U/Pb ages of ophiolites and arc-related plutons of the Norwegian Caledonides: implications for the development of Iapetus
Abstract
U/Pb zircon ages are reported for four ophiolites and three crosscutting arc-related plutons from the Norwegian Caledonides. Plagiogranite differentiated from gabbro of the Karmøy ophiolite is dated at 493+7/-4 Ma whereas arc-related trondhjemite cutting this ophiolite crystallized at 485+/-2 Ma. A crosscutting clinopyroxene-phyric gabbro intrusion is dated at 470+9/-5 Ma by near concordant magmatic titanite (sphene) and discordant U-rich (2903-6677 ppm) zircon. Lower intercepts of 247+/-68 and 191+/-70 Ma defined by the plagiogranite and clinopyroxene-phyric gabbro best-fit lines may reflect a real low-T alteration/rift-related event. A plagiogranite differentiate of the Gullfjellet ophiolite complex is dated at 489+/-3 Ma and a crosscutting arc-related tonalite is 482+6/-4 Ma. Both of these ages overlap with those of the correlative rocks at Karmøy suggesting that they are parts of one ophiolitic terrane with a common history. Trondhjemite associated with the Leka ophiolite is dated at 497+/-2 Ma, indicating that supra-subduction zone magmatism there may be coeval with spreading which formed the Karmøy axis sequence. The U/Pb zircon ages of Norwegian ophiolites reported here, combined with ages of other Appalachian-Caledonian ophiolite complexes in Britain and Canada, indicate a narrow age range for the generation of at least two marginal basins in the Tremadoc-Arenig. Two spreading episodes documented at Karmøy are separated in time by intrusion of arc-related trondhjemite magmas at 485+/-2 Ma and may correlate with two separate spreading events documented in other ophiolites. The Solund/Stavfjorden ophiolite, at 443+/-3 Ma, is the only late Ordovician ophiolite yet documented in the entire Appalachian-Caledonian Orogen and it probably represents a small, short-lived marginal basin late in the history of the Iapetus Ocean. It is correlative with Caradocian ensialic marginal basin magmatism in Wales and the Trondheim region, and with tholeiitic gabbro-diorite plutons that intruded Newfoundland ophiolites in a tensional regime after emplacement of the ophiolites over the continental margin.
- Publication:
-
Contributions to Mineralogy and Petrology
- Pub Date:
- January 1988
- DOI:
- 10.1007/BF00371904
- Bibcode:
- 1988CoMP...98...13D
- Keywords:
-
- Zircon;
- Ordovician;
- Continental Margin;
- Titanite;
- Marginal Basin