Ordovician and Silurian Magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland Nappe Complex, North-Central Norway

• Main Authors: Barnes, Calvin G., Reid, Kristin, Frost, Carol D., Barnes, Melanie, Allen, Charlotte M, Yoshinobu, Aaron S
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Geological Society Norway
• Subjects: Keywords: allochthon; batholith; dehydration; depositional sequence; emplacement; gneiss; magmatism; migmatization; nappe; Ordovician; shear zone; Silurian; zircon; Helgeland; Nordland; Norway; Bindal
• Online Access: http://hdl.handle.net/1885/32597; https://openresearch-repository.anu.edu.au/bitstream/1885/32597/5/NJG_3_2011_Barnes.pdf.jpg; https://openresearch-repository.anu.edu.au/bitstream/1885/32597/7/01_Barnes_Ordovician_and_Silurian_2011.pdf.jpg

The structurally lowest parts of the Upper Nappe of the Helgeland Nappe Complex consist primarily of migmatitic quartzofeldspathic gneiss with intercalated calc-silicate gneiss. Detrital zircons in the gneiss indicate an Ordovician depositional age. Migmatization via biotite dehydration reactions occurred at ~480 Ma at pressures of ~700 MPa; some migmatites were mobilized into cross-cutting diatexites. Diverse gabbroic to granitic plutons and dikes of the Bindal Batholith were emplaced from 448-424 Ma. They range from calc-alkaline to alkalic and most have initial 87Sr/86Sr <0.707 and εNd from 0 to -4. Pressures of emplacement were ca. 300 MPa. Shear zones with reverse shear sense cut 437 Ma intrusions, suggesting that contraction accompanied magmatism.