Mathematics – Logic
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.1899h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 8, pp.1899-1911
Mathematics
Logic
5
Scientific paper
Precise U-Pb zircon ages obtained for granitoid units in the Morkheia region, Norway, indicate that at least three periods of felsic magmatism can be discerned: 1.21, 1.19, and 1.13 Ga. The oldest period is represented by the Drihveia gneiss (1205 ± 9 Ma), followed by the Gjerstad augen gneiss (1187 ± 2 Ma) and three separate plutons within the Morkheia Complex, from south to north, that yield similar ages of 1132 ± 3, 1134 ± 2 and 1130 ± 2 Ma, respectively. The Morkheia Complex contacts are discordant to the regional foliation in the Telemark Sector; therefore, the 1.13 Ga age provides a minimum estimate for the timing of regional high-grade metamorphism and deformation. However, the complex is transected by the crustal-scale, Sector-bounding Porsgrunn-Kristiansand Shear Zone, documenting a major phase of Grenvillian ductile deformation post-1.13 Ga. Additional evidence for Grenvillian metamorphism in this region includes a 907 ± 14 Ma lower intercept age for a sample of Drivheia gneiss, that is controlled by a concordant titanite analysis. Previously reported Rb-Sr whole-rock results obtained for multiple samples collected at the same U-Pb sample localities yield geologically meaningless apparent ages. Re-evaluation of these data indicate that at least two main mechanisms are responsible for the disturbance; the anomalously old ages result from the mixing of two distinct isotopic components and the generation of a fictitious isochron at the time of emplacement, whereas the anomalously young ages generally reflect postcrystallization Rb addition during a pervasive low grade alteration event ca. 1050 Ma. Granitoid magmatism at the southeastern boundary of the Telemark Sector, Norway, may be part of a more widespread igneous event that occurred throughout Laurentia. Magmatism in the period between 1.21 and 1.13 Ga can be traced over distances of more than 4000 km in the former supercontinent and may represent the products of mantle melting during an anomalous thermal regime that existed prior to Mesoproterozoic continental rifting.
Heaman Larry M.
Smalley P. C.
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