A Rb-Sr isotope study of granitoids and related mylonites in the Tännäs Augen Gneiss Nappe, southern Swedish Caledonides
作者:
Stefan Claesson,
期刊:
Geologiska Föreningen i Stockholm Förhandlingar
(Taylor Available online 1980)
卷期:
Volume 102,
issue 4
页码: 403-420
ISSN:0016-786X
年代: 1980
DOI:10.1080/11035898009454497
出版商: Taylor & Francis Group
关键词: Rb-Sr;U-Pb;absolute age;dates;whole rock;mineral age;zircons;isochron;Concordia diagram;deformation;thrust faults;diffusion;recrystallization;granitoid;mylonite;augen gneiss;Tännäs Augen Gneiss Nappe;Veman Nappe;Middle Allochthon;Caledonides;Low
数据来源: Taylor
摘要:
The Tännäs Augen Gneiss Nappe and parts of the related, subjacent Veman Nappe are composed of K-feldspar megacryst bearing granodiorites, which have been subject to low-grade metamorphism and varying degrees of deformation. They form parts of the Middle Allochthon — a long-transported nappe unit in the central Scandinavian Caledonides. The least-deformed granodiorite has a Rb-Sr age of 1610 ± 85 Ma and Sr1= 0.7029, while zircons from the same rock give a discordia line with an upper intercept at 1685 ± 20 Ma. These dates reflect the age of intrusion of the rock. The granodiorite is comparable in composition and age with post-Svecokarelian granitoids and related volcanites east of the mountain belt. The more typically deformed augen gneiss does not yield isochrons, but shows disturbed Rb-Sr patterns both for specimens sampled regionally and on smaller scales. At the thrust contacts towards over- and underlying units, where the deformation was extreme, the gneisses have been transformed to dense mylonites. One specimen from the mylonite which separates the Tännäs Augen Gneiss Nappe from the overlying Särv Nappe has been divided into thin slices; these give a Rb-Sr isochron with an early Ordovician age of 485 ± 50 Ma, while mylonites collected on a larger scale scatter about this isochron. The age most probably reflects the time of development of the mylonite. The Sr isotopic equilibration during the Caledonian orogeny has approached completion only in the mylonite and even there only on a decimetre-scale. It has probably taken place mainly by grain boundary diffusion combined with extensive recrystallization.
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