A Strontium Isotopic Investigation of the Bjerkreim—Sokndal Layered Instrusion, Southwest Norway

Nielsen, Frank Møller; Campbell, Ian H.; McCulloch, Malcolm T.; Wilson, Joanne S.

doi:10.1093/petrology/37.1.171

Cited by 26 publications

(20 citation statements)

References 35 publications

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“…9. Accepting the Tjfrn jotunite of Duchesne and Hertogen (1988) and Vander Auwera and Longhi (1994) as the parental magma, and the LLD of Vander Auwera et al (1998) as describing the melt evolution, modelling of the cumulate/melt relationships by graphical methods gives a consistent view which confirms that: -cotectic compositions can be represented by average cumulate compositions provided the number of samples is large; -cumulates and related melt compositions from MCU II to MCU IV can be accounted for by the model; -a K 2 O contaminant must be involved in the formation of the acidic rocks of the upper part of the intrusion, in agreement with isotope data of Nielsen et al (1996); -the large volume of anorthosite in MCU I cannot be accounted for, which implies a different parental magma for MCU I or different crystallization conditions than those prevailing in the BKSK magma chamber;…”

Section: Discussionsupporting

confidence: 77%

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

Duchesne¹,

Charlier²

2005

Lithos

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Whole-rock major element compositions are investigated in 99 cumulates from the Proterozoic Bjerkreim-Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway), which results from the crystallization of a jotunite (Fe-Ti-P-rich hypersthene monzodiorite) parental magma. The scattering of cumulate compositions covers three types of cumulates: (1) ilmenite-leuconorite with plagioclase, ilmenite and Ca-poor pyroxene as cumulus minerals, (2) magnetite-leuconorite with the same minerals plus magnetite, and (3) gabbronorite made up of plagioclase, Ca-poor and Ca-rich pyroxenes, ilmenite, Ti-magnetite and apatite. Each type of cumulate displays a linear trend in variation diagrams. One pole of the linear trends is represented by plagioclase, and the other by a mixture of the mafic minerals in constant proportion. The mafic minerals were not sorted during cumulate formation though they display large density differences. This suggests that crystal settling did not operate during cumulate formation, and that in situ crystallization with variable nucleation rate for plagioclase was the dominant formation mechanism. The trapped liquid fraction of the cumulate plays a negligible role for the cumulate major element composition. Each linear trend is a locus for the cotectic composition of the cumulates. This property permits reconstruction by graphical mass balance calculation of the first two stages of the liquid line of descent, starting from a primitive jotunite, the Tjfrn parental magma. Another type of cumulate, called jotunite cumulate and defined by the mineral association from the Transition Zone of the intrusion, has to be subtracted to simulate the most evolved part of the liquid line of descent. The proposed model demonstrates that average cumulate compositions represent cotectic compositions when the number of samples is large (N 40). The model, however, does not account for the K 2 O evolution, suggesting that the system was open to contamination by roof melts. The liquid line of descent corresponding to the Bjerkreim-Sokndal cumulates differs slightly from that obtained for jotunitic dykes in that the most Ti-, P-and Fe-rich melts (evolved jotunite) are lacking. The constant composition of the mafic poles during intervals where cryptic layering is conspicuous is explained by a

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Section: Discussionsupporting

confidence: 77%

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

Duchesne¹,

Charlier²

2005

Lithos

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“…Figure 1C). Based on Nielsen et al (1996). Ca-rich pyroxene has a more restricted distribution than Ca-poor pyroxene: It is only found as a cumulus mineral in the upper parts of MCUs IB, III, and IV, as well as in the TZ, mangerite and quartz mangerite.…”

Section: Mineral Chemistrymentioning

confidence: 99%

“…A Sr isotope study of the Bjerkreim lobe of BKSK has recently been carried out by Nielsen et al (1996), whose results are shown in Figures 5 and 6. The variation in Sr 0 in Figure 5 is broadly parallel to the variation in An% in plagioclase and Mg# in Ca-poor pyroxene, showing that there is a close relationship between fractionation and contamination in BKSK.…”

