Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway) Part II. REE and the trapped liquid fraction

Rare earth elements in bulk cumulates and in separated minerals (plagioclase, apatite, Ca-poor and Ca-rich pyroxenes, ilmenite and magnetite) from the Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway) are investigated to better define the proportion of trapped liquid and its influence on bulk cumulate composition. In leuconoritic rocks (made up of plagioclase, Ca-poor pyroxene, ilmenite, ±magnetite, ±olivine), where apatite is an intercumulus phase, even a small fraction of trapped liquid significantly affects the REE pattern of the bulk cumulate, together with cumulus minerals proportion and composition. Contrastingly, in gabbronoritic cumulates characterized by the presence of cumulus Ca-rich pyroxene and apatite, cumulus apatite buffers the REE content. La/Sm and Eu/Eu* vs. P₂O₅ variations in leuconorites display mixing trends between a pure adcumulate and the composition of the trapped liquid, assumed to be similar to the parental magma. Assessment of the trapped liquid fraction in leuconorites ranges from 2 to 25% and is systematically higher in the north-eastern part of the intrusion. The likely reason for this wide range of TLF is different cooling rates in different parts of the intrusion depending on the distance to the gneissic margins. The REE patterns of liquids in equilibrium with primitive cumulates are calculated with mass balance equations. Major elements modelling (Duchesne, J.C., Charlier, B., 2005. 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. Lithos. 83, 299–254) permits calculation of the REE content of melt in equilibrium with gabbronorites. Partition coefficients for REE between cumulus minerals and a jotunitic liquid are then calculated. Calculated liquids from the most primitive cumulates are similar to a primitive jotunite representing the parental magma of the intrusion, taking into account the trapped liquid fraction calculated from the P₂O₅ content. Consistent results demonstrate the reliability of liquid compositions calculated from bulk cumulates and confirm the hypothesis that the trapped liquid has crystallized as a closed-system without subsequent mobility of REE in a migrating interstitial liquid.     

Marginal mafic intrusions as indicators of downslope draining of dense residual melts in anorthositic diapirs?

Geochemical and isotopic investigation of three small mafic intrusions (Løyning: 1250 × 150 m, Hogstad: 2000 × 200 m, Koldal: 1250 × 500 m) in the marginal zones of the Egersund-Ogna (Løyning, Koldal) and Åna-Sira massif-type anorthosites (Hogstad) (Rogaland Anorthositic Province, south Norway: 930 Ma) provides new insights into the late evolution of anorthositic diapirs. These layered mafic intrusions are essentially of norite, gabbronorite as well as leuconorite and display conspicuous evidence of subsolidus recrystallization. In Løyning and Hogstad, the modal layering is parallel to the subvertical foliation in the enclosing anorthosite. The northern part of the Koldal intrusion cuts across the foliation of the anorthosite, whereas in its southern part the subvertical layering is parallel to the anorthosite's foliation. The regularity of the layered structures suggests that the layering was initially acquired horizontally and later tilted during the final movements of the diapirs.

The least differentiated compositions of plagioclase and orthopyroxene in the three intrusions (An₅₉–En₆₈ in Løyning, An₄₉–En₆₄ in Hogstad and An₄₄–En₆₁ in Koldal) and the REE contents in apatite (Hogstad) indicate that their parent magmas were progressively more differentiated in the sequence Løyning–Hogstad–Koldal. Isotopic data (Løyning: ⁸⁷Sr/⁸⁶Sr: 0.70376–0.70457, ε_Ndt: + 6.8 to + 2.7; Hogstad: ⁸⁷Sr/⁸⁶Sr: 0.70537–0.70588, ε_Ndt: + 2.1 to − 0.5; Koldal: ⁸⁷Sr/⁸⁶Sr: 0.70659–0.70911, ε_Ndt: + 3.5 to − 1.6) also indicate that in this sequence, parent magmas were characterized by a progressively more enriched Sr and Nd isotopic signature. In Løyning, the parent magma was slightly more magnesian and anorthitic than a primitive jotunite; in Hogstad, it is a primitive jotunite; and, in Koldal, an evolved jotunite. Given that plagioclase and orthopyroxene of the three intrusions display more differentiated compositions than the orthopyroxene and plagioclase megacryts of the enclosing anorthosites, it is suggested that the parent magmas of the small intrusions are residual melts after anorthosite formation which were entrained in the anorthositic diapir during its rise from lower crustal chambers.

Calculated densities of primitive jotunites (2.73–2.74 at FMQ, 0.15% H₂O, 200 ppm CO₂, 435 ppm F, 1150 °C, 3 kb) and evolved jotunites (2.75–2.76 at FMQ, 0.30% H₂O, 400 ppm CO₂, 870 ppm F, 1135 °C, 3 kb) demonstrate that they are much denser than the plagioclase of the surrounding anorthositic crystal mush (2.61–2.65). Efficient migration and draining of dense residual melts through the anorthositic crystal mush could have taken place along sloping floors (zones of lesser permeability in the mush), which occur along the margins of the rising anorthositic diapirs. This process takes into account the restricted occurrence of the mafic intrusions in the margins of the massif anorthosites. In a later stage, when the anorthosite was nearly consolidated, the residual melts were more differentiated (evolved jotunites) and could have been extracted into extensional fractures in the cooling and contracting anorthositic body in a similar way as aplitic dikes are emplaced in granitic plutons. As in the Rogaland Anorthositic Province, these dikes are much more abundant than the small mafic intrusions, collection and transport along dikes was probably more efficient than draining through the crystal mush.     

The Fedorivka layered intrusion (Korosten Pluton, Ukraine): An example of highly differentiated ferrobasaltic evolution

This study documents the petrography and whole-rock major and trace element geochemistry of 38 samples mainly from a drill core through the entire Fedorivka layered intrusion (Korosten Pluton), as well as mineral compositions (microprobe analyses and separated mineral fraction analyses of plagioclase, ilmenite, magnetite and apatite) of 10 samples. The Fedorivka layered intrusion can be divided into 4 lithostratigraphic units: a Lower Zone (LZ, 72 m thick), a Main Zone (MZ, 160 m thick), and an Upper Border Zone, itself subdivided into 2 sub-zones (UBZ₂, 40 m thick; UBZ₁, 50 m thick). Igneous lamination defines the cumulate texture, but primary cumulus minerals have been affected by trapped liquid crystallization and subsolidus recrystallization. The dominant cumulus assemblage in MZ and UBZ₂ is andesine (An_39–42), iron-rich olivine (Fo_32–42), augite (En_29–35Fs_24–29Wo_42–44), ilmenite (Hem_1–6), Ti-magnetite (Usp_52–78), and apatite. The data reveal a continuous evolution from the floor of the intrusion (LZ) to the top of MZ, due to fractional crystallization, and an inverse evolution in UBZ, resulting from crystallization downwards from the roof. The whole-rock Fe/Mg ratio and incompatible element contents (e.g. Rb, Nb, Zr, REE) increase in the fractionating magma, whereas compatible elements (e.g. V, Cr) steadily decrease. The intercumulus melt remained trapped in the UBZ cumulates due to rapid cooling and lack of compaction, and cumulus mineral compositions re-equilibrated (e.g. olivine, Fe–Ti oxides). In LZ, the intercumulus melt was able to partially or totally escape. The major element composition of the MZ cumulates can be approximated by a mixing (linear) relationship between a plagioclase pole and a mafic pole, the latter being made up of all mafic minerals in (nearly) constant relative proportions. By analogy with the ferrobasaltic/jotunitic liquid line of descent, defined in Rogaland, S. Norway, and its conjugated cumulates occurring in the Transition Zone of the Bjerkreim-Sokndal intrusion (Rogaland, a monzonitic (57% SiO₂) melt is inferred to be in equilibrium with the MZ cumulates. The conjugated cumulate composition falls (within error) on the locus of cotectic compositions fixed by the 2-pole linear relationship. Ulvöspinel is the only Ti phase in some magnetites that have been protected from oxidation. QUIlF equilibria in these samples show that magnetite and olivine in MZ have retained their liquidus compositions during subsolidus cooling. This permits calculation of liquidus fO₂ conditions, which vary during fractionation from ΔFMQ = 0.7 to − 1.4 log units. Low fO₂ values are also evidenced by the late appearance of cumulus magnetite (Fo₄₂) and the high V³⁺-content of the melt, reflected in the high V-content of the first liquidus magnetite (up to 1.85% V).     

