Deformation history of anorthosite massifs: a comparison of Sognefjord and Tafjord, south west Norway
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Anorthosite massifs are a common feature in several geological provinces in Norway. In this paper the deformation history of the anorthosite massifs in Sognefjord and Tafjord, south west Norway, will be investigated. Relics of Precambrium deformation may be preserved in these anorthosite massifs. We want to find out if and where these relics are present. Furthermore deformation mechanisms of plagioclase under different metamorphic conditions will be examined and deformation mechanism maps for plagioclase will be constructed. Optical microscopy was used to study microstructures to determine recrystallization mechanisms. Grain size analysis is performed to infer the average recrystallized grain size, which are used to infer shear stress and strain rate. Three different paleopiezometers are used and compared. An estimate of the water content of the plagioclase was made using chemical reactions. Most of the anorthosites from Sognefjord have a composition which contains 20-35 percent mafic minerals. They were deformed during the Caledonian orogeny under retrograde amphibolite facies conditions, leading to mineral assemblages consisting of plagioclase, green hornblende, pyroxene and some residual garnet. Small scale shear zones are related to this deformation event. Recrystallization mechanisms obtained from microstructures associated with this event are bulging recrystallization and subgrain rotation recrystallization in partly to almost completely recrystallized fabrics. Three types of bands of recrystallized grains are found, monophase bands of plagioclase, with an average recrystallized grain size of 33 to 48 μm, multiphase bands of plagioclase-hornblende, or plagioclase-pyroxene, with an average recrystallized grain size of 23 to 41 μm, and monophase hornblende bands, which extend from hornblende porphroclasts, with a grain size that is too small to measure with optical microscopy. Also found in these anorthosites are predeformational, granulite facies conditions structures, like corona structures, which are probably post Sveconorwegian, and banded rocks with isoclinal folds, which are related to the Sveconorwegian orogeny. These granulite facies conditions structures have a composition consisting of plagioclase, pyroxene, mostly clinopyroxene, brown hornblende and garnet. The anorthosites from Tafjord are proper anorthosites, with only 5-10 percent mafic minerals. During the Caledonian orogeny these rocks were deformed under high grade metamorphic conditions. Microstructures are irregular shaped grains with subgrains, and interlobate grain boundaries, associated with high temperature grain boundary migration recrystallization and subgrain rotation recrystallization. The assemblages found in these anorthosites consist of plagioclase, biotite and hornblende, these are related to greenschist facies conditions, which is in contradiction with the high grade metamorphic conditions. Fast cooling after deformation to greenschist facies conditions, with static recrystallization, could be a solution for this contradiction. Grain growth by grain boundary area reduction during static recrystallization would also explain the relatively large recrystallized grain sizes, of approximately 74 μm, found in the samples from Tafjord. Due to the complete recrystallization of the rocks during the Caledonian orogeny no relics of older deformation, during the Sveconorwegian orogeny, were found in the samples from Tafjord. The shear stress during the Caledonian orogen inferred for Sognefjord is approximately 37 MPa and an associated shear strain rate was inferred of approximately 1·10-10. Due to static recrystallization after deformation, shear stresses inferred for Tafjord are an underestimate of the actual shear stress of deformation. Metamorphic conditions during the Caledonian orogeny were higher in Tafjord than in Sognefjord. Cooling after deformation was relatively fast at both locations. The rocks from Sognefjord and Tafjord have both undergone at least two deformation events after formation of the anorthosite massifs. First during the Sveconorwegian orogeny and later during the Caledonian orogeny. In Sognefjord relics of the Sveconorwegian orogeny are preserved, but in Tafjord the rocks have a completely recrystalized fabric, formed during the Caledonian orogeny. A water content of 0,3 weight percent seems accurate for the rocks in Sognefjord and Tafjord. After comparison of the three paleopiezometers, the one that gave the best fitting results was used to construct the deformation mechanism maps for plagioclase.