Bulletin of the Geological Society of Finland (Dec 2001)
Age of the Tokkalehto metagabbro and its significance to the lithostratigraphy of the early Proterozoic Kuusamo supracrustal belt, northern Finland
Abstract
The c. 30-km2 Tokkalehto metagabbro and associated sills and dykes intrude a large part of the western part of the early Proterozoic Kuusamo supracrustal belt in northern Finland. These rocks have previously been mapped as a metavolcanic unit, Greenstone Formation III, the type area of which occurs further east in the Kuusamo supracrustal belt. However, observations along the contacts of this unit (contact breccias, distinctive skarns and xenoliths) and within this unit (igneous layering and mafic enclaves) indicate that these rocks are intrusive. High resolution SIMS analysis of zircon from the metagabbro at Nurkamokumpu yields a crystallization age of 2216.2 ± 3.8 Ma. On the basis of this age and the lithological and petrographical characteristics of the metagabbro, it belongs to the c. 2.2 Ga karjalite or gabbro-wehrlite suite, which has a wide distribution in eastern and northern Finland. These results show that the strong aeromagnetic anomalies of the western part of the Kuusamo supracrustal belt do not represent Greenstone Formation III metavolcanics. A subdivision of the Kuusamo supracrustal belt into eastern and western halves is proposed along the Kitka Fault. Instead, these anomalies represent c. 2.2 Ga gabbroic intrusions, which intruded metasediments of the western Kuusamo supracrustal belt.
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