activity stage 4 low grade quartz pyrite gold mineralization formed at about 75 ma in the matrix of a breccia pipe that cuts 83 ma silicic and argillic ores adularia intergrown with stibnite orpiment pyrite quartz gold or realgar mineralization yields precise 40 ar 39 ar plateau ages of 42 11 or 0 43 and 41 90 or 0 25 ma for stage 5 this study documents the temporal complexity of mineralization and igneous activity at the getchell and twin creeks deposits minor gold in skarn type mineralization stages 1 and 2 associated with 95 and 92 ma igneous intrusions preceded and was not genetically related to carlin type gold mineralization stages 3 and 5 the carlin type deposits at getchell and twin creeks therefore represent a composite orebody formed by the overprinting of 83 ma stage 3 quartz pyrite gold by 42 ma stage 5 orpiment stibnite pyrite gold mineralization

nd isotope and geochemical constraints on the depositional setting of paleoproterozoic metasedimentary rocks along the margin of the archean hearne craton saskatchewan canada abstract the cree lake zone in the southwestern part of the hearne craton reworked during the paleoproterozoic trans hudson orogeny includes the wollaston group a sequence of paleoproterozoic metasedimentary rocks as these rocks have been multiply deformed and metamorphosed to high grade a geochemical and isotopic study was initiated to see through the overprinting events to better constrain the provenance and evolution of the wollaston group the wollaston group consists of thick sedimentary successions representing different depositional and tectonic environments that can be subdivided into two subgroups the lower subgroup is represented by deep water turbidite to shallow water marine deposits the upper subgroup separated from the lower one by a regional unconformity comprises molasse type assemblages ranging from talus to shallow marine lacustrine with subordinate volcanogenic rocks major and trace element data obtained from 31 psammopelitic rocks indicate that the clastic material was derived from sources consisting of both felsic and mafic material although felsic sources are more important in the samples from the lower subgroup sedimentary tectonic discrimination diagrams demonstrate that most of the wollaston group was deposited on an active continental margin rather than a passive margin the e nd values for the wollaston group psammopelites calculated at 1 92 ga the approximate age of the boundary between the lower and upper subgroups vary from 3 4 to 6 8 which yields an approximate 70 30 mixture between juvenile paleoproterozoic detritus and archean detritus from the hearne craton a secondary source of detritus may be 1 95 2 0 ga rocks of the taltson orogen which is exposed to the west of the archean rae hearne craton the wollaston group records the complete evolution of the western margin of the trans hudson orogen during the paleoproterozoic the rifting of an archean continent at ca 2 1 ga led to the deposition of the passive margin siliciclastic sediments comprising the lowermost but poorly preserved part of wollaston group the formation of a continental arc rottenstone along the hearne margin at ca 1 92 ga resulted in a change in tectonic environment from a passive margin to a back arc basin and detritus in most of the lower subgroup was then derived from both basement and newly formed magmatic arc collision with the juvenile la ronge arc led to uplift along the hearne margin which transformed the back arc basin into a retroarc foreland basin at ca 1 88 ga sedimentary starvation and shallowing of the basin led to widespread accumulation of carbonate and evaporite deposits in the west whereas rapidly uplifted orogenic belts in the east provided sources of coarse grained clastic detritus forming the wollaston upper subgroup closure and inversion of the sedimentary basin occurred by ca 1 86 ga coincident with the emplacement of mafic to felsic intrusive rocks in the cree lake zone and the emplacement of the voluminous ca 1860 ma andean type wathaman batholith into the continental margin closure was manifested by the onset of foreland directed thrusting of the wollaston group