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Cape Kiber on the Arctic coast of Chukotka, northeast Russia, consists of a granite intruding Devonian (and older?) strata in the core of a large southeast trending anticline. These strata are structurally overlain by Carboniferous and younger strata. AU-Pb age of 351.4 ±5.6 (2σ)Ma shows that the granite is Early Carboniferous in age. A large granite cobble extracted from a Carboniferous conglomerate produced a Late Devonian or Early Carboniferous U-Pb age of ~355-361 Ma. Also, a deformed and altered granitic dyke yielded an age of 363.7 ± 5.7 (2cr) Ma. Major and trace elements suggest a syn-collisional (orogenic) setting. The granite's (biotite) Ar release spectrum is reset. The granitic dyke also shows a disturbed Ar-Ar whole-rock spectrum implying an Early Cretaceous age (~122-130 Ma) for closure of the Ar system. We interpret this as due to widespread greenschist metamorphism accompanying regional deformation of the Anyuy-Chukotka Fold Belt that accompanied closure of the South Anyuy Ocean. Regionally, this event predates deposition of Aptian and Albian strata and the eruption of Okhotsk--Chukotsk magmatic rocks. An Ar-Ar (biotite) plateau age of 96.4 [+ or -]1.0 (2σ) Ma from a mildly deformed, lamprophyre dyke reflects intrusion in a setting of regional extension. Its deformation reflects a younger tectonic event. The record of Devonian--Carboniferous magmatism and early Carboniferous unroofing is younger and less complex than that of Arctic Alaska. However, evidence for Early Devonian (Caledonian) or Late Devonian (Ellesmerian) deformation could have been masked by intense Mesozoic deformation. Outcrop data and geochronology support and refine regional interpretations of Early Cretaceous deformation and mineral growth accompanying accretion of Chukotka to north Asia, followed by regional extension and subsequent convergent deformation. Resume : Le cap Kiber sur la cote arctique de Chukotka, dans le nord-est de la Russie, est forme d'un granite qui penetre des strates datant du Devonien (et plus anciennes?) dans le creur d'un grand anticlinal a pendage vers le sud-est. Ces strates sont recouvertes de maniere structurale par des strates datant du Carbonifere et plus recentes. Un age, determine par U-Pb, de 351,4 ± 5,6 (2σ) Ma montre que le granite date du Carbonifere precoce. Un gros caillou de granite, extrait d'un conglomerat datant du Carbonifere, a donne un age U-Pb de Devonien tardif ou de Carbonifere precoce de ~355-361 Ma. De plus, un dyke de granite deforme et altere a donne un age de 363.7 ± 5.7 (2σ) Ma. Les elements majeurs et en trace suggerent un environnement de collision synchrone (orogenique). Le spectre du relachement d'Ar du granite (biotite) est reinitialise. Le dyke de granite montre aussi un spectre Ar-Ar perturbe de la roche entiere, ce qui signifierait un age Cretace precoce (environ 122-130 Ma) pour la fermeture du systeme Ar. Nous interpretons cela comme etant du a un metamorphisme au facies des schistes verts etendu qui accompagne la deformation regionale de la zone de plissements Anyuy-Chukotka, laquelle accompagne la fermeture de l'ocean Anyuy sud. A l'echelle regionale, cet evenement a eu lieu avant la deposition des strates aptiennes et albiennes et l'eruption des roches magmatiques d'Okhotsk-Chukotsk. Un age plateau de 96,4 ± 1,0 (2σ) Ma, determine par Ar-Ar (sur une biotite) d'un dyke de lamprophyre legerement deforme, correspond a une intrusion dans un environnement d'extension regionale. Sa deformation est le reflet d'un evenement tectonique plus jeune. L'enregistrement de magmatisme au Devonien-Carbonifere et le retrait du toit au Carbonifere precoce est plus recent et moins complexe que celui de l'Alaska arctique. Toutefois, les evidences pour une deformation au Devonien precoce (Caledonien) ou au Devonien tardif (Ellesmerien) pourraient avoir ete masquees par une deformation intense au Mesozoique. Les donnees provenant des affleurements et de la geochronologie supportent et peaufinent les interpretations regionales d'une deformation et d'une croissance mineralogique au Cretace precoce qui accompagnaient l'accretion de Chukotka a l'Asie septentrionale; cette accretion a ete suivie d'une extension regionale et d'une deformation subsequente de convergence. [Traduit par le Redaction], Introduction As part of the Canada-Russia Science and Technology Exchange Agreement, co-operative field projects were undertaken to clarify the distribution and correlation of circum-Arctic Phanerozoic strata. One project, in 1989, [...]

