Rb-Sr Biotite Ages Research Articles

Geochronology, geochemistry, and geological evolution of the Troiseck-Floning and Rosskogel nappes (Eastern Alps): unraveling parallels between the Eastern Alps and Western Carpathians

The Troiseck-Floning and Rosskogel nappes are part of the Austroalpine Unit in the eastern part of the Eastern Alps. The nappes are in tectonic contact and comprise Permian to Mesozoic lower greenschist facies metamorphic metasediments, but only the Troiseck-Floning Nappe consists of a pre-Permian crystalline basement (Troiseck Complex) as well. LA-ICP-MS U–Pb zircon ages, Rb–Sr biotite ages and geochemical data unravel the geological evolution of these tectonic units from Neoproterozoic to Mesozoic times. Detrital U–Pb zircon analyses from siliciclastic metasediments of the Troiseck Complex indicate a late Ediacaran to early Cambrian deposition age of the volcanoclastic sequence. The age distribution correlates with a position along the northeastern Gondwana margin. A late Cambrian crystallization age (502.4 ± 6.8 Ma) of granitic intrusions together with evidence for Late Cambrian/Ordovician magmatism and metamorphism indicate a position at an active plate margin. Polyphase overprinting during the Variscan orogeny is recorded by Late Devonian/early Carboniferous pegmatite dikes (~ 353 Ma) that formed after an early Variscan event, while Pennsylvanian ages of overgrowth rims and inherited grains (~ 320 Ma) are evidence for late Variscan metamorphism. Rhyolitic to andesitic volcanic rocks from the Troiseck-Floning and Rosskogel nappes (271–264 Ma) concomitant with intrusions of porphyric granitoids now transformed to augen gneiss (271 Ma) yield evidence for Permian rift-related magmatism that is widely reported from the Eastern Alps and Western Carpathians. Rb–Sr biotite ages (75–74 Ma) indicate Late Cretaceous cooling below c. 300 °C. This relates to Late Cretaceous exhumation of the Troiseck-Floning Nappe following an Eo-Alpine metamorphic overprint at lower greenschist-facies metamorphic conditions. Based on the similar lithostratigraphy, analogous geological evolution and structure, the Troiseck-Floning Nappe represents the lateral extension of the Seckau Nappe. The new dataset also allows for correlations with other basement complexes that occur in the Western Carpathians.

Structural Evolution, Exhumation Rates, and Rheology of the European Crust During Alpine Collision: Constraints From the Rotondo Granite—Gotthard Nappe

AbstractThe rheology of crystalline units controls the large‐scale deformation geometry and dynamics of collisional orogens. Defining a time‐constrained rheological evolution of such units may help unravel the details of collisional dynamics. Here, we integrate field analysis, pseudosection calculations and in situ garnet U–Pb and mica Rb–Sr geochronology to define the structural and rheological evolution of the Rotondo granite (Gotthard nappe, Central Alps). We identify a sequence of four (D1–D4) deformation stages. Pre‐collisional D1 brittle faults developed before Alpine peak metamorphism, which occurred at 34–20 Ma (U–Pb garnet ages) at 590 ± 25°C and 0.9 ± 0.1 GPa. The reactivation of D1 structures controlled the rheological evolution, from D2 reverse mylonitic shearing at amphibolite facies (520 ± 40°C and 0.8 ± 0.1 GPa) at 18–20 Ma (white mica Rb–Sr ages), to strike‐slip, brittle‐ductile shearing at greenschist‐facies D3 (395 ± 25°C and 0.4 ± 0.1 GPa) at 14–15 Ma (white mica and biotite Rb–Sr ages), and then to D4 strike‐slip faulting at shallow conditions. Although highly misoriented for the Alpine collisional stress orientation, D1 brittle structures controlled the localization of D2 ductile mylonites accommodating fast (∼3 mm/yr) exhumation rates due to their weak shear strength (<10 MPa). This structural and rheological evolution is common across External Crystalline Massifs (e.g., Aar, Mont Blanc), suggesting that the European upper crust was extremely weak during Alpine collision, its strength controlled by weak ductile shear zones localized on pre‐collisional deformation structures, that in turn controlled localized exhumation at the scale of the orogen.

One billion years of tectonism at the Paleoproterozoic interface of North and South Australia

The Mount Woods Domain, in the northeastern Gawler Craton, occupies a tectonically important location in Proterozoic Australia, yet there is very little published U–Pb geochronology data from this region to underpin tectonic models. New LA-ICP-MS U–Pb monazite and detrital zircon geochronology reveal Archean to Paleoproterozoic basement in the central Mount Woods Domain, comprising metasedimentary rocks and garnet-bearing granite with protolith ages of c. 2550–2400 Ma and metasedimentary rocks deposited after c. 1855 Ma. The southern Mount Woods Domain contains younger metasedimentary sequences deposited after 1750 Ma. Metamorphic monazite and zircon geochronology combined with phase equilibria modelling show the rocks of the central Mount Woods Domain were metamorphosed to granulite facies between 1700 and 1670 Ma, reaching pressure and temperature conditions of 4.8–5.3 kbar and 800–840 °C. Monazite geochronology from samples located along major shear zones and in the westernmost Mount Woods Domain record amphibolite facies metamorphism and reworking at 1570–1550 Ma, with a further phase of shear zone activity along the northern margin of the Mount Woods Domain at c. 1480 Ma. Laser ablation inductively coupled plasma triple quadrupole mass spectrometry (LA-ICP-QQQ-MS) Rb–Sr biotite ages from across the Mount Woods Domain range between 1480 and 1390 Ma. The protracted geological history in the Mount Woods Domain from c. 2500–1400 Ma provides a piercing point linking different regions of Proterozoic Australia and western Laurentia during the tenure of the Nuna supercontinent.

Episodic alteration within a gold-bearing Archean shear zone revealed by in situ biotite Rb–Sr dating

Many economic gold deposits formed in the Neoarchean under greenschist facies conditions in association with ductile–brittle shear zones that were conduits for sulfur-bearing hydrothermal fluids. Although the large-scale architecture and formation history of Neoarchean structurally controlled gold deposits has been the subject of many studies, the timing, rates, and durations of mineralization-related alteration remain challenging to deconvolve owing to a lack of event-specific geochronometers and the potential multitude of overprinting processes. In an effort to better understand the alteration history along the mineralized Zuleika Shear Zone in the Eastern Goldfields Superterrane of the Yilgarn Craton in Western Australia, we present the results of biotite Rb–Sr from 13 drill core samples (seven with apatite Sr isotopes) from the Kundana gold camp and the Carbine area. The Kundana district is a lode gold deposit that represents an ideal case study area to test the longevity of mineralization-associated alteration systems. We use in situ laser ablation collision cell triple quadrupole mass spectrometry on biotite from a range of characteristic alteration assemblages with well-defined relative growth ages. Biotite Rb–Sr ages constrain the timing of four different alteration stages between ∼2700 and 2550 Ma. The correlation between relative alteration chronology and Rb–Sr age and the lack of any relationship between grain size and age supports an interpretation of Rb–Sr ratios reflecting biotite growth during hydrothermal alteration with subsequent rapid cooling. Initial 87Sr/86Sr ratios of biotite and apatite are similar and indicate compositions that are more radiogenic than chondritic values and generally slightly more radiogenic than modelled Neoarchean Bulk Silicate Earth values, consistent with a mixture of mantle and crustal fluid sources. Ti-in-biotite thermometry yields temperatures between 540 and 615 °C, peaking towards the main gold mineralization phase. Although it is not possible to link individual Rb–Sr ages to specific faulting events, the absolute age dating for the various alteration assemblages indicates a prolonged, multiphase alteration system lasting ∼150 Ma. Long-term permeability networks with episodic short-lived fluid pulses that outlast terrane- or camp-scale deformation may not only apply to Neoarchean lode gold deposits, but to any tectonically driven zone of weakness in the continental crust.

Apatite and biotite thermochronometers help explain an Arctic Caledonide inverted metamorphic gradient

Biotite generates significant radiogenic Sr and is thus suitable for Rb–Sr dating, which can be achieved directly in thin section via laser ablation and provides a means to directly date growth of this important fabric-forming mineral or its cooling. In contrast, apatite has an extremely low Rb/Sr ratio and hence offers a useful tool to estimate source 87Sr/86Sri. Plagioclase also contains significant Sr and may further assist in understanding the rocks Sr isotopic system. However, the fluid source of growing or recrystallizing apatite, plagioclase, and biotite need not be the same. Compounding the complexity, apatite may gain radiogenic Sr from biotite during recrystallization or dissolution–reprecipitation that accompanies metamorphism. In this contribution, we present a case study from the Kalak Nappe Complex in the Scandinavian Caledonides, Arctic Norway, where a combined geochronology program, critically supported by biotite Rb–Sr dating, helps to resolve the conspicuous inverted metamorphic field gradient found upwards through the nappe stack in this orogen. Apatite U–Pb ages of 417–413 Ma in the upper nappes are interpreted as cooling through the Pb retention zone. Such ages contrast with the suggestion of older, partially reset apatite ages in the lower nappes. Biotite Rb–Sr ages of c. 413 Ma in the upper nappes are similar to U–Pb ages of coexisting apatite but conspicuously different from c. 433 Ma biotite Rb–Sr ages in the lower nappes. Ages of c. 433 Ma are comparable to muscovite 40Ar/39Ar and titanite and monazite U–Pb ages throughout the complex and may directly date mineral growth at peak thermal conditions during the Scandian orogenic event. Apatite 87Sr/86Sr ratios also track the influence of metamorphism and imply more radiogenic signatures relative to relic plagioclase in samples which have greater overprinting in the upper nappes of the complex. Phase equilibrium modelling establishes Palaeozoic peak P–T conditions of 610–750 °C at <12 kbar in the upper nappes, comparable to peak conditions previously established for the lower nappes. Combined thermal modelling of the post-peak Scandian orogenic history based on published 40Ar/39Ar, paired with new Rb–Sr, and U–Pb data, implies a slower cooling pathway in the upper nappes relative to the lower nappes. We reconcile this thermal history with a geodynamic model of obduction of a hot back-arc, as represented by the Silurian Magerøy Nappe, onto the upper nappes of the Kalak Nappe Complex at c. 430 Ma. These results demonstrate that in-situ laser ablation Rb–Sr dating of biotite provides a powerful tool for resolving orogenic tectonothermal histories in orogens flushed with excess argon.

Polyphase deformation along the South Bohemian Batholith-Moldanubian nappes boundary – The Freyenstein Fault System (Bohemian Massif/Austria)

Abstract This work describes the Freyenstein Fault System, which extends over 45 km in the southeastern part of the Bohemian Massif (Lower Austria). It represents a ductile shear zone overprinted by a brittle fault located at the eastern edge of the South Bohemian Batholith towards the Moldanubian nappes. It affects Weinsberg- and a more “fine-grained” granite, interlayered aplitic granite and pegmatite dikes as well as paragneiss of the Ostrong Nappe System. The ductile shear zone is represented by approximately 500 m thick greenschist-facies mylonite dipping about 60° to the southeast. Shear-sense criteria like clast geometries, SCC`-type shear band fabrics as well as abundant microstructures show top to the south/ southsouthwest normal shearing with a dextral strike-slip component. Mineral assemblages in mylonitized granitoid consist of pre- to syntectonic muscovite- and biotite-porphyroclasts as well as dynamically recrystallized potassium feldspar, plagioclase and quartz. Dynamic recrystallization of potassium feldspar and the stability of biotite indicate upper green-schist-facies metamorphic conditions during the early phase of deformation. Fluid infiltration at lower greenschist-facies conditions led to local sericitization of feldspar and synmylonitic chloritisation of biotite during a later stage of ductile deformation. Finally, a brittle overprint by a north-south trending, subvertical, sinistral strike-slip fault that shows a normal component is observed. Ductile normal shearing along the Freyenstein Shear Zone is interpreted to have occurred between 320 Ma and c. 300 Ma. This time interval is indicated by literature data on the emplacement of the hostrock and cooling below c. 300°C inferred from two Rb-Sr biotite ages measured on undeformed granites close to the shear zone yielding 309.6 ± 3 Ma and 290.9 ± 2.9 Ma, respectively. Brittle sinistral strike-slip faulting at less than 300°C presumably took place not earlier than 300 Ma. Early ductile shearing along the Freyenstein Fault System may be genetically, but not kinematically linked to the Strudengau Shear Zone, as both acted in an extensional regime during late Variscan orogenic collapse. A relation to other major northeast-southwest trending faults of this part of the Bohemian Massif (e.g. the Vitis-Pribyslav Fault System) is indicated for the phase of brittle sinistral movement.

Constraints of zircon U-Pb and biotite Rb-Sr ages and P-T conditions on the emplacement and uplifting of the Late Mesozoic Jinan gabbro, eastern North China

The North China Craton underwent exceptional lithospheric thinning. A debate remains as to the mechanism and timing of this thinning partly as a result of the lack of information on the crustal uplift. Mantle-derived mafic rocks can carry key messages of lithospheric thinning and facilitate the determination of the time and mechanism. Gabbroic rocks from the Jinan area in eastern North China were emplaced in the peak period of the thinning and, for this reason, were chosen as the study object. Isotopic ages and P-T conditions of the Jinan gabbro pluton were measured to examine the uplifting and cooling history of the magma. Jinan gabbros were enriched in large ion lithophile elements and large rare earth elements but depleted in high field strength elements without a significant Eu anomaly. Their Sr-Nd-Pb isotopic features suggest that the magma was derived from the partial melting of an enriched lithospheric mantle (EMI). This pluton was emplaced at ~130 Ma, and crystallization temperatures of pyroxene and biotite were 909–1116 °C and 828–952 °C, respectively. The core and rim crystallization pressures of clinopyroxene and plagioclase are ~4.45 kbar and ~2.56–3.91 kbar, respectively. These might record the pressure changes during the uplift process from ~16.5 to 9.5 km. The Rb-Sr isotopic system of biotite was closed at ~121–117 Ma, constrained by the single–grain biotite Rb-Sr ages. This shows that the cooling depth of pluton is ~8.4 ± 2.4 km. Consequently, the pluton was rapidly uplifted from the ~16.5 km to ~8.4 km, at an uplift rate of 0.81 ± 0.24 km/Ma and/or a cooling rate of approximately 50 °C/Ma during 130–120 Ma. This fast uplift was related to the delamination and thinning of the lithosphere that caused activities associated with magmatism and mineralization in eastern North China during the late Mesozoic.

