International Geologiical Congress - Oslo 2008


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MPI-04 Mafic dyke swarms: A global perspective - Part 2


Magmatic and metamorphic constraints for the petrogenesis of the Jönköping Anorthositic suite


Linus Brander, University of Gothenburg (Sweden)
Johan Hogmalm, University of Gothenburg (Sweden)


The southern Fennoscandian Shield was subjected to wide-spread magmatism at 1.47-1.44 Ga, probably related to orogenesis along its southern margin. This magmatism was characterised by emplacement of mafic, mainly N-trending, dykes in the north and by granite plutons in the south. A member of this event is the 1455 Ma Jönköping Anorthositic Suite (JAS), which is positioned at the approximate transition between the regions dominated by mafic and felsic magmatism.
JAS is a suite of massif-type anorthosites in southern Sweden, represented by km-wide intrusions in a >30 x 30 km large area. The surrounding country rock is dominated by 1.71-1.66 Ga foliated granitoids of the Transscandinavian Igneous Belt. U-Pb ages of secondary zircon from these granitoids indicate high-grade metamorphism at 1.46-1.41 Ga.
The anorthositic rocks of the JAS exhibit a wide variety in the degree of alteration. Metamorphic mineral assemblages up to garnet amphibolite facies grade exist alongside relatively well-preserved igneous assemblages of plagioclase (average An55) + orthopyroxene + clinopyroxene + ilmenite + apatite. Several of the JAS bodies host deformational features (e.g. shear-zones and folds) that are attributed to the same deformation as seen in the country rock. Deformation in country rocks is cut by members of the 1.43 Ga Axamo Dyke Swarm (ADS), a swarm of gabbroic to granitic dykes associated with the JAS. These temporal and structural relations suggest that the age of the deformation is bracketed between 1455 and 1430 Ma.
The textural and mineralogical diversity of the JAS enable studies of emplacement conditions, the tectonic and metamorphic history, and the timing of these events. This contribution presents data and models attempting to provide 1) crystallization temperatures and pressure(s); 2) the composition of the parental liquid and its evolution throughout the crystallization path (phenocrystic opx - phenocrystic plag - matrix plag - matrix opx and cpx); 3) metamorphic conditions (PT); and 4) quantification of the three-component interaction of intrusion-country rock-deformation. We suggest that the anorthositic rocks of the JAS underwent polybaric crystallization, a statement based on the presence of cm-sized high-Al orthopyroxene inclusions within dm-sized reversely zoned plagioclase phenocrysts. The differentiation was probably controlled by orthopyroxene fractionation. The plagioclase phenocrysts are set in a fine- to medium grained noritic matrix indicating final emplacement at shallow levels.


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