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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, [...]

Depositional evidence of Early Pleistocene glaciations in British Columbia are documented at only a few sites. Near Kelowna, in southern British Columbia, a construction project exposed glacial sediments beneath Lambly Creek Basalt, providing a minimum age for this glaciation. The basalt is composed of a number of flows yielding ages that range from 0.76 ± 0.11 to 1.5 ± 0.1 Ma. The sediments consist of a diamicton, interpreted to be till, up to 3 m thick mantled by a weakly developed paleosol. The diamicton is underlain by fluvial sands up to 5 m thick, in places revealing injection features, and minor faulting. A unit of stratified gravel underlain by grey clay is inferred to underlie the exposed sediments, based on nearby outcrops and excavations. Sediments and overlying basalts are normally magnetized and are assigned to the Jaramillo normal subchron (1.069-0.987 Ma). The till is here referred to as the Westbank First Nation Till. It is Early Pleistocene in age and represents the earliest evidence of glaciation in the Okanagan Valley. Stone fabric analysis and clast lithologies suggest that ice movement was from northwest to southeast, and is here referred to as the West Kelowna Advance; we infer that this advance was part of a larger regional glaciation. Other Early Pleistocene glaciations in the Cordillera are briefly reviewed. En Colombie-Britannique, des indications de deposition de glaciations au Pleistocene precoce sont documentees a quelques sites seulement. A proximite de Kelowna, dans le sud de la Colombie-Britannique, un projet de construction a expose des sediments glaciaires sous le basalte de Lambly Creek, fournissant un age minimum pour cette glaciation. Le basalte comprend plusieurs coulees donnant des ages qui varient de 0,76 ± 0,11 a 1,5 ± 0,1 Ma. Les sediments comprennent un diamicton, qui est interprete comme etant un till, d'une epaisseur atteignant 3 m, recouvert d'un paleosol mal developpe. Le diamicton repose sur des sables fluviaux d'une epaisseur atteignant5met presentant, par endroits, des caracteristiques d'injection et des failles mineures. Une unite de gravier stratifie repose sur une argile grise; en se basant sur les affleurements avoisinants et les excavations, ce gravier serait sous les sediments affleurant. Les sediments et les basaltes qui les recouvrent sont normalement magnetises et ils sont assignes au sous-chron Jaramillo normal (1,069-0,987 Ma). Le till est ici nomme le till de la Premiere Nation de Westbank. Il date du Pleistocene precoce et il represente les premieres indications de glaciation dans la vallee de l'Okanagan. Selon une analyse de la fabrique de la roche et des lithologies des clastes, le mouvement de la glace aurait ete du nord-ouest au sud-est; ce mouvement est nomme l'avancee glaciaire de West Kelowna. Nous concluons que cette avancee glaciaire faisait partie d'une plus grande glaciation regionale. D'autres glaciations du Pleistocene precoce dans la Cordillere sont traitees sommairement. [Traduit par la Redaction], Introduction While Late Neogene paleoclimate records are well known from ocean cores depicting isotopic evidence of Pleistocene glaciations (Lisiecki and Raymo 2005), little is known about the glacial history of [...]

The Caribou Creek volcanic field lies along the continent-side edge of forearc basin rocks in south-central Alaska and consists of over 1000 m of shallow-dipping basalt and andesite lavas with minor mafic pyroclastic deposits. Dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas. The basalts are tholeiitic and strongly depleted in the light rare earth elements (La/ Yb = 0.18-1.5), with concentrations of high field strength elements (e.g., Zr, Hf, Ti, Y) similar to mid-ocean-ridge basalt and with variable enrichment in fluid-mobile elements (e.g., Cs, Ba, and Ph). Intermediate and felsic rocks show enrichment in the rare earth elements and fluid-mobile elements plus Rh and K, but retain low La/Yb ratios (0.48-3.6). A few andesite and dacite samples are strongly depleted in the heavy rare earth elements and arc geochemically similar to adakites (e.g., Sr/Y up to 52). Ten [sup.40]Ar/[sup.39]Ar ages for the Caribou Creek volcanic rocks range from 49.4 [+ or ] 2.2 to 35.6 [+ or ] 0.2 Ma. An adakite-like tuff beneath the other volcanic rocks yields an age of 59.0 [+ or ] 0.4 Ma. Caribou Creek basalts were derived from mid-ocean-ridge--like depleted mantle that was emplaced beneath the southern margin of Alaska through a slab window following spreading ridge subduction. Caribou Creek volcanism was coeval with oblique subduction, oroclinal bending, and right-lateral strike-slip faulting in south-central Alaska, all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas. The basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of Jurassic arc basement rocks to form the intermediate and felsic magmas. Enrichment of the basaltic parent magmas in fluid-mobile elements occurred by contamination from the Jurassic arc rocks and/or by contamination with metasomatic mantle remnant from preceding subduction. High heat flow through the slab window induced partial melting of garnet-bearing mafic parts of the Jurassic arc basement to form the adakite-like rocks. The Caribou Creek volcanic rocks demonstrate that slab windows can directly influence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime (e.g., crustal extension) and that the influence of a slab window on continental margin magmatism can be long-lived (>20 m.y.). Keywords: Eocene, volcanism, Alaska, Talkeetna Mountains, slab window, adakite.

[1] We report [.sup.40]Ar/[.sup.39]Ar ages from various tectonic units in the Aegean and westernmost Turkey. On the basis of published geochronologic data and our [.sup.40]Ar/[.sup.39]Ar ages we propose that the Aegean is made up of several high-pressure units, which were successively underplated from the Late Cretaceous until the Miocene. Ages for high-pressure metamorphism range from 80-83 Ma in parts of the Vardar-Izmir-Ankara suture zone in the north to 21-24 Ma for the Basal unit in the Cyclades and the external high-pressure belt on Crete in the south. Published seismic data suggest that high-pressure metamorphism is currently occurring underneath Crete. Younging of high-pressure metamorphism in a southerly direction mimics the southward retreat of the Hellenic subduction zone. We propose that distinct stages of high-pressure metamorphism were controlled by the underthrusting of fragments of mainly thinned continental crust and that these punctuated events were superposed on progressive slab retreat. By far most of the exhumation of the high-pressure units occurred early during the orogenic history in a forearc position. INDEX TERMS: 8110 Tectonophysics: Continental tectonics--general (0905); 8150 Tectonophysics: Plate boundary--general (3040); 1035 Geochemistry: Geochronology; 8030 Structural Geology: Microstructures; 3660 Mineralogy and Petrology: Metamorphic petrology; KEYWORDS: [sup.40.Ar/.sup.39] Ar dating, exhumation, microstructures, blueschists, Aegean, Greece. Citation: Ring, U., and P. W. Layer, High-pressure metamorphism in the Aegean, eastern Mediterranean: Underplating and exhumation from the Late Cretaceous until the Miocene to Recent above the retreating Hellenic subduction zone, Tectonics, 22(3), 1022, doi:10.1029/2001TC001350, 2003.

The petrogenesis of the 2800 [km.sup.3] of magma erupted as the youngest Toba Tuff has been investigated using experimental petrology and [sup.40]Ar/[sup.39]Ar dating of biotite, sanidine, hornblende, and plagioclase from the tuff. We find that hornblende does not crystallize experimentally from the magma at temperatures and pressures indicated by the natural mineral assemblage. Hornblende is also not in isotopic equilibrium with biotite and sanidine, both of which grew experimentally. Hornblende thus appears xenocrystic, despite being a major phase in the tuff. Some plagioclase is also xenocrystic, on the basis of Ar isotopes, but others are probably phenocrystic, because plagioclase grew experimentally. Crystal clots of hornblende + plagioclase observed in the tuff suggest that the xenocrysts came from a common source, which was at least 1.5 Ma (the oldest hornblende [sup.40]Ar/[sup.39]Ar age). Our results suggest that the Toba Tuff magma resided at nearly water-saturated pressures of 100-150 MPa and that xenocrysts were entrained as recently as 10 yr before the eruption. The ubiquitous presence of hornblende in the tuff indicates that entrainment occurred throughout the 2800 [km.sup.3] of magma. Keywords: argon-argon, isotopes, phenocrysts, xenocrysts, igneous petrology.