Zircon radiation damage ages

The suggestion that the accumulation of radiation damage in zircons could be used as a dating method was first proposed in the 1950s. In the original technique radiation damage was determined by X-ray diffraction however, this is not suitable for the small sample analysis needed in modern geochronology. It is now possible to measure radiation damage and U and Th contents on micron-sized areas on the polished surface of zircon grains using Raman spectroscopy and SIMS mass spectrometry. This opens the way for a reassessment of the potential of radiation damage ages and the purpose of this contribution is to re-examine the technique through its application to zircons from two granitoids from the Archean Yilgarn Craton and ancient detrital zircons from the Jack Hills in Western Australia. The three examples show internally consistent radiation damage ages that are also in accord with independent geochronological evidence. The 420±110Ma radiation damage age for the Darling Range granite is coincident with 500–400Ma biotite Rb–Sr ages in the same region of the Yilgarn Craton. Likewise a tonalite from the Youanmi Terrane in the Craton with a 1650±150Ma zircon radiation damage age, lies within the domain of a 1600Ma event recorded by biotite Rb–Sr ages. The Jack Hills zircons have a 1120±130Ma radiation damage age that is explained by a complex damage accumulation and annealing history culminating in a mild heating event indicated by biotite Ar–Ar ages of about 1140Ma. The positive results for the three case histories suggest that radiation damage ages could play a useful role in dating low temperature thermal events.

Occurrence of the high grade Thabsila metamorphic complex within the low grade Three Pagodas shear zone, Kanchanaburi Province, western Thailand: Petrology and geochronology

Within the NW–SE trending Three Pagodas shear zone, which is a result of the India–Eurasia collision, a narrow lenticular basement slice of high grade metamorphic rocks, called Thabsila metamorphic complex, is exposed. The Thabsila metamorphic complex is juxtaposed by fault contacts to the very low to non-metamorphic rocks of the shear zone. The high grade basement slice can be subdivided into four units based on lithology: (1) Unit A is composed of marble, mica schist and quartzite, (2) unit B comprises mylonites, (3) unit C is composed of calcsilicate, and (4) unit D comprises various varieties of gneisses. Classic geothermobarometry and pseudosection calculations reveal a P–T variation among the four units. Unit A experienced medium amphibolite facies conditions of 550–650°C and 5–6.5kbar while units B, C and D experienced upper amphibolite facies metamorphism, around 640–710°C and ∼5.5–8kbar. Age of metamorphism and the cooling history were constrained from LA–ICP–MS U–Pb zircon age and Rb–Sr biotite isochron age. Metamorphic zircon rims yield a metamorphic age of ∼51–57Ma, while Rb–Sr biotite cooling ages are ∼32–36Ma. These P–T–t data suggest that the Thabsila gneiss experienced peak upper amphibolite facies metamorphism around ∼51–57Ma during the early collision between India and Eurasia. Subsequently, whilst lateral southeastward extrusion of the Indochina terrane during Oligocene, it was exhumed due to strike-slip faulting along the Three Pagodas shear zone in the transtensional regime. The observed deformation stages D1 (constriction) and D2 (sinistral shearing) postdate the peak metamorphism and can be related to the exhumation stage. Final cooling of the basement rocks down to a temperature of ∼350–300°C is indicated by the biotite Rb–Sr ages at around 32–36Ma.

Surprisingly young Rb/Sr ages from the Simav extensional detachment fault zone, northern Menderes Massif, Turkey

The present paper demonstrates that exposed semi-brittle–brittle detachment fault zones, in addition to footwall mylonites and syn-extensional granitoids, can be used to date the timing of faulting and constrain the history and evolution of metamorphic core complexes. We employed Rb–Sr geochronology on micas, sampled directly from a part of the Simav detachment fault (SDF) zone in the northern Menderes Massif (western Turkey). The exposed part of the fault zone is marked by ∼3-m thick zone of low-grade mylonites/foliated cataclasites, in which mylonitic fabrics in orthogneisses are overprinted by fabrics of semi-brittle deformation. The low-grade mylonites/foliated cataclasites are characterized by coexistence of brown and green biotites. Rb–Sr ages on muscovite and brown and green biotite from the low-grade mylonites/foliated cataclasites are ca. 30 Ma, 17–13 Ma and 12–10 Ma, respectively; green biotite ages are interpreted as dating fluid-assisted deformation-induced dynamic recrystallization and suggest that a part of the SDF was active during a 12–10 Ma interval. The ca. 30 Ma muscovite ages date dynamic crystallization and probably beginning of extensional exhumation of the northern Menderes Massif. The coexistence of brown and green biotites in the same sample indicates retrogressive processes associated within a detachment faulting during which green biotites have recrystallized from primary brown biotites with an age of 19 ± 2 Ma in this area. This further means that the isotopic system became opened during faulting and that the green biotite ages therefore record the activity of the SDF. We have also dated an orthogneiss sample exposed well away from the detachment fault zone (devoid of any retrogressive processes); muscovites and biotites from this sample yield Rb–Sr ages of 45.7 ± 0.6 and 18.17 ± 0.18 Ma, respectively. The biotite age is in accord with regional biotite ages (19 ± 2 Ma) and record cooling of the footwall rocks of the detachment fault. We argue that the ca. 46 Ma muscovite age record synkinematic recrystallization during, and lend credibility to interpretation of, the prograde Barrovian-type regional Main Menderes Metamorphism as an Eocene event and that, prior to the onset of extensional detachment formation, the northern Menderes Massif (or at least the rocks we measured) experienced Eocene temperatures in excess of 500 °C to get this age. New brown biotite and secondary green biotite ages together with previously published age data argues that the footwall rocks reached to biotite cooling temperature conditions by ca. 18 Ma and stayed at similar conditions for a longer period until ca. 12 Ma. The ca. 18–12 Ma time interval corresponds to a period of tectonic quiescence during the Neogene and is attributed to the locking of SDF activity by the intrusion and rapid cooling of I-type syn-extensional granites. Secondary green biotite Rb–Sr ages indicate that the fault then become active during 12–10 Ma. Published low-temperature apatite–zircon fission track and apatite (U–Th)/He ages further suggest that displacement along the entire fault zone has continued until 8.0 ± 0.5 Ma. Combined with previous radiometric and low-temperature thermochronologic ages, the new ages argue for episodic core-complex formation and the Simav detachment fault activity between ca. 30 and 8 Ma. The data therefore argues for a complex history of core-complex formation in the northern Menderes Massif; it occurs in pulses and involves periods of alternating rapid and slow cooling/denudation rates.

