Liquid ilmenite or liquidus ilmenite: a comment on the nature of ilmenite vein deposits

The interstitial character of ilmenite in common gabbroidic rocks has led to the idea that the last drop of liquid to crytallize in a rock could be pure ilmenite. When injected into enclosing rocks this liquid could give rise to ilmenite-pure veins. Fractional crystallization or immiscibility cannot produce a pure ilmenite melt. Actually, ilmenite is a liquidus mineral, which is the first or among the first mafic minerals to crystallize in Fe-Ti rich magmas. This is clearly shown by the sequence of crystallization in the Bjerkreim-Sokndal layered intrusion, by quantitative modelling of the liquid line of descent of jotunitic (hypersthene monzodioritic) magmas, and by experimental work. The interstitial character of ilmenite is therefore acquired subsolidus. Deformation can enhance the process. The synemplacement deformation linked to the diapiric uprise of anorthosite massifs is a suitable environment for the formation of layers of Cr- and Mg-rich ilmenite and their subsequent deformation and mobilization in discordant veins of pure ilmenite

Peculiar characteristics of the Bjerkreim-Sokndal intrusion (Rogaland anorthosite province, S. Norway) and implications for the layering formation

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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. P2O2 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 Bjerkreiin-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. (c) 2005 Elsevier B.V. All rights reserved

The Farsund Shear Zone: geochemical evidence for lithological diversity in the wall rock of the Rogaland anorthosite province, South Norway

peer reviewedThe Rogaland anorthosite province (RAP) – a typical Proterozoic Anorthosite-Mangerite-Charnockite (AMC) plutonic complex exposed in the Sveconorwegian orogen in South Norway – was emplaced diapirically through the crust, along a shear zone, at c. 933–916 Ma. The shear zone, recently defined as the Farsund Shear Zone, is outcropping along the eastern flank of the anorthosite province, and it is made up of banded gneisses, strongly foliated and verticalized. The banded gneisses comprise a diversity of lithologies: metabasites, granitoid gneisses, augen gneisses and kinzigitic gneisses. Major and trace elements composition of samples mostly from the banded gneisses and the neighbouring granite gneiss permit to unravel the nature of the various protoliths. The kinzigitic gneiss result from metamorphism of pelitic sediments, the augen gneisses belong to the high-K calc-alkaline series similar to the Feda suite (c. 1050 Ma), a major component of the Sirdal magmatic belt (SMB, 1070–1020 Ma). The metabasites comprise jotunites comparable to the intermediate rocks of the AMC suite (933–916 Ma) as well as amphibolites and norites with trace element signatures consistent with an oceanic origin. Charnockitic gneisses could result from anatexis in CO2-rich conditions or from fractionation of felsic magma similar to those of the charnockite-granite Farsund intrusion (931–926 Ma). Some leucogranitic layers have typical REE distribution of migmatitic leucosome. Other granite layers can be distinguished from the massive granite gneiss that has higher Th, Pb, Rb concentrations and [La/Yb]N ratios, but all granites display high-K calc alkaline affinities compatible with island arc origins. The granite gneiss is comparable to the SMB rocks and granite layers to the Suldal arc lithologies (c. 1520–1480 Ma). The Farsund shear zone hosts thin interleaved layers of rocks corresponding to the major lithologies exposed at regional scale. This includes jotunites and charnockites probably genetically related to intrusion of the c. 930 Ma anorthosite province, and a variety of granitic gneiss, metasediments, augen gneisses, and mafic rocks with protolith age ranging from c. 1020 Ma to c. 1500 Ma