The Hardangervidda-Rogaland Block within southwest Norway is host to ~1.52 to 1.48 Ga continental building and variable reworking during the ~1.1 to 0.9 Ga Sveconorwegian orogeny. Due to the lack of geochronological and geochemical data, the timing and tectonic setting of early Mesoproterozoic magmatism has long been ambiguous. This paper presents zircon U-Pb-Hf-O isotope data combined with whole-rock geochemistry to address the age and petrogenesis of basement units within the Suldal region, located in the centre of the Hardangervidda-Rogaland Block. The basement comprises variably deformed grey gneisses and granitoids that petrologically and geochemically resemble mature volcanic arc lithologies. U-Pb ages confirm that magmatism occurred from ~1,521 to 1,485 Ma, and conspicuously lack any xenocrystic inheritance of distinctly older crust. Hafnium isotope data range from eHf(initial) +1 to +11, suggesting a rather juvenile magmatic source, but with possible involvement of late Palaeoproterozoic crust. Oxygen isotope data range from mantle-like (δ[sup.18]O ~5 ‰) to elevated (~10 ‰) suggesting involvement of low-temperature altered material (e.g., supracrustal rocks) in the magma source. The Hf-O isotope array is compatible with mixing between mantle-derived material with young low-temperature altered material (oceanic crust/sediments) and older low-temperature altered material (continent-derived sediments). This, combined with a lack of xenoliths and xenocrysts, exposed older crust, AFC trends and S-type geochemistry, all point to mixing within a deep-crustal magma-generation zone. A proposed model comprises accretion of altered oceanic crust and the overlying sediments to a pre-existing continental margin, underthrusting to the magma-generation zone and remobilisation during arc magmatism. The geodynamic setting for this arc magmatism is comparable with that seen in the Phanerozoic (e.g., the Sierra Nevada and Coast Range batholiths), with compositions in the Suldal Sector reaching those of average upper continental crust. As within these younger examples, factors that drive magmatism towards the composition of the average continental crust include the addition of sedimentary material to magma source regions, and delamination of cumulate material. Underthrusting of sedimentary materials and their subsequent involvement in arc magmatism is perhaps a more widespread mechanism involved in continental growth than is currently recognised. Finally, the Suldal Arc magmatism represents a significant juvenile crustal addition to SW Fennoscandia. Keywords Fennoscandia * Zircon * Arc magmatism * Crustal recycling * Hf-O isotopes, Introduction The recycling of pre-existing continental crust, including sedimentary material, to arc magmas has long been recognised (e.g., Armstrong 1971; Hildreth and Moorbath 1988; Plank and Langmuir 1993). Numerous studies [...]

[1] This study reassesses the stratigraphy, sedimentology, and provenance of the Indus Basin sedimentary rocks, deposited within the Indus Tsangpo Suture Zone (ITSZ) during the early phases of India-Eurasia collision. Using field observations, biostratigraphy, and petrographic and isotopic analyses we create a paleo-depositional reconstruction within the paleotectonic setting of the early phases of India-Eurasia collision. We then re-examine existing constraints to the timing of India-Eurasia collision previously interpreted from the earliest occurrence of mixed Indian- and Eurasian-derived detritus in the succession. From mid-Cretaceous to early Paleocene times the Jurutze and Sumda Formations were deposited within an arc-bounded marine basin between the Dras and Kohistan-Ladakh Island arcs. The

The Area Drime Massif (ADM) is an elongate north-south trending antiformal feature that extends ~70 km north across the crest of the South Tibetan Himalaya and offsets the position of the South Tibetan Detachment system. A detailed U(-Th)-Pb geochronologic study of granulitized mafic eclogites and associated rocks from the footwall of the ADM yields important insights into the middle to late Miocene tectonic evolution of the Himalayan orogen. The mafic igneous precursor to the granulitized eclogites is 986.6 [+ or -] 1.8 Ma and was intruded into the paleoproterozoic (1799 [+ or -] 9 Ma) Ama Drime orthogneiss, the latter being similar in age to rocks previously assigned to the Lesser Himalayan Series in the Himalayan foreland. The original eclogite-facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite facies metamorphism at 750[degrees]C and 0.7-0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with synkinematic sillimanite-grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at < 13.2 [+ or -] 1.4 Ma. High-grade metamorphism was followed by postkinematic leucogranite dyke emplacement at 11.6 [+ or -] 0.4 Ma. This integrated data set indicates that high-temperature metamorphism, decompression, and exhumation of the ADM postdates mid-Miocene south directed midcrustal extrusion and is kinematically linked to orogen-parallel extension.

U-Pb ages of zircon and allanite from the coesite-bearing ultrahigh-pressure (UHP) units in the Kaghan Valley, northern Pakistan, demonstrate that peak UHP metamorphism along the northern margin of the Indian plate occurred at 46.4 [+ or -] 0.1 Ma at peak pressure-temperature conditions of >27.5 kbar (>100 km) and 720-770 [degrees]C. Much lower pressure retrogressive growth of titanite took place between 46.4 and ca. 44 Ma, indicating that the eclogites were exhumed to 35 km depth at or before 44 Ma, implying very rapid exhumation rates within the mantle of ~30-80 mm/yr or more, comparable to rapid plate velocities. Once entrained in the base of the crust, the eclogites followed a slower cooling history from 45 Ma, similar to the amphibolite facies gneisses of the Pakistan Himalaya. Keywords: ultrahigh-pressure metamorphism, exhumation, Pakistan, Himalaya, allanite, UTh-Pb dating, zircon, coesite, continental collision.

Clastic sediments deposited in foreland basins and offshore fans allow the evolution of an orogen to be reconstructed--provided their source regions are properly characterized. Isotopic data from the lithotectonic units of the eastern Himalaya (Bhutan) indicate clear isotopic differences directly comparable with the equivalent units from the central Himalaya. Zircons from metaquartzites of the High Himalayan Series that range in age from 980 to 1820 Ma and an orthogneiss intruded at 825 [+ or -] 9 Ma bracket deposition to between 816 and 980 Ma. Model ages derived from Nd isotopes of associated metapelites range from 1700 to 2200 Ma. In contrast, zircons from a metaquartzite from the Lesser Himalayan Series range from 1850 to 2550 Ma. A Paleoproterozoic deposition age (ca. 1750 Ma) is inferred from the age of a metarhyolite associated with the sediments. Model Nd ages of metapelites from the Lesser Himalayan Series range from 2500 to 2600 Ma. The Bhutanese Himalayan units can be correlated more than 1000 km westward along strike from catchments of the Brahmaputra to the headwaters of the Sutlej on the basis of their structural position, provenance, and isotopes. Because current discharge from the Brahmaputra carries the dominant sedimentary flux into the Bengal Fan, results from this work validate the interpretation of isotopic variations observed in offshore deposits, in terms of the unroofing history of the component lithologies of the Himalaya. Keywords: Himalaya, U-Pb ages, zircon, isotope geochemistry.

The presence of high-pressure (P) eclogites along the northeast margin of the Arabian continental plate beneath the Semail ophiolite demonstrates that subduction of thinned continental crust to pressures of 20 kbar (~80 km depth) has occurred. Current debate centers on whether this high-P metamorphism occurred prior to, during, or following ophiolite emplacement, which is known to have started ca. 95-93 Ma. Five concordant U-Pb zircon ages from a garnet-clinopyroxene-phengite-crossite eclogite from As Sifah, northeast Oman, precisely constrain the age of high-P metamorphism at 79.1 [+ or -] 0.3 Ma. This age demonstrates that high-P metamorphism occurred during continental subduction 15 m.y. after formation of the ocean floor that subsequently became the Semail ophiolite. Keywords: Oman ophiolite, subduction, Arabian margin, eclogites, U-Pb geochronology

Ladinian (Middle Triassic) strata of the Balaton Highland (west central Hungary) comprise interbedded marine carbonate and volcaniclastic rocks. The sediments are noted for their rich ammonoid faunas, which allow detailed biostratigraphic subdivision and correlation. For the first time, isotopic dating of the tuff layers was carried out to calibrate the age of ammonoid zones and subzones. Four successive horizons were dated from the Felsoors section, a candidate stratotype for the base of the Ladinian stage. Within the Reitzi Zone, which is interpreted here as the basal Ladinian unit, the following biostratigraphically tightly constrained U-Pb zircon ages were obtained: Felsoeoersensis Subzone 241.1 [+ or -] 0.5 Ma; Liepoldti Subzone 241.2 [+ or -] 0.4 Ma; Reitzi Subzone 240.5 [+ or -] 0.5 and 240.4 [+ or -] 0.4 Ma. A redeposited tuff from the Gredleri Zone at Liter yielded an additional U-Pb age of 238.7 [+ or -] 0.6 Ma. The new isotopic ages are in agreement with published U-Pb dates from the Southern Alps. Together, they allow estimation of the start of the Ladinian at 241.5 Ma and of the end of the stage at 237 Ma. A duration of Keywords: Middle Triassic, U-Pb geochronology, biochronology, time scale.

U-Pb dating of metamorphic and igneous rocks from the Hunza Valley and Baltoro regions of the Karakoram Mountains in northern Pakistan addresses the thermal and magmatic evolution of the thickened Asian plate crust before, during, and after the collision of the Kohistan arc and the Indian plate. Crustal thickening and high-temperature, sillimanite-grade metamorphism in the southern Karakoram Mountains followed the collision and accretion of the Kohistan arc during the Late Cretaceous. U-Pb ages of metamorphic monazites from sillimanite gneisses in the Hunza Valley are 63.3 [+ or -] 0.4 Ma, ca. 50-52 Ma, and 44.0 [+ or -] 2.0 Ma, and monazites from a kyanite-grade schist from the Baltoro region are 28.0 [+ or -] 0.5 Ma. Metamorphic monazites from a highly graphitic garnet + staurolite schist from the Hunza Valley yield a crystallization age of 16.0 [+ or -] 1.0 Ma. Sillimanite gneisses from the Dassu dome have magmatic zircons of 1855 [+ or -] 11 Ma, reflecting a Proterozoic continental crustal source, and metamorphic monazites of 5.4 [+ or -] 0.2 Ma. Magmatism was also sporadic; early granodiorite, monzogranite, and leucogranite dikes yield zircon, monazite, and uraninite ages of 50-52 Ma and 35.0 [+ or -] 1.0 Ma. Widespread lower crustal melting during the latest Oligocene--early Miocene culminated with emplacement of the Baltoro Plutonic Unit in the Karakoram batholith that cuts deformation fabrics in the high-grade gneisses to the south. The youngest magmatic phase dated is the 9.3 [+ or -] 0.2 Ma Sumayar leucogranite pluton. On the basis of detailed structural field studies combined with U-Pb geochronology, sillimanite-grade metamorphism was either a protracted event lasting as long as 20 m.y. (64-44 Ma) or peaked at different times within the lower crust following collision of first, the Kohistan arc, and later, the Indian plate. We also present evidence for southward propagation of peak metamorphism and postmetamorphic thrusting and folding of isograds within the past 5 m.y. Detailed geochronology shows that deformation, metamorphism, and magmatism in the middle and lower crust of the south Asian margin has been occurring within the Karakoram metamorphic complex for more than 60 m.y. Similar processes may also have affected the unplumbed depths of the south Tibetan crust. Keywords: Karakoram, Pakistan, tectonics, uranium-lead geochronology.

