Valérie Bosse

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

Abstract A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.

Partial resetting of the U-Th-Pb systems in experimentally altered monazite: Nanoscale evidence of incomplete replacement

Alteration experiments on natural monazite crystals (Manangotry standard, Madagascar) under alkali conditions at 300, 400, 500 and 600 °C and 200 MPa were conducted to clarify mechanisms behind incomplete resetting of U-Th-Pb geochronological systems in monazite replaced by dissolution and precipitation processes. Above 400 °C, experimental products show typical replacement textures: a compositionally distinct monazite rim, referred as altered rim, surrounds the primary monazite (Mnz1). Isotopic and electron microprobe U-Th-Pb in situ dating of the altered rim yields intermediate ages between pristine monazite (555 Ma) and complete experimental resetting (0 Ma). Lead is systematically detected in altered rims, with concentration decreasing from 400 °C to 600 °C. The origin of incomplete resetting is elucidated with transmission electron microscope images that reveal an incomplete replacement of Mnz1 by a secondary monazite (Mnz2) within the altered rim. With increasing temperature, the size and volume of the Mnz2 within the altered rim become more important. Because no structural Pb or Pb nanoinclusions were observed, Pb in the altered rim is attributed to the Mnz1 component. Partial resetting of U-Th-Pb systems depends on the nanomixture of different Mnz1 proportions in the analyzed volume, and explains the higher rejuvenation at 600 °C than at lower temperatures. Although microanalytical techniques have the spatial resolution to date micrometer-sized rims, they are unable to resolve a nanoscale mixture of pristine and secondary monazite that could occur in altered rims formed by fluid-driven replacement, especially at low temperatures. Porosity and/or inclusions and complex age scattering in zoned monazite are significant markers that can indicate a possible nano-sized partial replacement.

Eocene ultra high temperature (UHT) metamorphism in the Gruf complex (Central Alps): constraints by LA-ICPMS zircon and monazite dating in petrographic context.

The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic ultra-high temperature (UHT) metamorphism in the world, but its age is still a matter of debate. Here we present LA-ICPMS dating in a petrographic context of zircon and monazite from a UHT restitic granulite. Zircons and monazites are both included in large crystals and in retrograde symplectites. In such restitic rocks, partial melting or fluid interactions are unlikely, precluding resetting of the monazite chronometers. Zircon cores yield Permian ages, which are interpreted as the age of charnockitization. They are sometimes surrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma age interpreted as the time of complete (re-)crystallization during the UHT paragenesis. We propose that the zircons dated a post-Hercynian metamorphism which is responsible for the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluid-absent melting event produced refractory lithologies, such as restites in charnockites. We suggest that Gruf UHT paragenesis is alpine in age and crystallized from these refractory lithologies. We conclude that the lower restitic crust produced in the Permian had the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time. Supplementary material: Analytical procedures for monazite analysis and dating, plus details of the major elements of the minerals, isotope data and trace element measurements in zircon are available at https://doi.org/10.6084/m9.figshare.c.4123619

Two successive phases of ultrahigh temperature metamorphism in Rogaland, S. Norway: Evidence from Y-in-monazite thermometry

Handling Editor: Michael Brown Abstract In Rogaland, South Norway, a polycyclic granulite facies metamorphic domain surrounds the late-Sveconorwegian anorthosite–mangerite–charnockite (AMC) plutonic complex. Integrated petrology, phase equilibria modelling, monazite microchemistry, Y-in-monazite thermometry, and monazite U–Th–Pb geochronology in eight samples, distributed across the apparent metamorphic field gradient, imply a sequence of two successive phases of ultrahigh temperature (UHT) metamorphism in the time window between 1,050 and 910 Ma. A first long-lived metamorphic cycle (M1) between 1,045 8 and 992 11 Ma is recorded by monazite in all samples. This cycle is interpreted to represent prograde clockwise P–T path involving melt production in fertile protoliths and culminating in UHT conditions of ~6 kbar and 920°C. Y-in-monazite thermometry, in a residual garnet-absent sapphirine–orthopyroxene granulite, provides critical evidence for average temperature of 931 and 917°C between 1,029 9 and 1,006 8 Ma. Metamorphism peaked after c. 20 Ma of crustal melting and melt extraction, probably supported by a protracted asthenospheric heat source following lithospheric mantle delamination. Between 990 and 940 Ma, slow conductive cooling to 750–800°C is characterized by monazite reactivity as opposed to silicate metastability. A second incursion (M2) to UHT conditions of ~3.5–5 kbar and 900–950°C, is recorded by Y-rich monazite at 930 6 Ma in an orthopyroxene– cordierite–hercynite gneiss and by an osumilite gneiss. This M2 metamorphism, typified by osumilite paragenesis, is related to the intrusion of the AMC plutonic complex at 931 2 Ma. Thermal preconditioning of the crust during the first UHT metamorphism may explain the width of the aureole of contact metamorphism c. 75 Ma later, and also the rarity of osumilite-bearing assemblages in general.

Coeval Mantle-Derived and Crust-Derived Magmas Forming Two Neighbouring Plutons in the Songpan Ganze Accretionary Orogenic Wedge (SW China)

The Songpan Ganze accretionary orogenic wedge (Eastern Tibetan Plateau, China) is characterized by midto late Triassic syn-orogenic magmatism displaying a diversity of petrological and geochemical characteristics: I-type lowto high-K calc-alkaline, A-type alkaline, and S-type peraluminous granitoids. To better understand the genesis and emplacement of magmas associated with tectonic accretion along convergent plate boundaries, we present a comprehensive petrological, geochemical, and geochronological study of the neighbouring Markam and Taiyanghe granitoid plutons (separated by less than 2 km) intrusive into Songpan Ganze metasediments. The main volume of the Markam pluton is dominated by an S-type peraluminous mediumto coarse-grained granite [QtzþPlþKfs(6Mc)þBtþMsþGrtþChlþTur] which was emplaced over a period of 20 Myr from 231 to 209 Ma based on U–Pb zircon ages, at an estimated minimum depth of 11 km (300 MPa). Markam granites [0 709405< Sr/Sr(t)< 0 711113; –9 02< eNd(t)< –7 99] result from the melting of the Songpan Ganze metasediments under amphibolite-facies conditions at a depth of at least 20 km. The magmatic series has evolved from the parental melt by fractional crystallization yielding a residual S-type, peraluminous, coarse-grained granite with feldspar phenocrysts (QtzþKfsþPlþBtþMsþAln) forming the southeastern branch of the Markam pluton with U–Pb zircon ages ranging from 230 to 200 Ma (peak at 207 Ma). The main volume of the Taiyanghe pluton is dominated by I-type, high-K calc-alkaline, fineto coarse-grained diorite and syenodiorite (QtzþPlþKfsþBtþAmp 6 Chl 6 Grt), and was emplaced over a period of 40 Myr (239–202 Ma) at an estimated minimum depth of about 11 km. Taiyanghe rocks have isotopic signatures [0 708037< Sr/Sr(t)< 0 708665; –6 27< eNd(t)< –3 62] typical of magmas resulting from partial melting in a subduction context of an ultramafic source under eclogite-facies conditions and were only slightly geochemically influenced by Songpan Ganze metasediments. This magmatic series has experienced limited fractional crystallization from the parental magma, leading to the formation of a (PlþAmp)-rich cumulate. The Taiyanghe pluton also includes I-type, low-K calc-alkaline hornblende-gabbros [Amp(Hbl)þPlþQtzþBt], that have to our knowledge not been recognized before in the Songpan Ganze terrane, which we interpret as a residual mush resulting from the differentiation of the Taiyanghe magmas, intruding the pluton soon after its formation. These data provide evidence that the Triassic plutons intruding the Songpan Ganze accretionary orogenic wedge result from partial melting at different levels of mantle and crustal protoliths leading to magmas that show limited interactions despite their nearby emplacement, over a 30 Myr period. VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.comã 2221 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 11, 2221–2256 doi: 10.1093/petrology/egy007 Advance Access Publication Date: 3 February 2018