Provenance and exhumation of an exotic eclogite-bearing nappe in the Caledonides: a U–Pb and Rb–Sr study of the Jæren nappe, SW Norway

Abstract: The provenance and exhumation history of paragneisses that enclose the 471–458 Ma eclogites in the Caledonian Jæren nappe, SW Norway, were constrained by performing U–Pb analysis on detrital zircon and Rb–Sr dating of mica. Zircon grains from four samples were analysed by laser ablation inductively coupled plasma mass spectrometry, providing detrital U–Pb ages between 3130 and 620 Ma. These data suggest Laurentian provenance, implying the existence of an early Caledonian (Taconian) HP fragment of Laurentian crust in the southern Scandinavian Caledonides. Several rims and recrystallized zircon patches yielded U–Pb ages averaging at 469 ± 6 Ma (2σ). These are identical to the Lu–Hf ages of the eclogites and underscore the importance of Taconian metamorphism in this part of the Caledonides. The main phengite Rb–Sr age cluster at 424.4 ± 4.6 Ma provides time constraints on the amphibolite-facies recrystallization of former high-Si phengite. Five out of seven biotite Rb–Sr ages were identical, yielding a weighted mean age of 413.6 ± 2.6 Ma. Low-Si phengite and biotite in three samples gave c . 400 Ma ages. The Rb–Sr data record the mid-crustal emplacement of the exotic Jæren nappe and its subsequent unroofing along the western edge of Baltica during the backsliding and collapse of the Scandian orogenic front. Supplementary material: The sample locations, U–Pb isotope data and ages for zircon, and the phengite and biotite major element data are available at http://www.geolsoc.org.uk/SUP18448 .

Metamorphism and diachronous cooling in a contractional orogen: the Strandja Massif, NW Turkey

AbstractThe southern part of the Strandja Massif, northern Thrace, Turkey, comprises a basement of various gneisses, micaschists and rare amphibolite, and a cover of metaconglomerate and metasandstone, separated from each other by a pre-metamorphic unconformity. Metamorphic grade decreases from the epidote–amphibolite facies in the south to the albite–epidote–amphibolite/greenschist-facies transition in the north. Estimated P–T conditions are 485–530°C and 0.60–0.80 GPa in the epidote–amphibolite facies domain, and decrease towards the transitional domain between greenschist- and epidote–amphibolite facies. Rb–Sr muscovite ages range from 162.9 ± 1.6 Ma to 149.1 ± 2.1 Ma, and are significantly older (279–296 Ma) in the northernmost part of the study area. The Rb–Sr biotite ages decrease from 153.9 ± 1.5 Ma in the south to 134.4 ± 1.3 Ma in the north. These age values in conjunction with the attained temperatures suggest that the peak metamorphism occurred at around 160 Ma and cooling happened diachronously, and Rb–Sr muscovite ages were not reset during the metamorphism in the northernmost part. Structural features such as (i) consistent S-dipping foliation and SW to SE-plunging stretching lineation, (ii) top-to-the-N shear sense, and (iii) N-vergent ductile shear zones and brittle thrusts suggest a N-vergent compressional deformation coupled with exhumation. We tentatively ascribe this metamorphism and subsequent diachronous cooling to the northward propagation of a thrust slice. The compressional events in the Strandja Massif were most probably related to the coeval N-vergent subduction/collision system in the southerly lying Rhodope Massif.

