Exhumation of the Central Alps: evidence from 40Ar/39Ar laserprobe dating of detrital white micas from the Swiss Molasse Basin

⁴⁰Ar/³⁹Ar single-grain laserprobe dating of detrital white micas from early Oligocene to middle Miocene (31–14 Ma) sedimentary rocks of the central Swiss Molasse basin reveals three distinct clusters of cooling ages for the hinterland. Two Palaeozoic age clusters reflect cooling after the Variscan orogeny with only limited reheating during the Alpine orogeny. The third Tertiary age cluster reflecting late Alpine cooling is restricted to sediments younger than 20 Myr old. Micas with cooling ages < 30 Myr are interpreted to originate from the footwall of the Simplon detachment fault, thus representing formerly exposed upper levels of the present-day Lepontine metamorphic dome. Erosion of these levels is reflected by an increase of low-grade metamorphic lithic grains in the sandstones. This interpretation puts constraints on the timing of exhumation as well as on the evolution of the drainage pattern of the Central Alps.     

40Ar/ 39Ar dating of Penninic Front tectonic displacement (W Alps) during the Lower Oligocene (31–34 Ma)

Direct absolute dating of the Penninic Frontal Thrust tectonic motion is achieved using the ⁴⁰Ar/³⁹Ar technique in the Pelvoux Crystalline Massif (Western Alps). The dated phengites were formed syn-kinematically in shear zones. They underline the brittle-ductile stretching lineation, pressure-shadow fibres and slickensides consistent with underthrusting of the European continental slab below the propagating Penninic Thrust. Chlorite–phengite thermobarometry yields 10–15 km and T ∼280 °C, while ⁴⁰Ar/³⁹Ar phengite ages mainly range between 34 and 30 Ma, with one younger age at 27 Ma. This Early Oligocene age range matches a major tectonic rearrangement of the Alpine chain. Preservation of prograde ⁴⁰Ar/³⁹Ar ages is ascribed to passive exhumation of the Pelvoux shear zone network, sandwiched between more external thrusts and the Penninic Front reactivated as an E-dipping detachment fault. Partial resetting in the Low Temperature part of argon spectra below 24 Ma is ascribed to brittle deformation and alteration of phengites.     

Geochemistry and timing of post-metamorphic dyke emplacement in the Mersin Ophiolite (southern Turkey): New age constraints from 40Ar/39Ar geochronology

The Mersin ophiolite, which is a relic of the late Cretaceous Neotethyan ocean domain in the eastern Mediterranean, is situated on the southern flank of the central Tauride belt. The ophiolite body is cross-cut at all structural levels by numerous mafic dyke intrusions. The dykes do not intrude the underlying melange of platform carbonates. Therefore, dyke emplacement post-dates the formation of the opholite and metamorphic sole but pre-dates the final obduction onto the Tauride platform. The post-metamorphic dyke swarms suggest the geochemical characteristics of Island Arc Tholeiites (IAT). ⁴⁰Ar/³⁹Ar geochronology of the post-metamorphic microgabbroic-diabasic dykes cutting both mantle tectonites and metamorphic sole revealed ages ranging from 89.6 ± 0.7–63.8 ± 0.9 Myr old, respectively, indicating widespread magmatic activity during the Late Cretaceous-early Palaeocene in the Neotethyan ocean. These data suggest that island arc development in the Neotethyan ocean in southern Turkey was as early as Late Cretaceous.     

40Ar/39Ar Constraints on the tectonic history and architecture of the ultrahigh-pressure Sulu orogen

New ⁴⁰Ar/³⁹Ar ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane and surrounding areas. Combined with U-Pb ages, Sm-Nd ages, Rb-Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino–Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K-feldspar ⁴⁰Ar/³⁹Ar ages. Subsequent unroofing at rates of 5–25 km Myr⁻¹ brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by ⁴⁰Ar/³⁹Ar ages in the 219–210 Ma range, implying cooling at crustal depths at rates of 50–200 °C Myr⁻¹. Ages in the 210–170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166–149 Ma plutonism was followed by cooling at rates of c. 15 °C Myr⁻¹, suggesting relatively deep crustal conditions, whereas Cretaceous 129–118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr⁻¹, suggesting relatively shallow crustal depths.     

Evidence from U–Pb zircon and 40Ar/39Ar muscovite detrital mineral ages in metasandstones for movement of the Torlesse suspect terrane around the eastern margin of Gondwanaland

New U–Pb detrital zircon ages from Triassic metasandstones of the Torlesse Terrane in New Zealand are compared with ⁴⁰Ar/³⁹Ar muscovite data and together, reveal four main source components: (i) major, Triassic–Permian (210–270 Myr old) and (ii) minor, Permian–Carboniferous (280–350 Myr old) granitoids (recorded in zircon and muscovite data); (iii) minor, early middle Palaeozoic, metamorphic rocks, recorded mainly by muscovite, 420–460 Myr old, and (iv) minor, Late Precambrian–Cambrian igneous and metamorphic complexes, 480–570 Myr old, recorded by zircon only. There are also Proterozoic zircon ages with no clear grouping (580–1270 Myr). The relative absence of late Palaeozoic (350–420 Myr old) components excludes granitoid terranes in the southern Lachlan Fold Belt (Australia) and its continuation into North Victoria Land (East Antarctica) and Marie Byrd Land (West Antarctica) as a potential source for the Torlesse. The age data are compatible with derivation from granitoid terranes of the northern New England Orogen (and hinterland) in NE Australia. This confirms that the Torlesse Terrane of New Zealand is a suspect terrane, that probably originated at the NE Australian, Permian–Triassic, Gondwanaland margin and then (200–120 Ma) moved 2500 km southwards to its present New Zealand position by the Late Cretaceous (90 Ma). This sense of movement is analogous to that suggested for Palaeozoic Mesozoic terranes at the North American Pacific margin.     

Dating cleavage formation in slates and phyllites with the 40Ar/39Ar laser microprobe: an example from the western New England Appalachians, USA

Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high-spatial-resolution ⁴⁰Ar/³⁹Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S₂, S₃, and S₄) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S₃ and S₄ are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative ⁴⁰Ar/³⁹Ar geochronology. Mean ⁴⁰Ar/³⁹Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S₃ and 345.5 ± 1.7 Myr for S₄. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S₃ and S₄, these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region.     

Superimposed tectonic and hydrothermal events during the late-orogenic extension in the Western French Massif Central: a structural and 40Ar/39Ar study

In the Western French Massif Central, the Argentat fault is a major structure through which As–Au fluids percolated in the Late Carboniferous along brittle fractures. New petrostructural investigations show that an early ductile normal-dextral faulting, coeval to leucogranite emplacement took place during the Late Visean syncollisional extension of the belt and was accompanied by a hydrothermal event marked by the growth of muscovites whose ⁴⁰Ar/³⁹Ar ages cluster around 335 Ma. This early fluid channelling is associated with brittle deformation only in the hangingwall of the Argentat fault, whereas ductile deformation is restricted to the footwall. These results provide new evidence for the upper crust implication during the syncollisional extension in the French Massif Central. This study stresses the interest of a detailed multimethod analysis to characterize hydrothermal processes, especially in basement areas where the tectonic, plutonic and metamorphic evolution is polyphased.     

Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south-west Japan: evidence from 40Ar/39Ar whole-rock phyllite ages

Abstract Five whole-rock ⁴⁰Ar/³⁹Ar plateau ages from low-grade sectors of the Sambagawa belt (Besshi nappe complex) range between 87 and 97 Ma. Two whole-rock phyllite samples from the Mikabu greenstone belt record well-defined ⁴⁰Ar/³⁹Ar plateau ages of 96 and 98 Ma. Together these ages suggest that a high-pressure metamorphism occurred in both the Sambagawa and Mikabu belts at c. 90–100 Ma. The northern Chichibu sub-belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north ( c. 110 Ma) to south ( c. 215 Ma). The northern Chichibu sub-belt is correlated with the Kuma nappe complex (Sambagawa belt). Two whole-rock phyllite samples from the Kurosegawa terrane display markedly older metamorphic ages than either the Sambagawa or the Chichibu belts.
Accretion of Sambagawa-Chichibu protoliths began prior to the middle Jurrasic. Depositional ages decrease from middle Jurassic (Kuma-Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units. The Kurosegawa belt and the geological units to the south belong to distinctly different terrances than the Sambagawa-Chichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous.     

40Ar/39 mineral ages from the Scituate Granite, Rhode Island: implications for Late Palaeozoic tectonothermal activity in New England

The alkalic Scituate Granite was emplaced into crystalline sequences within the New England Esmond–Dedham terrane in the Late Devonian ( c. 370 Ma). Variably recrystallized amphibole (iron-rich, hastingsite–hastingsitic hornblende) from four variably deformed samples of the pluton record south-westerly younging ⁴⁰Ar/³⁹Ar plateau ages ranging between 276 and 263 Ma. These are interpreted to date diachronous cooling through temperatures appropriate for intracrystalline retention of argon following late Palaeozoic orogenic activity. Iron-rich biotite concentrates from the samples record only slightly younger ages, and therefore suggest relatively rapid post-metamorphic cooling. The ⁴⁰Ar/³⁹Ar ages indicate that the late Palaeozoic tectonothermal overprint was much more regionally pervasive than was previously considered. The apparent timing of this activity is similar to previous estimates for the chronology of high-grade metamorphism throughout the adjacent Hope Valley terrane and for phases of ductile movement on the intervening Lake Char–Honey Hill fault system.     

Reconsideration of the age of blueschist facies metamorphism on the Seward Peninsula, Alaska, based on phengite 40Ar/39Ar results

Abstract ⁴⁰Ar/³⁹Ar step-heating and single-grain laser fusion ages from phengites from the polydeformed and polymetamorphosed blueschist-greenschist facies Nome Group fall into two groups. Samples from the upper part of the structural section that have experienced a relatively weak metamorphic and deformational post-blueschist facies overprint and one sample from the Cape Nome orthogneiss yield plateau ages of 116-125 Ma. More intensely overprinted samples yield hump-shaped spectra with minimum ages of 123 Ma and maximum ages of 334 Ma. Samples with hump-shaped spectra are derived from a greater structural depth than most samples with plateau ages. Unreasonably old maximum ages from some of the disturbed spectra suggest that the hump-shaped spectra result from the incorporation of excess ⁴⁰Ar. This interpretation conflicts with previous interpretations of similarly disturbed spectra from the Brooks Range, which have been argued to provide minimum ages for blueschist facies metamorphism. Since the maximum temperatures achieved by all samples were probably above the blocking temperature of Ar in phengite, the 116-125 Ma plateau ages are a minimum age for blueschist facies metamorphism on the Seward Peninsula, Alaska.     

40Ar-39Ar Spectra of Alunite from Phreatic Craters in the Muine Volcano, Hokkaido, Japan

Abstract. ⁴⁰Ar-³⁹Ar analyses of two alunite samples from phreatic craters in the Pliocene Muine volcano in southwest Hokkaido, Japan, were carried out. The alunite with 17.4 permil δ³⁴S_{V_CDT} value in hydrothermal breccia from the Nagaoyama crater and that with 14.3 permil δ³⁴S_{V_CDT} value in silicified andesite from the Konuma crater give total fusion ages of 1.40 ± 0.04 Ma (la uncertainty) and 1.24 ± 0.08 Ma, respectively. However, the spectra of these samples indicate they have been effected by thermal overprinting and/or the existence of excess argon. These preliminary ⁴⁰Ar-³⁹A analyses suggest that the alunite underwent multiple hydrothermal activity by magmatic gas and vapor subsequent to the main hydrothermal activity.     

