Från gabbro till granatamfibolit : en studie av metamorfos i Åkermetabasiten, väster om Protoginzonen, Småland

Sammanfattning: The Protogine Zone is the easternmost major deformation zone in the southern part of the Sveconorwegian Province. Over the past 20 years discussions have concerned its tectonic significance, e.g. whether this zone is extensional or compressional. Several generations of mafic intrusives are situated along the Protogine Zone. The metamorphic and deformational history of these intrusions provides a key for understanding the Protogine Zone. The Åker metabasite, a strongly deformed garnet-amphibolite (1'8 km large), is situated in the westernmost part of the Protogine Zone c. 5 km west of Skillingaryd, Småland. The protolith age is 1565 Ma and the main deformation most likely relates to Sveconorwegian reworking. Along a 20 m long road-cut the basic rock undergoes a distinct transition. In a central part (ca. two metres) the primary texture and mineralogy(An60 + Cpx +Ol + Ilm) of the rock is well-preserved. This central gabbro core is mantled by a 5 dm wide zone of metamorphosed but undeformed metagabbro with amphibolite facies minerals (Pl + Zoi + Amf +Bt + Gt) pseudomorphing the primary minerals. The metabasite is successively more deformed and recrystallized until it becomes a strongly foliated garnet-amphibolite (An18 + Hbl + Bt + Gt + Qz ± Ti).Through the road-cut it is thus possible to document the metamorphic reactions and the relations between deformation and metamorphism. In the penetratively deformed garnet-amphibolite most reactions have reached equilibrium. P-T estimates (Gt + Hbl + Pl + Qz)have yielded pressures of 10.5 - 12 kbar and temperatures around 600 °C. P-T estimates from metabasites further south-west record similar pressures but significantly higher temperatures (680- 800 °C). The P-T estimates from the Åker metabasite shows that metamorphism close to the Protogine Zone occurred at pressures corresponding to ca. 40 km depth. In the metamorphosed but undeformed part ilmenite has started to break down and metamorphic zircon has formed as colliers along the rims. When ilmenite (which can contain large amounts of Zr)is consumed zirconium is released to form zircon provided that SiO2 is available. In the penetratively deformed metagabbro zircon aggregates form elongated domains together with biotite, titanite and remnants of magmatic ilmenite. Since the crystallization of zircon is texturally related to a metamorphic reaction the age of these zircons will give an age of the metamorphic and deformational event.