Detrital zircon provenance of the Bremnes Migmatite Complex on Bømlo, SW Norway
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Detrital zircon provenance, geochemistry, and field descriptions from the Bremnes Migmatite Complex provide new insights in the origin and evolution of the Early Ordovician ophiolitic terrane of SW Norway. The Bremnes Migmatite Complex developed as the Laurentian continental margin was subducted sufficiently below the ophiolitic terrane to commence partial melting of the sedimentary protolith. The migmatization of the complex at 477 ± 7 Ma, occurred prior to the intrusion of the Vardafjell Gabbro (472 ± 2 Ma), and simultaneously to the intrusion of the S-type granitoids of the West Karmøy Igneous Complex (474 +3/-2 Ma). Appinites related to an early volatile-rich phase of the Vardafjell Gabbro intruded the migmatite complex, and most likely marks the cessation in subduction of the Laurentian continental margin. Zircons of the Bremnes Migmatite Complex are dominated by Archean (~2700 Ma), Paleoproterozoic (~1800 Ma), and Mesoproterozoic (~1100 Ma) ages, and show major similarities with the provenance signatures of the S-type granitoids of the West Karmøy Igneous Complex, the Vikafjord Group, and the Mundheim Group of the Early Ordovician ophiolitic terrane. Combined, the provenance signature of the Bremnes Migmatite Complex, and the other units of the ophiolitic terrane, resemble sedimentary successions that accumulated at the Laurentian margin in Mid Neoproterozoic (c. 700 Ma) to Early Paleozoic (c. 520 Ma). Our results constrain the continental component of the Early Ordovician ophiolitic terrane to be sourced in a sedimentary succession best described by the Dalradian Supergroup of the Scottish Caledonides. Major- and trace-element composition of the Bremnes Migmatite Complex indicates that the complex primarily developed from fine-grained sedimentary rocks sourced in the Dalradian Supergroup. This is consistent with the formation of time-equivalent migmatites and granitoids exposed within the Scottish Caledonides, that have inherited their continental component from the Dalradian Supergroup, suggesting that the Early Ordovician ophiolitic terrane of SW Norway and time-equivalent units exposed in the Scottish Caledonides were closely related both temporally and spatially. These findings reveal that the continental component present within the Bremnes Migmatite Complex, and several other units of the Early Ordovician ophiolitic terrane, were derived from the Laurentian continental margin, and provides new insights into key parts of the tectono-magmatic evolution of the ophiolitic terrane of SW Norway.
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