Laser ⁴⁰Ar/³⁹Ar ages of single detrital white mica grains related to the exhumation of Neoproterozoic and Late Devonian high pressure rocks in the Southern Urals (Russia)

Abstract

Single grains of detrital white mica from two different synorogenic sediments in the Southern Urals were analysed using the in situ ultraviolet laser ablation Ar-Ar dating technique to discriminate between age signatures associated with a high-pressure signal (phengites) from those related to muscovite only. Two disparately aged sandstone formations of Neoproterozoic (Upper Vendian) and Upper Devonian (Famennian) age were formed by the erosion of high-relief source areas with contemporaneously exhumed high-pressure rocks. A bimodal distribution of ages and chemical compositions can be detected in the two detrital populations. There is no age overlap between the two populations, reflecting completely different source areas containing high-pressure rocks of different ages. Within the Upper Vendian sandstones, detrital white mica from a 571-609 Ma age group is phengitic in composition (Si 3.3-3.41 per formula unit), while an older 645-732 Ma age group is comprised of muscovite composition grains only. The first group is compatible with the time of late exhumation and emplacement of a source area containing high-pressure rocks, the Neoproterozoic Beloretzk terrane. The older age range is compatible with a long history of cooling and the allochthonous nature of this terrane. Detrital white mica from the Famennian sandstones (Zilair Formation) comprises one age group (342-421 Ma) containing phengite (Si 3.21-3.39 per formula unit) and muscovite, and a second group (446-496 Ma) containing muscovite only. While the derivation of the second group cannot be correlated with any as yet known regional data, the first age group indicates the earliest arrival of high-pressure rocks at the surface along the suture zone after Late Devonian arc-continent collision.

P–Tevolution and timing of a late Palaeozoic fore-arc system and its heterogeneous Mesozoic overprint in north-central Chile (latitudes 31–32°S)

“…Following similar findings by Willner et al . (2004 a ) and Willner (2005), the exotic intercalation of garnet mica-schist in the CMC with the highest P–T conditions and oldest ages can be interpreted as being derived from the subduction channel below the mantle wedge of the convergent margin. Hence, in both rocks (06CH45 and 02CH44) about 35–40 Ma of retrograde mineral formation during exhumation are documented.…”

“…We envisage an in situ dissolution/precipitation effect by hydrous fluids penetrating the rocks at different times and depths. Willner et al 2004b). 6a, b).…”

Intrusion-Hosted Mineralization in the Charters Towers Goldfield, North Queensland: New Isotopic and Fluid Inclusion Constraints on the Timing and Origin of the Auriferous Veins

“…After irradiation in the H5 position of the nuclear reactor at McMaster University, Canada, the samples were analyzed, using the infrared laser ablation total fusion technique (e.g., Hodges, 1998). Analytical methods and equipment were similar to those described by Willner et al (2004).…”

Baltica in the Cryogenian, 850–630 Ma

“…That said, this change from an extensional to convergent regime is recorded across Baltica's eastern region: (i) fold and thrust tectonics are well documented by a prePaleozoic unconformity traced along the entire western slope of the Urals, and it is evident that the structural trends of the Timanian and Uralian fold belts do not coincide (Fig. 4); (ii) Cambrian strata are almost absent in the Urals (Stratigraphic schemes of the Urals, 1993); (iii) Ediacaran (Early Vendian) sediments, in contrast to the older Neoproterozoic sediments, are represented by polymict molasse (Ablizin et al, 1982;Willner et al, 2001Willner et al, , 2003Willner et al, , 2004; (iv) Orogeny was accompanied by regional greenschist metamorphism which is traced continuously along the deformation front of the Timanides, with amphibolite and even lower-eclogite facies metamorphism recorded locally (Getsen et al, 1987;Rusin et al, 1989;Glasmacher et al, 2001;Alexeev et al, 2002;Rusin, 2004;Lorenz et al, 2004).…”