Evidence for compositionally zoned magma during crystallisation of some Norwegian layered intrusions
|Location||International Geological Congress,oslo 2008|
|Author||Wilson, J. Richard|
|Holding Date||21 September 2008|
Some of the first evidence for compositional zoning in magma chambers parental to layered intrusions came from Norway in the mid 1980s. Deformation, uplift and erosion of two Caledonian intrusions exposed fortuitous sections that allowed access to their floor and wall(s). The asymmetric arrangement of cumulate layers within ~20 cyclic units in Intrusion 2 of the Honningsvåg Suite (HS-I2) was explained by crystallisation of compositionally zoned magma on an inwardly-sloping floor. Repetition of units occurred as a result of influx of new, dense magma that elevated the zoned magma column, accompanied by magma chamber expansion.
In the Fongen-Hyllingen (FH) layered intrusion there is a discordant relationship between modal and cryptic layering. Cumulus minerals become increasingly evolved along the strike of modal layering approaching the margin. Over a distance of ~7 km, olivine and plagioclase gradually change composition from Fo75:An63 to Fo13:An42 and magnetite, apatite and zircon progressively become cumulus phases. The apparent angle between cryptic and modal layering is usually <20°. This is likewise explained by the crystallisation of compositionally zoned magma along a sloping floor.
In the Late Proterozoic Bjerkreim-Sokdal (BKSK) layered intrusion there is a discordant contact between megacyclic units (MCU). The central portions of the base of MCU IV overlie ilmenite magnetite norites of MCU III, whereas the marginal parts overlie apatite magnetite ilmenite gabbros. This is explained by crystallisation of compositionally zoned magma parental to MCU III in a bowl-shaped chamber along a saucer-shaped crystallisation front. Relatively evolved rocks were crystallising from the buoyant, evolved magma near the flanks at the same time as more primitive assemblages were forming towards the central, deeper part of the chamber. Relatively dense replenishing magma elevated the compositionally zoned magma column and produced an abrupt compositional reversal at the MCU III/IV boundary.
In the uppermost part of BKSK there is a zone of country rock inclusions with a variety of lithologies that can be followed along strike for considerable distances; they indicate an episode of roof collapse. This zone is systematically discordant to the contact between mangerite and quartz mangerite which is concordant with cryptic and phase layering. The nature of this discordance is consistent with that at the MCU III/IV boundary and it implies crystallisation of mangerite in the central part of the chamber at the same time as more evolved quartz mangerite was crystallising towards the margins.
The primary evidence for crystallisation of HS-I2 from compositionally zoned magma is based on field relations; in FH it is mainly based on the relationship between modal and cryptic layering; in BKSK it is based on a variety of evidence. The development of compositional zoning in these three magma chambers is undoubtedly related to the process of repeated magma addition.