The Apophysis of the Bjerkreim–Sokndal layered intrusion (Rogaland anorthosite province, SW Norway): A composite pluton build up by tectonically-driven emplacement of magmas along the margin of an AMC igneous complex

The Late-Proterozoic Bjerkreim–Sokndal layered intrusion (BKSK) is connected to a foliated, sheet-like igneous body (the Apophysis), that is a potential feeder for the BKSK magma chamber. Field, petrographical, geochemical and structural data are used to demonstrate that the Apophysis is a composite igneous body, constructed by coeval mafic to felsic magmas that were collected in a sub-vertical shear zone. Three liquid lines of descent are distinguished in the main Apophysis component (a felsic series, predominantly quartz mangeritic) and in coeval felsic rocks from the upper part of the BKSK. Minor mineralogical and geochemical discrepancies between these three trends are indicative of distinct sources and crustal contaminants, as well as slight differences in the differentiation mechanisms. Jotunitic to noritic cumulates or crystal-laden magmas, associated with their trapped melts, mingled with the felsic series in two distinct portions of the Apophysis. In one area, this association is dominated by a FTP (Fe–Ti–P-rich) jotunite, interpreted as an accumulation of pyroxenes + Fe – Ti oxides + apatite + plagioclase. In the second area, the melt dominates over the associated cumulate; it is a primitive (MgO-rich and K₂O-poor) jotunite, that was also involved in the genesis of another igneous body in the vicinity of the Apophysis. Magma mixing, in addition to mingling, was also potentially important in the petrogenesis of some jotunite rocks.     

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

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 Tjörn 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 (> 40). The model, however, does not account for the K₂O 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 compositional balance between the Fe–Ti oxide minerals, which decrease in Fe content in favour of Ti, and the pyroxenes which increase in Fe.     

Relationship between the Layered Series and the overlying evolved rocks in the Bjerkreim-Sokndal Intrusion, southern Norway

The Bjerkreim-Sokndal layered intrusion (BKSK) consists of a > 7000-m-thick Layered Series comprising anorthosites, leuconorites, troctolites, norites, gabbronorites and jotunites (hypersthene monzodiorites), overlain by an unknown thickness of massive, evolved rocks: mangerites (hypersthene monzonites; MG), quartz mangerites (QMG) and charnockites (CH). The Layered Series is subdivided into six megacyclic units that represent the crystallisation products of successive major influxes of magma. We have studied a ca. 2000-m-thick section that straddles the sequence from the uppermost part of the Layered Series to the QMG in the northern part of the intrusion. Mineral compositions in 37 samples change continuously in the lower part of the sequence up to the middle of the MG-unit (plagioclase An37-18; olivine Fo40-7; Ca-poor pyroxene Mg#57-15; Ca-rich pyroxene Mg#65-21). Above this compositions are essentially constant in the upper part of the MG-unit and in the QMG (An21-13; Fo6-4; Mg#opx17-13; Mg#cpx25-20). The amount of interstitial quartz and the amount of normative orthoclase, however, both increase systematically upwards through the QMG-unit, implying that these rocks are cumulates. There is no evidence of a compositional break in the MG-QMG sequence that could reflect influx of relatively primitive magma.

Two types of QMG/CH are known in the uppermost part of BKSK. Olivine-bearing types are comagmatic with the underlying Layered Series; the studied stratigraphic sequence belongs to this suite. Two-pyroxene QMG and amphibole CH define a separate compositional lineage related to jotunites. An intrusive unit of dominantly two-pyroxene QMG is discordant to the olivine-bearing jotunite-MG-QMG sequence near Rapstad, confirming the presence of two compositionally distinct suites of QMG and related lithologies in the upper part of BKSK.

A xenolith-rich unit near the olivine-bearing MG-QMG boundary represents a major collapse of the roof of the magma chamber during the final stages of crystallisation.