Absolute dating of brittle fault movements: Late Permian and late Jurassic extensional fault breccias in western Norway

⁴⁰Ar/³⁹Ar geochronological and palaeomagnetic dating methods applied to fault breccias in western Norway have isolated two brittle reactivation episodes of the syn-post-Caledonian, extensional Nordfjord-Sogn Detachment. These events, of latest Permian and latest Jurassic–Early Cretaceous ages, demonstrate temporal relationships between development of chemical remanent magnetism and partial resetting of Ar isotopic systems during distinct breccia-forming episodes. A third event of Carboniferous age was also identified in the breccias with the ⁴⁰Ar/³⁹Ar technique and is a relict unroofing signature inherited from the fault wall-rocks. These brittle faults are significant time markers and become relevant to interpretations of offshore seismic data which attempt to place ages on faults that have undergone multiple reactivation episodes.     

40Ar/39Ar evidence that formation of blueschists in continental crust was synchronous with foreland fold and thrust belt deformation, western Brooks Range, Alaska

Abstract ⁴⁰Ar/³⁹Ar ages from white mica in rocks of the internal zone of the Brooks Range contractional orogen indicate that the Nanielik antiformal duplex developed at about 120 Ma and was remobilized on its southern boundary at c . 108 Ma. Blueschist facies metamorphism accompanied development of the antiform. The timing of the blueschist facies event and creation of the antiform overlap the period of shallow-seated deformation in the foreland fold and thrust belt and sedimentation in the foreland basin of the Brooks Range. Blueschist facies P-T conditions may therefore characterize the thicker parts of orogenic wedges in some orogenic systems; ancient blueschists need not necessarily be interpreted as indicators of active subduction or continent-continent collision.
Microprobe analysis using quantitative wavelength-dispersive and electron backscattered electron imaging methods was used to characterize the composition of white micas in the dated samples. None of the samples was compositionally homogeneous; many contained 2-3 populations of white mica, including both potassic and sodic varieties. Samples which had undergone (in sequence) amphibolite, albite-epidote amphibolite and blueschist facies metamorphic events retained muscovites relict of the amphibolite facies event. Samples that had undergone only the blueschist facies event also contained multiple populations of mica, some probably from detrital sources.     

Sr-Nd-Pb isotope ratios, geochemical compositions, and 40Ar/39Ar data of lavas from San Felix Island (Southeast Pacific): Implications for magma genesis and sources

We present the first trace element and age data combined with new Sr, Nd, and Pb isotope ratios on lavas from San Felix Island in the Southeast Pacific. A ⁴⁰Ar/³⁹Ar plateau age of 421 ± 18 ka implies young intraplate volcanic activity in this region relative to the ∼22 Ma old volcanism on the neighbouring Easter seamount chain (ESC). The incompatible element compositions of the San Felix magmas are similar to those of EM1-type basalts from Gough, although the isotopic compositions differ. San Felix formed some 20 Ma after the ESC plume affected the plate in this region but no chemical signature of the ESC material is observed in the young volcanic rocks. The composition of the San Felix basalts indicates a mantle source containing old continental lithospheric material from either metasomatized mantle or recycled sediments, which ascends in a weak mantle plume.     