Early Miocene post-collisional magmatism in NW Turkey: geochemical and geochronological constraints

The Alaçam region of NW Turkey lies within the Alpine collision zone between the Sakarya continent and the Menderes platform. Four different tectonic zones of these two continents form imbricated nappe packages (including the Afyon zone), intruded by the Alaçam granite. Newly determined U-Pb zircon ages of this granite are 20.0 ± 1.4 and 20.3 ± 3.3 Ma, indicating early Miocene emplacement. Rb-Sr biotite ages of the granite are 20.01 ± 0.20 and 20.17 ± 0.20 Ma, suggesting fast cooling at a shallow crustal level. Geochemical characteristics show that the Alaçam granite is similar to numerous EW-trending plutons in NW Anatolia. Gneissic granites of the Afyon tectonic zone were intruded by the Miocene Alaçam granite and have been interpreted in earlier studies as sheared parts of the Alaçam granite, which formed along a crustal-scale detachment zone under an extensional regime. We determined a U-Pb zircon age of 314.9 ± 2.7 Ma for a gneissic granite sample of the Afyon zone, demonstrating that these rocks are unrelated to the Miocene Alaçam granite. The early Miocene granitic plutons bear post-collisional geochemical features and are interpreted as products of Alpine-type magmatism along the Izmir–Ankara suture zone in NW Turkey, and seem to have no genetic relation to the detachment zone.

Tectonic evolution of the Arctic Norwegian Caledonides from a texturally- and structurally-constrained multi-isotopic (Ar-Ar, Rb-Sr, Sm-Nd, U-Pb) study

This study presents the results of an integrated Ar-Ar, Sm-Nd, U-Pb, and Rb-Sr isotopic and structural investigation from the Kalak Nappe Complex (KNC) and overlying Magerøy Nappe in Finnmark, Arctic Norway. The results indicate a Paleozoic tectonometamorphic history dissimilar to that previously inferred for the region. The Kalak Nappe Complex (KNC) has been classically regarded as a major component of the allochthonous margin of Baltica variably affected by two Paleozoic tectonometamorphic events separated widely in time -an earlier Finnmarkian event (late Cambrian, ∼490 -- 510 Ma) first defined from field relationships of country rocks to gabbroic intrusions, and a later over printing Scandian event (Silurian to early Devonian, ∼400 --428 Ma). A foliated granitic dike within the Magerøy Nappe yields a U-Pb zircon age of 438 ± 3 Ma, an age identical to intrusive bodies that cross cut early deformation structures within this nappe. An early generation of muscovite within the KNC produces a weighted mean Ar-Ar cooling age of 438 ± 2 Ma and garnet from a folded dike, also in the KNC, yields a Sm-Nd crystallization age of 436 ± 21 Ma. Thus early Silurian metamorphism within the KNC occurred at the same time as deformation and magmatism within the overlying Magerøy Nappe of the Upper / Uppermost Allochthon. The Scandian continent-continent collision is recorded both within the Magerøy Nappe and within the KNC. U-Pb dating of hydrothermal zircon in the KNC at 428 ± 4 Ma is interpreted to date high temperature conditions during Scandian metamorphism. Ar-Ar muscovite ages of 422 ± 2 Ma, associated with overprinting textures, and Rb-Sr biotite ages of 413 ± 3 Ma record cooling after this event. The Engesfjellet Granite in the Magerøy Nappe yields an Ar-Ar amphibole age of 423 ± 8 Ma, ascribed to cooling after the Scandian collision and thus provides a minimum age for the time of emplacement of this unit onto the KNC. A late, undeformed cross-cutting pegmatite within the KNC yields a U-Pb zircon age of 425 ± 5 Ma and indicates that the Scandian collision was completed prior to this time. Biotite Ar-Ar furnace step-heating analyses have pervasive excess radiogenic argon contamination. Ar-Ar UV laser-probe biotite analyses are also contaminated with excess argon but reveal unaffected portions of grains with ages of 401 ± 6 Ma. In one sample from the Magerøy Nappe, two apparent ages, one at 410 ± 8 Ma and another at 404 ± 4 Ma, correspond to different fabrics with distinctive trapped argon components. The older ages are affected by excess argon. However analyses of the younger biotite fabric reveal ^40^Ar/^36^Ar values of atmospheric composition and are interpreted as dating cooling in biotite. This is consistent with the pervasive excess argon flux recorded throughout northern Norway having developed during a short-lived event in the Scandian Orogeny. These results have significant implications for understanding the development of the Caledonian orogenic belt in North Norway, but do not support the premise of a Finnmarkian Orogeny (in the original sense of the term) within its type locality. The results show that the KNC was affected by a tectonometamorphic event some 10 Ma prior to the climactic Scandian collision, likely related to slab roll back in the subducting Iapetus Ocean.

