Cryptic layering in the Sokndal lobe of the Bjerkreim-Sokndal Intrusion, Rogaland Anorthosite Province, SW Norway

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Robins, Brian; Chiodoni, Federico
Holding Date 21 September 2008

The Layered Series (LS) in the Sokndal Lobe of the late Proterozoic Bjerkreim-Sokndal Intrusion (BSI) is deformed into a deep composite syncline due to postmagmatic gravitational collapse into underlying anorthosite. It consists of sequential zones of leucotroctolite/leuconorite, ilmenite leuconorite, ilmenite-magnetite leuconorite, apatite-bearing gabbronorite, an olivine-bearing Transition Zone (TZ) and mangerite and quartz mangerite in the uppermost part of the intrusion. The Layered Series is ~3400 m thick in the axial region of the syncline to the base of the mangerite but thins into the eastern and western limbs of the syncline as the lowermost two zones wedge out against the floor of the intrusion. No roof is preserved above the quartz mangerite.
The cryptic layering has been investigated in a series of sections in the core and limbs of the syncline. Plagioclase varies from An49 in the leucotroctolite/leuconorite to An26 (antiperthite) in the TZ. Mangerite and quartz mangerite contain a single exsolved ternary feldspar (mesoperthite). The Ca-poor pyroxene (mainly orthopyroxene but inverted pigeonite appears in the uppermost part of the apatite gabbronorite) varies in Mg# from 75 to 45. Olivine is present in the lowest part of the LS (Fo76-70), is absent in the ilmenite leuconorite, ilmenite-magnetite leuconorite and apatite-bearing gabbronorite, but reappears in the TZ and becomes rapidly enriched in iron upwards into the mangerite (Fo44-9). Cumulus Ca-rich pyroxene appears relatively late in the LS and varies in Mg# from 40 at the base of the gabbronorite to ~16 in the mangerite. All sampled sections through the LS show basal reverse cryptic variations through sequences of cumulates up to 400 m thick. The preservation and compositional variation of magnesian olivine in mafic layers in the lowest part of the LS and the anomalous amounts of apatite suggests that the apparent basal reversals are a consequence of upwardly diminishing amounts of trapped melt in the cumulates due to a progressively decreasing rate of cooling into the floor rocks during accumulation of the LS.
The cryptic variation in the Sokndal lobe is comparable to that exhibited by the uppermost Macrocyclic Unit IV in the >7 km thick LS of the northern Bjerkreim Lobe of the intrusion and appears to reflect uninterrupted fractional crystallisation of a diopside-poor, jotunitic parental magma. However, the onlap of successive zones from the axial onto the more distal parts of the floor of the intrusion suggests that at least the lower part of the magma was compositionally zoned upon emplacement. It is inferred that the magma flowed laterally from the north into a trough-shaped southern extension of the Bjerkreim chamber that opened in response to an important episode of magma influx.