Astronomical tuning of the Tortonian 87Sr/86Sr curve in the Mediterranean basin

ABSTRACT This work presents a detailed ⁸⁷Sr/⁸⁶Sr isotope curve for the interval 7.5–9.7 Ma obtained by a high-resolution analysis (sampling spacing of about 40 kyr) of an astronomically calibrated land-based sedimentary sequence exposed in the central Mediterranean area (Gibliscemi section, southern Sicily). The main aim is to verify a synchronous response of the Mediterranean seawater Sr isotope record to the oceanic forcing on the basis of multiple comparisons of the Gibliscemi record with published coeval ⁸⁷Sr/⁸⁶Sr curves. A good correlation with the ⁸⁷Sr/⁸⁶Sr data from the ODP site 926 (equatorial Atlantic ocean), considered to be the Sr chemostratigraphic reference section for the Late Miocene, and from the Pacific DSDP site 590B was registered. Conversely, the comparison of the Gibliscemi Sr isotope data with ⁸⁷Sr/⁸⁶Sr ratios from the coeval segment of the land-based Sardella section (eastern Mediterranean) shows important differences highlighting a local control on the seawater Sr isotope changes in semi-isolated subbasins within the Late Miocene Mediterranean.     

Contact metamorphism in the Malashan dome, North Himalayan gneiss domes, southern Tibet: an example of shallow extensional tectonics in the Tethys Himalaya

Combined petrographic, structural and geochronological study of the Malashan dome, one of the North Himalayan gneiss domes, reveals that it is cored by a Miocene granite, the Malashan granite, that intruded into the Jurassic sedimentary rocks of Tethys Himalaya. Two other granites in the area are referred to as the Paiku and Cuobu granites. New zircon SHRIMP U-Pb and muscovite and biotite ⁴⁰Ar-³⁹Ar dating show that the Paiku granite was emplaced during 22.2–16.2 Ma (average 19.3 ± 3.9 Ma) and cooled rapidly to 350–400 °C at around 15.9 Ma. Whole-rock granite chemistry suggests the original granitic magma may have formed by muscovite dehydration melting of a protolith chemically similar to the High Himalayan Crystalline Sequence. Abundant calcareous metasedimentary rocks and minor garnet-staurolite-biotite-muscovite ± andalusite schists record contact metamorphism by three granites that intruded intermittently into the Jurassic sediments between 18.5 and 15.3 Ma. Two stages of widespread penetrative ductile deformation, D1 and D2, can be defined. Microstructural studies of metapelites combined with geothermobarometry and pseudosection analyses yield P – T conditions of 4.8 ± 0.8 kbar at 550 ± 50 °C during a non-deformational stage between D1 and D2, and 3.1–4.1 kbar at 530–575 °C during syn- to post-D2. The pressure estimates for the syn- to post-D2 growth of andalusite suggest relatively shallow (depth of ∼15.2 km) extensional ductile deformation that took place within a shear zone of the South Tibetan Detachment System. Close temporal association between intrusion of the Malashan granite and onset of D2 suggests extension may have been triggered by the intrusion of the Malashan granite.     

Alpine metamorphism in the south-east Tauern Window, Austria: 2. Rates of heating, cooling and uplift

Abstract Existing geochronological data are reviewed and new Rb-Sr, K-Ar and ³⁹Ar–⁴⁰Ar ages are presented, including a suite of 33 mica ages from a 20 km north–south tunnel section. These data are discussed in relation to the thermal history from the overthrusting of the Autroalpine nappes c. 65 Myr ago to the present. The earliest phase of metamorphism, involving lawsonite crystallization, is associated with emplacement of these nappes. Subsequently, temperatures in the rocks beneath rose, at a mean rate of 3–6°C/Myr, until the climax of metamorphism.
At high structural levels, published data indicate an age > 35 Myr for the metamorphic climax. In contrast, a new ³⁹Ar–⁴⁰Ar step-heating age of 23.8 ± 0.8 Myr on amphibole, from near the base of Peripheral Schieferhülle, closely approximates the age of metamorphism and provides the first clear indication that the climax of metamorphism occurred later at deeper structure levels. Following the climax, near-isothermal uplift and erosion reduced pressure to c. 1 kbar before white mica closure at 19 Myr; this implies uplift at >3 mm/yr.
Along the tunnel section, white mica K-Ar ages vary systematically from 24 Myr to 16.5 Myr with position relative to a late 4 km amplitude dome whereas biotite Rb-Sr ages are uniform at 16.5 Myr across the whole profile; doming is thus dated at 16.5 Myr with transient uplift rates >5 mm/yr. At other times uplift rates were <1 mm/yr.     

10Be chronology of the last deglaciation of County Donegal, northwestern Ireland

A well-preserved moraine on the northern coast of County Donegal, Ireland, has played a critical role in our understanding of the glacial history of this sector of the Irish Ice Sheet (IIS). Because of a lack of numerical dating of the moraine, however, previous interpretations of its age and significance to the glacial history of this region have varied widely. Here we report eight in situ cosmogenic ¹⁰Be ages on boulders sampled from the moraine. Two of these ages are outliers, with the remaining six ranging from 18.8±1.0 ¹⁰Be kyr to 20.9±1.3 ¹⁰Be kyr, with an uncertainty-weighted mean age of 19.4±0.3 ¹⁰Be kyr (19.4±1.2 kyr accounting for production rate uncertainty). Our results confirm one previous ¹⁰Be age obtained from the moraine, with the combined data ( n =7) constraining the age of initial deglaciation of the IIS from its LGM position on the continental shelf to be 19.3±0.3 ¹⁰Be kyr (19.3±1.2 kyr accounting for production rate uncertainty). These ages are in excellent agreement with calibrated ¹⁴C ages that constrain retreat of the IIS margin from the continental shelf elsewhere in northwestern and western Ireland and the Irish Sea Basin associated with the start of the Cooley Point Interstadial (≥20–≤18.2 cal. kyr BP), suggesting widespread deglaciation of the IIS ∼19.5–20 kyr ago.     

40Ar/39Ar dating of Usol'skii sill in the south-eastern Siberian Traps Large Igneous Province: evidence for long-lived magmatism

Main part of the Siberian Traps Large Igneous Province was formed in a short time-span at the Permo-Triassic boundary c. 250 Ma. New ⁴⁰Ar/³⁹Ar dating results for the Usol'skii dolerite sill in south-eastern part of the province indicate its probable emplacement c. 6 Myr after the main Permo-Triassic magmatic phase. Compilation of the published ⁴⁰Ar/³⁹Ar and U-Pb ages implies that basaltic and related magmatism lasted in total as long as 22–26 Myr. Therefore, similar to other large igneous provinces, magmatism of the Siberian Traps combined voluminous short-lived and less prominent long-lived events.     

Fluid inclusion evidence for basement decompression during Permo-Triassic extension, SE New England, USA

Abstract CO₂-bearing fluid inclusions in strongly lineated but weakly foliated late Precambrian gneisses within the Hope Valley Shear zone of Connecticut and Rhode Island are of mixed composition ( X _co2± 0.1; 7 wt% NaCl equivalent) and variable density (0.59–0.86 g/ml) and occur mainly as isolated inclusions. Also present are dilute (3 wt% NaCl equivalent) aqueous inclusions which occur on healed fractures related to greenschist facies retrograde metamorphism. Isochores for dense isolated CO₂-bearing inclusions indicate pressures of 7.5–9 kbar at 500–600° C, the estimated temperature conditions of peak metamorphism. Published ⁴⁰Ar/³⁹Ar hornblende plateau age spectra indicate cooling through about 500° C at 265 ± 5 Ma. Isochores for low-density CO₂-bearing inclusions and aqueous inclusions intersect at the conditions of retrograde metamorphism (325–400° C) and indicate pressures of 3–4 kbar. Published ⁴⁰Ar/³⁹Ar biotite plateau ages indicate cooling through about 300° C at 250 ± 5 Ma. These data define a P–T uplift curve for the region which is convex towards the temperature axis and indicate uplift rates between 0.4 and 3.3 mm/year in Permian time. Exhumation of basement gneisses was coeval with normal (west-down) motion along the regional basement–cover contact (Honey Hill–Lake Char–Willimantic fault system), and is interpreted as due to post-orogenic extensional collapse of the Alleghanian orogeny.