Magma influx and mixing in the Bjerkreim-Sokndal layered intrusion, South Norway: evidence from the boundary between two megacyclic units at Storeknuten

Abstract

. Magma influx and mixing in the Bjerkreim-Sokndal layered intrusion, South Norway: evidence from the boundary between two megacyclic units at Storeknuten. Lithos, 29:311-325.The Bjerkreim-Sokndal layered intrusion belongs to the Proterozoic anorthositic province in the Rogaland area of southern Norway. The northwestern part of the intrusion comprises a ca. 6 km-thick Layered Series made up of megacyclic units (MCU) arranged in a syncline; each megacyclic unit reflects the influx of fresh magma into the chamber. The boundary between megacyclic units III and IV has been studied in detail at Storeknuten on the southern flank of the syncline. The megacyclic units can be subdivided into a series of cumulate stratigraphic zones; the interval from the top of zone IIIe to the base of zone IVd is exposed in the Storeknuten area. Modally layered plagioclase-hypersthene-ilmenite-magnetite-augite-apatite cumulates belonging to zone IIIe are overlain by 30 m of massive plagioclase-rich rocks (commonly containing ilmenite and/or hypersthene) constituting zone IVa. The entry of cumulus olivine defines the base of zone IVb (dominantly plagioclase-olivine-ilmenite cumulates) which is about 100 m thick. Many of the olivines are partly or completely replaced by Ca-poor pyroxene/Fe-Ti oxide symplectites. This massive leucotroctolitic zone is overlain by modally layered, laminated plagioclasehypersthene-ilmenite cumulates of zone IVc. The successive entry of magnetite, apatite (accompanied by Ca-rich pyroxene) and inverted pigeonite defines zones IVd, e and f respectively. The entry of Kfeldspar (accompanied by Fe-rich olivine) defines the base of a jotunitic transition zone which passes upwards into mangerites and quartz mangerites.There is a compositional regression through zone IVa. The upper part of zone IIIe has Ca-poor pyroxene with about En68, plagioclase with An44-48 and a Sr-isotope ratio of about 0.7062, while the base of zone IVb has olivine with Fo75 together with En78, An53 and 0.7050 respectively. Similar reversals are shown by the minor element compositions of plagioclase and Fe-Ti oxides. Sr-isotope ratios increase systematically up through zone IVb (reaching 0.7058 in zone IVd) while An% and Sr in plagioclase and Ni and Cr in Fe-Ti oxides decrease. Olivine compositions vary unsystematically and are believed to have changed their Fe:Mg ratios as a result of trapped liquid shift.The magma residing in the chamber when the influx at the base of megacyclic unit IV took place was compositionally zoned, and assimilation of gneissic country rock at the roof had resulted in the St-isotope ratio increasing up through the magma column. The new magma had a Sr-isotope ratio of about 0.7050 while the resident magma had a ratio of 0.7062 at the floor, increasing upwards. The new magma mixed with the basal layer(s) of the compositionally zoned resident magma and crystallization of this hybrid magma during influx and mixing produced the compositional regression in zone IVa. When magma influx

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway). Part I: Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent

“…5;Wilson et al, 1996), together with troctolites (pimo-C) present near the base of MCU III and IV. The troctolite formation is linked to magma replenishment episodes (Jensen et al, 1993(Jensen et al, , 2003 …”

“…In the Bjerkreim lobe (Fig. 4), the sequence of cumulates is subdivided into megacyclic units (MCU IA, IB, II, III and IV), which result from separated magma influxes (Duchesne, 1972a;Jensen et al, 1993;Wilson et al, 1996;Barling et al, 2000). Homogeneous layers up to hundreds of meters thick are typical of the lower and medium parts of the megacyclic units; modally graded layers are conspicuous in the upper parts of MVU III and IV (Fig.…”

Fe–Ti–V–P ore deposits associated with Proterozoic massif-type anorthosites and related rocks

“…The Layered Series of the Bjerkreim-Sokndal layered intrusion is subdivided into five megacyclic units (MCUs), the base of each displaying a compositional reversal to more primitive mineral compositions as a result of a new magma influx (Duchesne, 1972;Jensen et al, 1993;Wilson et al, 1996). Each MCU is subdivided into zones according to cumulus mineral assemblages.…”

Trace element compositions of apatite from the middle zone of the Panzhihua layered intrusion, SW China: Insights into the differentiation of a P- and Si-rich melt

“…It is well known that there are compositional reversals of An content and Sr concentration for plagioclase, Fo content for olivine, Mg# value for clinopyroxene and Cr concentration for magnetite in different height levels of layered intrusions elsewhere, such as the Bushveld Complex (Cawthorn and Ashwal, 2009;Cawthorn and McCarthy, 1980;Tegner et al, 2006), the Sept Iles intrusion (Namur et al, 2010) and the Bjerkrein-Sokndal intrusion in Norway (Jensen et al, 1993). These compositional reversals are often attributed to the mixing of evolved magmas with new primitive magmas in magma chambers (Ashwal et al, 2005;Jensen et al, 1993;Namur et al, 2010), or evolved magmas with less differentiated magmas from the upper part of the magma chamber due to magma convection (Cawthorn and Ashwal, 2009;Cawthorn and McCarthy, 1980;Tegner et al, 2006). In general, mixing of evolved magmas with new primitive magmas would decrease P in the mixed magmas because P is highly undersaturated in primitive magmas.…”