Article (Scientific journals)
Dual origin of Fe-Ti-P gabbros by immiscibility and fractional crystallization of evolved tholeiitic basalts in the Sept Iles layered intrusion
Namur, O.; