Geochronological constraints for a two-stage history of the Sveconorwegian rare-element pegmatite province formation

Most pegmatites of southern Norway seem to be derived from anatectic melting of metamorphic rocks during the Sveconorwegian orogeny rather than to be highly evolved residual melts derived from granites. We test this hypothesis by providing new age data for thirteen pegmatites and one granite. Based on these new age data, we distinguish two age groups of Sveconorwegian pegmatites (>1,000 m3 in size); Group 1: 1100-1030 Ma and Group 2: 930-890 Ma. All pegmatites except those from the Ostfold area crystallized significantly earlier or later than adjacent granites. The Tordal granite, yielding an age of 946 +/- 4 Ma, is about 40 Ma older than the adjacent pegmatites. Field evidence and the age difference between pegmatites and granites supports an anatectic origin for these pegmatites. Sources of these pegmatite melts are biotite-and biotite-amphibole gneisses and am-phibolites. Group 1 pegmatites formed in transpressional regimes after peak metamorphism, whereas Group 2 pegmatites formed in an extensional regime and the required heat for partial melting was provided by mafic magma underplating.Differences in the rheological behavior of amphibolite and granitic gneiss during extensional tectonics are the major reason why Group 2 pegmatites occur preferentially in large amphibolite bodies. Under mid-crustal conditions, amphibolite reacts brittle to semi-brittle forming open structures in an extensional tectonic regime where partial melts drained into. Granitic gneisses react in a ductile manner and do not have the ability to drain partial melt.Pegmatite formation in the Grenville Province, i.e., the Laurentian part of the Grenville-Sveconorwegian orogenic belt, formed between ca. 1090 and 980 Ma peaking at 1010 to 980 Ma. Thus, the Grenville peak postdates the Sveconorwegian Group 1 peak by about 30 Ma. These pegmatites formed in similar orogenic settings, implying that similar tectono-metamorphic developments along the Grenville-Sveconorwegian orogenic belt were diachronous.We conclude that local anatexis is the major pegmatite-melt forming process in the Sveconorwegian as well as Grenville orogen. Local anatexis also may be important in other pegmatite provinces.

Automated summary

Most pegmatites in southern Norway are not highly evolved residual melts derived from granites, but rather formed from anatectic melting of metamorphic rocks during the Sveconorwegian orogeny. New age data for thirteen pegmatites and one granite support this hypothesis, showing two age groups of Sveconorwegian pegmatites: Group 1 (1100-1030 Ma) and Group 2 (930-890 Ma). The age difference between pegmatites and adjacent granites, as well as field evidence, suggests an anatectic origin for these pegmatites.