Gilles Ruffet

Plateau ages and excess argon in phengites: an 40Ar39Ar laser probe study of Alpine micas (Sesia Zone, Western Alps, northern Italy)

Abstract Bulk samples and single grains of high-pressure white micas and a biotite from two restricted areas (Mucrone and Marine) 10 km apart in the Sesia Zone (Western Alps, Italy) were analysed by the conventional (step-heating) and continuous laser probe (step-heating and spot fusion ) 40 Ar 39 Ar techniques, respectively. The analysed minerals crystallized during the Eoalpine, eclogitic facies metamorphism. The phengites from the Mucrone area display ages which scatter between ∼ 104 and ∼ 180 Ma. In the Marine area (a 50-m-wide outcrop) the phengites from four samples with different bulk-rock compositions display precise plateau ages, which scatter between 69.4 ± 0.7 and 76.9 ± 0.6 Ma, and homogeneous age maps, whereas two minerals (phengite and biotite) from the same rock sample provide two very “discordant” plateau ages, 69.4 ± 0.7 and 140.5 ± 0.6 Ma, respectively. In the light of the present knowledge of the history of the Alps, ages as old as 180 Ma are unrealistic and are related to excess argon, whereas ages in the range 70–80 Ma, perhaps even 140 Ma, could be significant. We suggest that excess argon, probably conveyed by a fluid phase and trapped in defects within the structure of the phengite grains at the time of their isotopic closure, is the best explanation for the observed spread in apparent ages on a regional to single grain scale, in spite of a nearly universal existence of plateau ages and homogeneous age maps. We propose that the age of closure of the phengites to argon loss is ⩽ 69.4 ± 0.7 Ma.

Structural and Geochronological Study of High‐Pressure Metamorphic Rocks in the Kekesu Section (Northwestern China): Implications for the Late Paleozoic Tectonics of the Southern Tianshan

Blueschist‐ and eclogite‐facies high‐ to ultrahigh‐pressure (HP/UHP) metamorphic rocks occur in the southern Tianshan Belt. Their deformation and metamorphic history is important for understanding the Paleozoic tectonics of the Central Asian Orogenic Belt. Our study focuses on the structural analysis and geochronology of the HP metamorphic rocks and the surrounding rocks in the Kekesu Section in the southern Chinese Tianshan. Geometric and kinematic analyses indicate three ductile deformation events: a top‐to‐the‐north thrusting, a top‐to‐the‐south shearing, and a dextral wrenching. New 40Ar/39Ar laser probe plateau ages were obtained on white mica from retrograde blueschist ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Comparison of 40Ar-39Ar conventional and laser dating of biotites from the North Trégor Batholith

316\pm 2

Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar- 40 Ar dating of supergene K-Mn oxides

\end{document} and \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French–Spanish project

331\pm 1

40Ar/39Ar dating of West African lateritic cryptomelanes

\end{document} Ma; 1σ) and greenschist‐facies metasediments ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape