Isotopic investigations of the Nordingrå rapakivi massif, north-central Sweden
Titel:
Isotopic investigations of the Nordingrå rapakivi massif, north-central Sweden
Auteur:
Welin, Eric Lundqvist, Thomas
Verschenen in:
GFF
Paginering:
Jaargang 106 (1984) nr. 1 pagina's 41-49
Jaar:
1984-10-31
Inhoud:
Granite, gabbro and anorthosite together constitute the Nordingrå massif. These igneous rocks have intruded a sequence of Svecokarelian metasediments and postdate the regional metamorphism and folding. Available field observations demonstrate that the granite intrudes the gabbro-anorthosite complex. Zircons separated from the granite and the anorthosite have an age of 1578 ± 19 Ma, which is considered to indicate the time of magmatic crystallization. The Rb-Sr whole rock age 1552 ± 18 Ma of the gabbro and anorthosite is slightly lower than the zircon age, whereas the Rb-Sr reference age of the granite, 1416 Ma, is considerably lower than the age of the zircons. Hypothetical models of genesis and history are discussed to explain this discordancy. It is suggested that post-crystallization hydrothermal fluids, circulating in the bedrock during the uplift and cooling of the orogenic belt and/or during subsequent epeirogenic movements, transported Rb and Sr isotopes in the whole rock system. This led to an apparent low Rb-Sr age and a high initial 87 Sr/86 ratio. The conclusions are supported by δ18O analyses, which indicate that the granite has been depleted in 18O, and by microscopical observation of mineral alterations. K-Ar ages of separated minerals from granite and gabbro may also have been influenced by these alterations. A thermal imprint on the bedrock at the intrusion of the Jotnian dolerite sheets may also have contributed to the observed considerable spread of the K-Ar ages from 1600 to 1100 Ma.
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