Geochronology of Eocene plutonism and metamorphism in northwest

In northwest Turkey Eocene calc-alkaline intrusions with isotopic ages of 53 Ma to 35 Ma form a WNW-ESE trending belt, ~400 km long and ~60 km wide. In the east, the plutons intrude blueschists representing a subducted passive continental margin sequence with Late Cretaceous (~80 Ma) metamorphic ages. In the vicinity of one of these granitoids a low pressure - high temperature metamorphism has resulted in complete transformation of the blueschist metapelites into andalusite micaschists. The andalusite + cordierite + biotite + muscovite + quartz + K-feldspar + plagioclase paragenesis in the micaschists indicates a pressure of 2 ± 1 kbar and a temperature of 575 ± 50° C during the metamorphism. Rb-Sr muscovite and biotite ages of a micaschist sample are 46 ± 3 and 39 ± 1 Ma, respectively, similar to the 45.0 ± 0.2 Ma U-Pb zircon and Rb-Sr biotite ages from the neighbouring pluton. Development of folds and a new foliation in the andalusite micaschists suggest regional rather than contact metamorphism. The Eocene granitic magmatism and associated LP/HT metamorphism in northwest Turkey have probably formed above a NE directed subduction zone located along the Vardar suture.

Development of high precision Rb–Sr phlogopite and biotite geochronology; an alternative to 40Ar/ 39Ar tri-octahedral mica dating

Potassium–Ar and Rb–Sr dating of minerals was fundamental in early efforts to date magmatic and metamorphic processes and paved the way for geochronology to become an important discipline within the earth sciences. Although K–Ar and, in particular, 40Ar/ 39Ar dating of micas is still widely applied, Rb–Sr dating of micas has declined in use, even though numerous studies demonstrated that tri-octahedral mica yields geologically realistic, and more reliable and reproducible Rb–Sr ages than the K–Ar or 40Ar/ 39Ar system. Moreover, a reduction of uncertainties typically reported for Rb–Sr ages (ca. 1%) can now be achieved by application of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) rubidium isotope dilution measurements (<0.3%). Replicate Rb–Sr biotite ages from the Oslo rift, Norway, yield an external reproducibility of ±0.3% ( n=4) and an analytical error of ±0.8 Ma for individual ages that vary between 276.9 and 275.5 Ma. Conventional thermal ionisation mass spectrometry (TIMS) Rb analysis on the same mineral separates yields ages between 276.1 and 271.7 Ma, three times the spread compared to Rb MC-ICPMS data. Biotite and phlogopite from the central Nagssugtoqidian orogen, West Greenland, yield 40Ar/ 39Ar plateau ages (ca. 1700 Ma) with a spread of ±150 Ma, while Rb–Sr ages on either biotite or phlogopite separates have a much narrower range of ±10 Ma. This comparison of Rb–Sr and 40Ar/ 39Ar ages demonstrates the robustness of the Rb–Sr system in tri-octahedral micas and cautions against the sole use of 40Ar/ 39Ar tri-octahedral mica ages to date geological events. Analytical errors of 16 Ma for these Rb–Sr mica ages determined by TIMS are reduced to <±5 Ma when the Rb concentration is determined by MC-ICPMS. All the TIMS and MC-ICPMS data from the Nagssugtoqidian orogen agree within assigned analytical uncertainties. However, high precision Rb–Sr dating by MC-ICPMS can resolve geological information obscured by TIMS age determinations. TIMS data for seven phlogopite samples form an isochron age of 1645±6 Ma, and thus, no differentiation in age between the different samples can be made. In contrast, MC-ICPMS Rb measurements on the same samples reveal two distinct populations with ages of 1633±3 or 1652±5 Ma. Combining the mica Rb–Sr geochronological data with the well-constrained thermal history of this ancient orogen, we estimate the closure temperature of the Rb–Sr system in 1–2 mm slowly cooled phlogopite crystals, occurring in a matrix of calcite and plagioclase to be ∼435 °C, and at least 50 °C above that of biotite.

Incomplete retention of radiation damage in zircon from Sri Lanka

A suite of 18 zircon gemstones from placers in the Highland/Southwestern Complex, Sri Lanka, were subjected to a comprehensive study of their radiation damages and ages. The investigation included X-ray diffraction, Raman and PL spectroscopy, electron microprobe, PIXE and HRTEM analysis, as well as (U-Th)/He and SHRIMP U-Th-Pb age determinations. Zircon samples described in this study are virtually homogeneous. They cover the range from slightly metamict to nearly amorphous. Generally concordant U-Th-Pb ages averaging 555 ± 11 Ma were obtained. Late Ordovician zircon (U-Th)/He ages scattering around 443 ± 9 Ma correspond reasonably well with previously determined biotite Rb-Sr ages for rocks from the HSWC. Slightly to moderately metamict zircon has retained the radiogenic He whereas only strongly radiation-damaged zircon (calculated total fluences exceeding ~3.5 × 10 18 α-events/g) has experienced significant He loss. When compared to unannealed zircon from other localities, Sri Lanka zircon is about half as metamict as would correspond to complete damage accumulation over a ~555 m.y. lasting self-irradiation period, suggesting significant annealing of the structural radiation damage. Insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon. We suggest to estimate “effective α-doses” for Sri Lanka zircon by multiplying total α-fluences, which were calculated using the zircon U-Th-Pb age, by a correction factor of 0.55. This conversion may be applied to literature data as well, because all gem-zircon samples from Sri Lanka (this work and previous studies) seem to reveal the same general trends of property changes depending on the radiation damage. The use of “effective α-doses” for Sri Lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation-damaged zircon.

Crustal response to continental collisions between the Tibet, Indian, South China and North China Blocks: geochronological constraints from the Songpan‐Garzê Orogenic Belt, western China

AbstractWe report SHRIMP U–Th–Pb monazite, conventional U–Pb titanite, Sm–Nd garnet and Rb–Sr muscovite and biotite ages for metamorphic rocks from the Danba Domal Metamorphic Terrane in the eastern Songpan‐Garzê Orogenic Belt (eastern Tibet Plateau). These ages are used to determine the timing of polyphase metamorphic events and the subsequent cooling history. The oldest U–Th–Pb monazite and Sm–Nd garnet ages constrain an early Barrovian metamorphism (M1) in the interval c. 204–190 Ma, coincident with extensive Indosinian granitic magmatism throughout the Songpan‐Garzê Orogenic Belt. A second, higher‐grade sillimanite‐grade metamorphic event (M2), recorded only in the northern part of the Danba terrane, was dated at c. 168–158 Ma by a combination of U–Th–Pb monazite and titanite and Sm–Nd garnet ages. It is suggested that M1 was a thermal event that affected the entire orogenic belt while M2 may represent a local thermal perturbation. Rb–Sr muscovite ages range from c. 138–100 Ma, whereas Rb–Sr biotite ages cluster at c. 34–24 Ma. These ages document regional cooling at rates of c. 2–3 °C Myr−1 following the M1 peak for most of the terrane. However, those parts of the terrane affected by the higher‐temperature M2 event (e.g. the migmatite zone) experienced initially more rapid (c. 8 °C Myr−1) cooling after peak M2 before joining the regional slow cooling path defined by the rest of the terrane at c. 138 Ma. Regional slow cooling between c. 138 and c. 30 Ma is thought to be the result of post‐tectonic isostatic uplift after extensive crustal thickening caused by collision of the South and North China Blocks. The clustering of biotite Rb–Sr ages marks the onset of rapid uplift across the entire terrane commencing at c. 30–20 Ma. This cooling history is shared with many other regions of the Tibet Plateau, suggesting that uplift of the Tibet Plateau (including the Songpan‐Garzê Orogenic Belt) occurred predominantly in the last c. 30 Myr as a response to the continuing northwards collision of India with Eurasia.

Geodynamic evolution of southern Costa Rica related to low-angle subduction of the Cocos Ridge: constraints from thermochronology

The Late Tertiary shallow subduction of the Cocos ridge under the Caribbean plate controlled the evolution of the Cordillera de Talamanca in southeast Costa Rica, which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting. Fission track thermochronology using zircon and apatite, as well as 40Ar– 39Ar and Rb–Sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the Cordillera de Talamanca and the age of onset of subduction of the Cocos ridge. Shallow intrusion of granitoid melts resulted in fast and isobaric cooling. A weighted mean zircon fission track age (13 analyses) and Rb–Sr biotite ages of about 10 Ma suggest rapid cooling and give minimum ages for granitoid emplacement. In some cases 40Ar– 39Ar and Rb–Sr apparent ages of amphibole and biotite are younger than the zircon fission track ages, which can be attributed to partial resetting by hydrothermal alteration. Apatite fission track ages range from 4.8 to 1.7 Ma but show no correlation with the 3090-m elevation span over which they were sampled. The apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes. The combination of the apatite fission track ages with subduction parameters of the Cocos plate such as subduction angle, plate convergence rate and distance of the Cordillera de Talamanca to the trench implies that the Cocos ridge entered the Middle America Trench between 5.5 and 3.5 Ma.

Pan-African intraplate deformation in the northern Prince Charles Mountains, east Antarctica

New structural and metamorphic data coupled with U–Pb SHRIMP zircon and Rb–Sr step-leach biotite ages help constrain a period of Early Palaeozoic (Pan-African) deformation recognised in the northern Prince Charles Mountains, east Antarctica. This period of deformation is accommodated along discrete northeast trending mylonites that preserve up-dip reverse kinematics with dominantly southeast over northwest vergence. Ambient P–T conditions of 524±20°C and 7.6±4 kbar accompanied deformation. This phase of deformation post-dated the intrusion of planar felsic dykes that yield ages of c. 550 Ma and pre-dated Rb–Sr biotite ages of c. 475 Ma that record cooling of the terrane below c. 300°C. These mylonites are identical in age to continental collisional events recognised in the southern Prince Charles Mountains and Prydz Bay, which lie to the south and east of the northern Prince Charles Mountains, and similar in age to orogenesis recognised to the west in Lützow-Holm Bay. These belts represent sutures between the component lithospheric blocks of east and west Gondwana. The northern Prince Charles Mountains lie between these sutures. Consequently, the mylonites we report here are interpreted to have formed in an intraplate setting and developed in response to stresses applied along the plated margins as a consequence of continental collision during the amalgamation of Gondwana.