TY - JOUR
T1 - Precambrian multi-stage crustal evolution in the Bamble sector of south Norway
T2 - Pb isotopic evidence from a Sveconorwegian deep-seated granitic intrusion
AU - Andersen, T.
AU - Hagelia, P.
AU - Whitehouse, M. J.
PY - 1994/10/5
Y1 - 1994/10/5
N2 - The Precambrian Ubergsmoen augen gneiss in the Bamble sector of south Norway is a dry, syntectonic charnockitic intrusion, which was emplaced at amphibolite- to granulite-facies conditions in the Sveconorwegian orogeny (∼ 1120 Ma). The present-day 206Pb 204Pb and 207Pb 204Pb ratios show restricted variation ( 206Pb 204Pb = 17.566-17.938, 207Pb 204Pb = 15.459-15.590 in whole rocks; and 206Pb 204Pb = 17.159-17.350, 207Pb 204Pb = 15.451-15.487 in feldspar), whereas the 208Pb 204Pb ratio has a wider range of variation in whole rocks (36.571-42.200), but not in feldspars (36.487-37.054). These values reflect a 238U 204Pb (μ) of 2-6 since the Sveconorwegian, and a variable Th/U ratio (2-20). At the time of final U-Pb differentiation, homogenization of the Pb isotopes was imperfect. The data therefore do not allow Pb-Pb isochron dating, but are well suited for model calculations. The isotopic evolution of Pb can be reproduced by a three-stage model, involving: (1) a mantle stage ending at 1.9-1.6 Ga; (2) extraction of a crustal precursor at 1.9-1.6 Ga, and differentiation of a LILE-enriched component (μ = 14-22); and (3) anatexis, igneous differentiation, emplacement and subsequent metamorphism at ≈ 1.12 Ga. The granitic intrusion acquired its low U/Pb ratio by magmatic differentiation processes, rather than by fluid-induced U loss at high-grade metamorphic conditions.
AB - The Precambrian Ubergsmoen augen gneiss in the Bamble sector of south Norway is a dry, syntectonic charnockitic intrusion, which was emplaced at amphibolite- to granulite-facies conditions in the Sveconorwegian orogeny (∼ 1120 Ma). The present-day 206Pb 204Pb and 207Pb 204Pb ratios show restricted variation ( 206Pb 204Pb = 17.566-17.938, 207Pb 204Pb = 15.459-15.590 in whole rocks; and 206Pb 204Pb = 17.159-17.350, 207Pb 204Pb = 15.451-15.487 in feldspar), whereas the 208Pb 204Pb ratio has a wider range of variation in whole rocks (36.571-42.200), but not in feldspars (36.487-37.054). These values reflect a 238U 204Pb (μ) of 2-6 since the Sveconorwegian, and a variable Th/U ratio (2-20). At the time of final U-Pb differentiation, homogenization of the Pb isotopes was imperfect. The data therefore do not allow Pb-Pb isochron dating, but are well suited for model calculations. The isotopic evolution of Pb can be reproduced by a three-stage model, involving: (1) a mantle stage ending at 1.9-1.6 Ga; (2) extraction of a crustal precursor at 1.9-1.6 Ga, and differentiation of a LILE-enriched component (μ = 14-22); and (3) anatexis, igneous differentiation, emplacement and subsequent metamorphism at ≈ 1.12 Ga. The granitic intrusion acquired its low U/Pb ratio by magmatic differentiation processes, rather than by fluid-induced U loss at high-grade metamorphic conditions.
UR - http://www.scopus.com/inward/record.url?scp=0028574944&partnerID=8YFLogxK
U2 - 10.1016/0009-2541(94)90023-X
DO - 10.1016/0009-2541(94)90023-X
M3 - Article
AN - SCOPUS:0028574944
SN - 0009-2541
VL - 116
SP - 327
EP - 343
JO - Chemical Geology
JF - Chemical Geology
IS - 3-4
ER -