New monazite U-Pb geochronological data from the Everest region suggest that approximately 20-25 m.y. elapsed between the initial India-Asia collision and kyanite-sillimanite-grade metamorphism. Our results indicate a two-phase metamorphic history, with peak Barrovian metamorphism at 32.2 +/- 0.4 Ma and a later high-temperature, low-pressure event (620 degrees C, 4 kbar) at 22.7 +/- 0.2 Ma. Emplacement and crystallization of the Everest granite subsequently occurred at 20.5-21.3 Ma. The monazite crystallization ages that differ by 10 m.y. are recorded in two structurally adjacent rocks of different lithology, which have the same postcollisional pressure-temperature history. Scanning electron microscopy reveals that the younger monazite is elaborately shaped and grew in close association with apatite at grain boundaries and triple junctions, suggesting that growth was stimulated by a change in the fluid regime. The older monazite is euhedral, is not associated with apatite, and is commonly armored within silicate minerals. During the low-pressure metamorphic event, the armoring protected the older monazites, and a lack of excess apatite in this sample prevented new growth. Textural relationships suggest that apatite is one of the necessary monazite-producing reactants, and spots within monazite that are rich in Ca, Fe, Al, and Si suggest that allanite acted as a preexisting rare earth element host. We propose a simplified reaction for monazite crystallization based on this evidence. Keywords: monazite, U-Pb dating, Everest, Himalaya.

The hanging wall of the South Tibetan detachment system was originally thought to be caused by gravity, as characterized by its regional-scale, northeast-verging folds. Recent studies have offered a different interpretation of the movement and suggests a fan-structure occurrence which was estimated to have happened sometime between the late Eocene to Oligocene stratigraphic geological period. A new model for studying the northeast verging folds of the Kali Gandaki is proposed.

Stikinia is a tectonostratigraphic terrane in the Canadian Cordillera that formed in a volcanic arc environment during Paleozoic and Mesozoic time. Until now, the Paleozoic rocks that form a semicontinuous belt along its western margin (Stikine assemblage) were only recognized in a restricted area in northern British Columbia, between the Stikine and Taku river areas. In contrast, Mesozoic Stikinia rocks form an almost continuous belt that extends much farther to the north, leading some authors to question the nature of the unexposed Paleozoic basement north of the Taku River area. The following correlations have significant implications for tectonic reconstructions of the northern Cordillera because they suggest that Stikinia's Paleozoic volcanic-sedimentary basement is more widespread than previously thought. On the basis of similar rock types and lithologic associations, six new U-Pb zircon dates, and the common intrusive relationship with 184-195 Ma plutons, the Stikine assemblage is correlated with the Boundary Ranges suite, a metamorphosed Paleozoic volcanic assemblage exposed in the Tagish Lake area, north of the Taku River and south of the Yukon-British Columbia border. The recognition of the Boundary Ranges suite and the Jurassic plutons that intruded it (Tagish Lake suite) as part of Stikinia has implications for the age and character of the Stikinia-Tracy Arm terrane boundary because the Boundary Ranges and Tagish Lake suites form the footwall of a major Middle Jurassic shear zone that carried the continental margin-like rocks of the Tracy Arm terrane in its hanging wall. This correlation also implies that the late Paleozoic basement to the Mesozoic Stikinia are is not a continental margin assemblage, at least as far north as the British Columbia-Yukon border, and possibly farther. The Boundary Ranges suite, and therefore the Stikine assemblage, are also tentatively correlated with parts of the Yukon-Tanana terrane in Yukon (Aishihik Lake areal, parts of the Taku terrane in southeast Alaska, and undivided metamorphic rocks in west-central British Columbia. Differences in the isotopic signatures of these rocks may reflect along-strike changes in the character of the basement rocks of the late Paleozoic Stikinia volcanic arc.

The Main Central thrust of the central Himalaya juxtaposes the Lesser Himalayan sequence (footwall) with the Greater Himalayan sequence (hanging wall), both of which are regarded as components of the northern passive margin of India. Considerable uncertainty surrounds their respective ages and sedimentary derivation. U-Pb and Sm-Nd isotopic studies of samples from the Langtang area, central Nepal, demonstrate important distinctions between the two rock packages. The Greater Himalayan sequence had a sedimentary provenance that included a major source of 0.8-1.0 Ga zircons, implying a Late Proterozoic age. The source of the Precambrian part of the Lesser Himalayan sequence contained 1.87-2.60 Ga zircons, and its depositional age is probably Middle Proterozoic. Nd isotopic characteristics of the two sequences are different: at 21 Ma, the Greater Himalayan rocks had [[Epsilon].sub.Nd] values between -14.6 and -18.5 while their Lesser Himalayan counterparts had values between -21.4 and -25.9, giving rise to major differences in their model ages. The Lesser and Greater Himalayan sequences at Langtang are interpreted as correlative to the lower and upper parts of the Vindhyan Supergroup, currently exposed south of the Gangetic plain in northern India. On the basis of available age data, the exposed hanging wall of the Main Central thrust in central and eastern Nepal contains no Archeart-Early Proterozoic basement. If the fault does root into the basement at depth, it must do so farther north than recently published cross sections indicate. This implies a total displacement on the fault that is substantially greater than the 140-210 km required by tectonic overlap in the eastern Himalayas.

The Insular composite terrane and the Intermontane terrane contract to reveal crustal reflectivity. The reflection images indicate an upper crustal reflectors and a lower crustal ramp with east dips. An early phase of contraction directed from the west caused the two-side Cretaceous Coast belt. Crustal boundinage causes the crust flanking in the east of the Coast Mountains of the Canadian Cordillera to possess a structural character. There is good correlation between structural belts and the crustal reflectivity traversing the southern Coast Mountains.

A metamorphosed assemblage of continental margin-type strata, termed the Nisling terrane, exists as a narrow terrane silver in the northern Coast Mountains of British Columbia. It could be a fragment rifted from North America and later accreted, or, alternatively, it could be allochthonous and far traveled. The Nisling terrane is tectonically bounded by younger, oceanic, arc-dominated terranes on both sides - the Alexander terrane to the west and metamorphosed Paleozoic rocks of Stikinia to the east. The Nisling terrane was accreted to Stikinia along a sinistral transpressive ductile shear zone between 185 and 170 Ma, during or before the accretion of Stikinia to the ancestral margin of North America. Along this tectonic boundary, intense deformation was accompanied by lower amphibolite-facies metamorphism. Previous inferences by other workers that the Nisling-Stikinia accretion occurred in the Triassic are incorrect, as is the supposition that the Nisling terrane formed the depositional basement to Stikinia. Although the Nisling terrane exists for approximately 1000 km along strike on the west side of Stikinia, its connection to either North America or another continent remains uncertain and tectonically puzzling. The middle Cretaceous collision of the Nisling-Stikinia terrane composite with the Alexander terrane overprinted the Jurassic deformational features and produced a complex belt of tectonic slivers involving all of these terrane fragments.

Images of structures at all levels of the southern Canadian Cordillera, provided by the lithoprobe deep seismic reflection data, reveals a complex and highly reflective crust. The area covered is the interval from the eastern most metamorphic core complexes near Arrow Lakes to the Fraser River. The reflection Moho is the base of the crustal reflectivity. The reflection data indicates that during Cenozoic extension, the Maho and lower crust acted as a zone of detachment. The three major crustal antiforms, namely, the Monashee complex, the Central Nicola horst and the Vernon antiform are also interpreted.

Individual detrital monazite grains from the Bonner Formation of the Belt Supergroup in southwestern Montana have been analyzed using U-Ph and Sm-Nd isotopes. Combined with single-grain U-Pb analyses of detrital zircon, these data place constraints on the age and isotopic characteristics of the source area, previously inferred to be to the west and south of the Belt basin. U-Pb ages from eight single zircon grains range from 1.70 to 1.86 Ga; two grains were ca. 2.6 Ga. U-Pb ages from five monazite grains range from 1.64 to 1.79 Ga. Sm-Nd analyses of these same monazite grains yield [[epsilon].sub.Nd] at the time of crystallization of +0.7 to -14.5. These data suggest that the source area for this part of the Bonner Formation was an Early Proterozoic magmatic (orogenic?) belt with some reworked Archean material. It is postulated that this source area may have been the western edge of Archean crust (Wyoming craton) that was involved in younger magmatism, possibly continuous with rocks of similar age in southcentral Montana, the subsurface of Alberta, or the southwestern United States.