Section: Initialmentioning

confidence: 99%

“…The gradual increase in Sr 0 through MCU IV, ending with fairly constant values in the mangerite and quartz mangerite, combined with the field, mineralogical and cryptic variations, implies that the two parts of the complex are related. Nielsen et al (1996).…”

Section: Initialmentioning

confidence: 99%

“…The Storeknuten profile is part of profile F in Figure 3 which is based on Nielsen and Wilson (1991). Further details of the Storeknuten section have been studied by Nielsen et al (1996). Some of the results of these studies are included here.…”

Section: Variations Across the Mcu πI-iv Boundarymentioning

confidence: 99%

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The Bjerkreim-Sokndal Layered Intrusion, Southwest Norway

Wilson

Robins

Nielsen

et al. 1996

Developments in Petrology

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The Bjerkreim-Sokndal Layered Intrusion is a large (~230 km 2 ), discordant, Late Protero-zoic, post-orogenic pluton in the Egersund-Farsund Igneous Province. The intrusion was em-placed shortly after massif-type anorthosite plutons and is cut by jotunite dykes. It contains a >7000 m thick Layered Series consisting of rocks belonging to the anorthosite kindred: andes-ine anorthosite, leuconorite, troctolite, norite, gabbronorite, mangerite, and quartz mangerite. Cumulates in the Layered Series are organized in 6 megacyclic units (MCU 0 to IV), individually up to 1800 m thick, but varying considerably in thickness and development along strike. The highest-temperature cumulates are troctolites containing plagioclase of ~An 54 and olivine of ~Fo 77 · Phase contacts in the macrocyclic units reflect crystallization of the silicate minerals in the order plagioclase (± olivine), orthopyroxene, Ca-rich pyroxene, pigeonite. Il-menite crystallized early and apatite appeared as a cumulus mineral at about the same time as Ca-rich pyroxene. Cumulus magnetite followed orthopyroxene and preceded Ca-rich pyroxene in MCU III and ΓV, but crystallized after Ca-rich pyroxene in MCU IB. MCUs 0, IA and II do not contain cumulates with cumulus magnetite or Ca-rich pyroxene. Olivine (~Fo 50 ) reappears in the uppermost part of the Layered Series where there is a rapid stratigraphic transition to mangerite and quartz mangerite. The basal parts of MCUs III and ΓV are characterized by thin sequences of plagioclase, plagioclaseorthopyroxene-ilmenite and orthopyroxene-ilmenite cumulates in which there are systematic upward decreases in initial Sr isotope ratios. They are overlain by troctolite (plagioclase-olivine cumulate) and are believed to have crystallized from hybrid magmas. The MCUs, the discordant geometry of phase contacts, the stratigraphic variations in initial 87 Sr/ 86 Sr ratio (0.7049-0.7085), and the abundance of xenoliths suggest crystallization of the cumulates at the base of a periodically-replenished, compositionally-zoned magma chamber that was continually assimilating country rocks. The parent, as indicated by medium-grained jotunite along country-rock contacts, appears to have been an evolved, Ti-rich magma similar to ferrobasalt, but poor in diopside components. Systematic stratigraphic variations in initial 87 Sr/ 86 Sr ratio at the base of MCU IΠ and MCU IV suggest that influx of magma into the chamber was accompanied by mixing with resident, contaminated magma.

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Two‐stage magmatism and mineralization of Tulaergen nickel‐copper deposit in eastern Tianshan, North‐west China: Evidence from bulk rock geochemistry and in situ mineral chemistry

et al. 2021

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A Strontium Isotopic Investigation of the Bjerkreim—Sokndal Layered Instrusion, Southwest Norway

Cited by 26 publications

References 35 publications

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

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

The Bjerkreim-Sokndal Layered Intrusion, Southwest Norway

Two‐stage magmatism and mineralization of Tulaergen nickel‐copper deposit in eastern Tianshan, North‐west China: Evidence from bulk rock geochemistry and in situ mineral chemistry

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