Age determination is an essential part of many geological investigations. The ages of Phanerozoic sedimentary rocks are typically determined by palaeontological stud-ies (often with high precision), but for Precambrian rocks as well as for younger igneous intrusions, pre-cise ages can only be obtain...
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| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.607.6125 http://www.geus.dk/publications/review-greenland-96/gsb176p55-59.pdf |
| Summary: | Age determination is an essential part of many geological investigations. The ages of Phanerozoic sedimentary rocks are typically determined by palaeontological stud-ies (often with high precision), but for Precambrian rocks as well as for younger igneous intrusions, pre-cise ages can only be obtained by isotopic analysis of minerals or rocks. Isotopic (radiometric) age determi-nation of Greenland rocks began in the early 1960s and has continued since with gradually improving meth-ods. In this contribution, the development of geochrono-logical knowledge of the Precambrian of Greenland is described in historical perspective, and an outline of new results is given. The history of geochronology can be roughly divided into three periods: 1) a period of single-sample K-Ar and Rb-Sr mineral or whole-rock age determinations; 2) a time when most ages were determined with the help of Rb-Sr and Pb-Pb whole-rock isochrons and multi-grain zircon U-Pb isotope data; 3) the present, where ‘single ’ zircon U-Pb data are the preferred method to obtain rock ages. These stages in the development of radiometric dating methods partly overlap in time, and each has yielded very significant contributions to the knowledge of Precambrian evolution in Greenland. The early years The first results of K-Ar and Rb-Sr single-sample dat-ing for Greenland rocks were published around 1960. Moorbath et al. (1960) presented Rb-Sr and K-Ar data for micas from two intrusions belonging to the Gardar igneous province of South Greenland (Fig. 1), from which they calculated ages between c. 1100 and 1250 Ma (Ma = million years). Until then it was believed these intrusions could be of Palaeozoic age (Ussing, 1912; Moorbath et al., 1958). Moorbath et al. (1960) also obtained a minimum age of c. 1600 Ma for the Julianehåb granite, the country rock to the Gardar intru-sions. Subsequent isotopic data have demonstrated that the age of the Julianehåb granitoids is about 1800 Ma (see p. 56-57). Shortly after, the first evidence was found Age determination of ... |
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