The Kaap Valley pluton is one of several early Archean (3200-3500 Ma) tonalitetrondhjemite plutons that surround the Barberton Greenstone belt, southern Africa. Precise dating using single-grain [Pb.sup.207]/[Pb.sup.206] evaporation of zircon and [Ar.sup.40] /[Ar.sup.39] laser step-heating of hornblende and biotite indicates that, in its interior, the Kaap Valley pluton preserves a memory of its initial intrusion and cooling, which spanned a time from 3225 to 3142 Ma. The pluton also records the effect of a low-temperature thermal event at its margin as seen by a [Ar.sup.40/[Ar.sup.39] biotite age of 3035 Ma, which is perhaps related to hydrothermal activity and gold mineralizatin in the adjacent Barberton Greenstone belt. These pre-3000 Ma ages are not in agreement with results of dating studies from sedimentary rocks in the Barberton Greenstone belt and plutons south of the belt which show evidence of having been overprinted by late Archean events (2650-2700 Ma), and the Bushveld Complex intrusion (2050 Ma). These events have been interpreted as affecting most of the Kaapvaal craton. That the Kaap Valley pluton has escaped these and all other events since 3035 Ma with temperatures never reaching 250 [degrees]C implies that these large-scale events did not affect the entire craton and the overprinting seen elsewhere is of a wore local nature. Thus, it is possible to determine the intrusive and cooling history of the pluton; these data can be used in developing models of heat flow, paleomagnetic remanence acquisition, and deformation events.

We present a new 1:80,000-scale geologic map of the Acoculco caldera (Ac) located between the states of Puebla and Hidalgo in eastern México. The map, encompassing an area of 856 km2, is grounded on an ArcMap data set and is supported by nine new 40Ar/39Ar dates. The caldera lies upon Cretaceous limestones and Miocene to Pliocene volcanic rocks (13–3 Ma). The caldera consists of 31 lithostatrigraphic units formed between 2.7 and 0.06 Ma that include a wide variety of volcanic landforms (cinder cones, lava domes). The caldera has a semi-circular shape (18–16 km) bounded by the Atotonilco scarp to the north, the NW–SE Manzanito fault to the west, and scattered vents to the east and southern parts. The distribution of the Acoculco ignimbrite, the lithic breccia, and lacustrine sediments define the caldera ring fault. Late Pleistocene activity and pervasive hydrothermal alteration suggest a high geothermal potential in the area. [ABSTRACT FROM AUTHOR]

The Ust'-Belaya terrane (Chukotka, NE Russia), belonging to the West Koryak fold belt, is made of mantle and crustal ultramafic-mafic complexes which originated during several discrete episodes of magmatic activity in the Neoproterozoic, the late Neoproterozoic-Cambrian and the early-middle Triassic and accreted to the Asian continental margin in the early Cretaceous. This paper focuses on the latest magmatic episode expressed in intrusion of microgabbro dikes and nearly coeval metamorphism superimposed on both ancient complexes and the dikes. We present original 40Ar/39Ar ages, mineral and bulk-rock chemistry of the microgabbro dikes, metamorphosed dike and vein cutting ultramafic-mafic complexes. Dike microgabbros resemble differentiated arc-tholeiitic magmas originated in a subduction setting. Differentiated magmas intruded into much older spinel peridotites and dunites located at mid-crustal levels. Intrusion and crystallization of these magmas was followed by down-going movement of spinel peridotites and rocks of the mantle-crust transition zone towards a mantle wedge where they were metamorphosed at high-P conditions. This metamorphism resulted in transformation of microgabbro to garnet amphibolite, diorite to albite-zoisite-paragonite-pargasite rock and spinel peridotites to metaperidotites. P-T parameters of this metamorphism reconstructed based on mineral assemblage of garnet amphibolite correspond to those of epidote amphibolite – amphibolite – amphibole eclogite facies transition. The peculiar zoisite (clinozoisite)-paragonite mineral assemblage typical of metamorphosed vein rock indicates high-P metamorphic conditions of epidote-amphibolite facies. [ABSTRACT FROM AUTHOR]

