Reworking of deep‐seated gabbros and associated contact metamorphosed paragneisses in the southeastern part of the Seiland Igneous Province, northern Norway

• Published in: Journal of Metamorphic Geology
• Main Authors: ELVEVOLD, S., REGINIUSSEN, H., KROGH, E. J., BJØRKLUND, F.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1994
• Subjects: Geochemistry and Petrology; Geology; Norway; Seiland; Northern Norway
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1994.tb00041.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1994.tb00041.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1994.tb00041.x

ABSTRACT The Seiland Igneous Province of the North Norwegian Caledonides consists of a suite of deep‐seated rift‐related magmatic rocks emplaced into paragneisses during late Precambrian to Ordovician time. In the south‐eastern part of the province, contact metamorphism of the paragneisses and later reworking of intrusives and associated contact aureoles have resulted in the development of three successive metamorphic stages. The contact metamorphic assemblage (M1) Opx + Grt + Qtz + Pl + Kfs + Hc + Ilm ± Crd is preserved in xenolithic rafts of paragneiss within metagabbro. Geothermobarometric calculations yield 930‐960d̀ C and 5‐6.5 kbar for the contact metamorphism. M1 was followed by cooling, accompanied by strong shearing, formation of the gneiss foliation and recrystallization at intermediate‐ P granulite facies conditions (M2). Stable M2 phases are Cpx + Opx + Pl +Ilm ± Hbl in metagabbro and Grt ± Sil ± Opx + Kfs + Qtz + Pl ± Bt + Ilm in host paragneiss. The M2 conditions are estimated to 700‐750d̀ C and 5‐7 kbar. A subsequent pressure increase is recorded in the M3 episode, which is associated with recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. M3 is defined by the assemblages Grt + Cpx ± Opx + Pl + Ru + Qtz in metagabbro, and Grt ± Ky + Qtz + Pl ± Kfs + Bt + Ru in host paragneiss. M3 conditions are estimated to 650‐700d̀ C and 8‐10 kbar. The substantial pressure increase related to the M2 → M3 transition is interpreted to be a result of (early?) Caledonian overthrusting. Chemical zoning in cordierite and biotite suggest rapid cooling following the M3 event. The proposed P ‐ T ‐ t evolution implies that the tectonic evolution of the Seiland Igneous Province was long (at least 330 Ma) and complex and involved initial rifting and extension followed by crustal thickening and compression.