Polybaric evolution of the Hakefjorden Complex, southwestern Sweden, deduced from partial dissolution in andesine megacrysts

Abstract

Disequilibrium textures in andesine megacrysts, showing re‐sorbed morphologies, of the post‐kinematic Sveconorwegian Hakefjorden Complex provide important information about the emplacement mechanism. These textures consist of fine cellular plagioclase inter‐growths occurring as 0.4–2.5 mm mantles around the megacrysts and as entirely cellular grains. Outside the cellular mantle the megacrysts are covered by a normally zoned rim. The cellular mantles consist of two phases, one relatively Na‐rich (An₄₅) and one relatively Ca‐rich (An₅₉). Compositionally, the Na‐rich phase is similar to the non‐cellular megacryst core (An₄₆) and the Ca‐rich phase is similar to the inner part of the normally zoned rim (An₆₀). This rim grades from An₆₀) to An₃₀ which is about the same as the matrix plagioclases of the surrounding norite. The cellular network of plagioclase intergrowths is interpreted as a result of partial dissolution caused by decompression during the emplacement. The emplacement mechanism can be summarised as follows: (1) Crystallisation of plagioclase, from a mantle derived magma, deep in the crust. (2) Partial dissolution of the early crystallised plagioclase related to decompression as the magma rise to a higher level chamber. (3) Renewed crystallisation at lower pressure creating the normally zoned rim at the same time and with the same composition as the matrix plagioclase in the surrounding norite.

Årebäck, H. & Stigh, J., 1997: Polybaric evolution of the Hakefjorden Complex, southwestern Sweden, deduced from partial dissolution in andesine megacrysts. GFF, Vol. 119 (Pt. 2, June), pp. 97–101. Stockholm. ISSN 1103–5897.

Keywords:

• Andesine megacryst
• anorthosite
• cellular plagioclase
• emplacement of anorthosites
• partial dissolution
• southwestern Sweden