The Cyclades in the backarc region of the present Hellenic subduction zone are known for widespread Late Cretaceous to Eocene high-pressure metamorphism in the Cycladic blueschist unit. We report [sup.40]Ar/[sup.39]Ar and Rb/Sr phengite ages of 24-21 Ma for high-pressure metamorphism (8-10 kbar, 350-400 [degrees] C) in the lowest tectonic unit in the Cyclades, the Basal unit, which structurally underlies the Cycladic blueschist unit. The Basal unit is correlated with the Tripolitza unit of the External Hellenides in the forearc region of the Hellenic subduction zone. The Tripolitza unit is unmetamorphosed on Crete, where it is separated from the underlying high-pressure (8-10 kbar, 300-400 [degrees] C) Plattenkalk and Phyllite-Quartzite units by the extensional Cretan detachment. The age for high-pressure metamorphism in the latter units is similar to our age for the Basal unit in the Cyclades. Because pressure-temperature conditions in the Plattenkalk and Phyllite-Quartzite units on Crete and the Basal unit in the Cyclades are also similar, they must have been in close proximity in the early Miocene Hellenic subduction zone. A palinspastic reconstruction suggests a subsequent displacement of [is greater than] 100 km on the Cretan detachment. This is one of the greatest displacement magnitudes ever reported from detachment faults. Because of this large offset, the Cretan detachment was an efficient agent for exhuming high-pressure rocks. Keywords: detachment fault, displacement, exhumation, high-pressure metamorphism, geochronology, Aegean, Greece.

Samples from plutonic belts and individual intrusions in the Sakha Republic (Yakutia), northeastern Russia, were analyzed using [sup.40]Ar/[sup.39]Ar geochronology and trace element geochemistry. Four phases of activity were identified: a subduction event along the southwest margin of the Kolyma-Omolon superterrane with apparent ages of ca. 160-140 Ma, a superposed collisional event dated at 143-138 Ma, a subduction event along the northern margin of the superterrane dated at 130-123 Ma with a slightly older to contemporaneous extensional event to the west; and intrusions associated with east-west extension between 106 and 92 Ma. These phases are interpreted to represent (1) subduction under and (2) accretion to Asia of Kolyma-Omolon followed by local extension, (3) subduction along the northern edge of this superterrane, and (4) regional extension following closure of the South Anyui suture and/or the start of subduction associated with the Okhotsk-Chukotka volcanic belt. Keywords: [sup.40]Ar/[sup.39]Ar, geochemistry, geochronology, Russia, tectonics, terranes, Yakutia Region.

Abstract: While the offshore post‐Caledonian extensional history of the north Norwegian passive margin is well constrained, the tectonic relationship between onshore and offshore regions is less clear because of limited age constraints on the timing of rifting onshore. 40Ar/39Ar dating of K‐feldspar from hydrothermally altered fault rocks in a Precambrian gneiss complex in northern Norway was used to study the timing of extensional faulting onshore. In addition, 40Ar/39Ar dating of K‐feldspar from the host rock provided insight into the regional rock cooling history prior to brittle deformation. Results indicated a dominant Late Permian–Early Triassic (~265–244 Ma) faulting event and found no evidence for later reactivation, which has been documented offshore. The region cooled to below the closure temperature for 40Ar/39Ar K‐feldspar in the Carboniferous to Early Permian, prior to the main brittle faulting event. 40Ar/39Ar dating of fault zone K‐feldspar products provided a means to date brittle faulting events. [ABSTRACT FROM AUTHOR]