HR: 1340h
AN: V43A-1107    [Abstracts]
TI: Ilmenite-Rich Layers and Sequences in Regressive Intervals in the Bjerkreim Layered Series and Their Relation to Magma Mixing During Episodes of Chamber Replenishment.
AU: * Chiodoni, F
EM: federico.chiodoni@geo.uib.no
AF: Department of Earth Science, University of Bergen, Allegt. 41, Bergen, 5007, Norway
AU: Robins, B
EM: brian.robins@geo.uib.no
AF: Department of Earth Science, University of Bergen, Allegt. 41, Bergen, 5007, Norway
AB: The Bjerkreim-Sokndal Layered Intrusion and the closer massif-type anorthosite plutons make up the Egersund- Farsund Igneous Province emplaced in Rogaland south Norway. The layered Intrusion on the base of the rock composition is divided in three main portions: anorthosite and leuconorite, troctolite and leuconorite, mangerite and qtz-mangerite. The main aim of the present work is to characterize the boundary between Megacyclic Units (MCU) II and III along the North-Eastern side of the Intrusion in order to construct a model for mixing in the magma chamber that led to the crystallization of the ilmenite-rich sequences at the base of MCU III. In the Bjerkreim Layered Series ilmenite- rich layers and sequences are rare and restricted to the regressive zones at the bases of MCU III and IV. Cumulates elsewhere in the Layered Series contain limited amounts of ilmenite, as expected during the cotectic crystallization of plagioclase, orthopyroxene and ilmenite. In a restricted area along the eastern flank of the Bjerkreim lobe of the intrusion, ilmenite-rich cumulates form thin, strongly modally-layered sequences near the base of MCU III. Here, they are closely associated with rather massive leuconorite, rare troctolite and a sulphide- bearing layer of melanorite or ilmenite orthopyroxenite. Jensen et al. (2000) inferred that the ilmenite orthopyroxenite or melanorite that marks the base of MCU III was related to the initiation of replenishment and the consequent hybridization of inflowing jotunite (hypersthene monzodiorite) magma and resident magmas. At this time the floor of the chamber was characterised by a deep axial trough and a much shallower eastern flank limited to the north by a pronounced transverse ridge. The inflowing magma was envisaged as being less dense than the lower part of the differentiated and stratified resident magma and orthopyroxene and ilmenite crystallised from an extensive layer of hybrid magma some distance above the temporary floor of the chamber. Dilute suspensions of pyroxene, ilmenite and subordinate plagioclase were postulated to have been carried from the layer of hybrid magma to the floor of the chamber in intermittent plumes. The layer of hybrid magma thickened as replenishment and mixing in the buoyant plume continued and it eventually reached the floor of the chamber. Replenishment culminated in the crystallisation of troctolite (plagioclase-olivine-ilmenite-magnetite cumulate) on the deeper parts of the chamber floor while the crystallisation of lower-temperature norite (plagioclase-hypersthene-ilmenite cumulate) continued on the elevated portions. The results of a more detailed investigation of the stratigraphy, cryptic layering and whole-rock major and trace- element compositions of the cumulates forming the basal portion of MCU III on the north-eastern flank of the Bjerkreim lobe shows a striking correlation between the mineral composition and the stratigraphic distribution of cumulus ilmenite and orthopyroxene. Ilmenite–rich layers and sequences are always characterised by more primitive mineral compositions than the enclosing leuconorite. This is clear evidence that cumulates at the base of MCU III crystallised from two different magmas. Furthermore the shape of the floor of the magma chamber seems to have played a fundamental role in the formation of the ilmenite-rich layers since they are preserved only on the shallow eastern flank of the intrusion. Enrichment in ilmenite in excess of normal cotectic proportions appears to be a consequence of either the sequence of crystallisation in the hybrid magma or mineral sorting in descending plumes.
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
DE: 3643 Layered magma chambers
DE: 3690 Field relationships (1090, 8486)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting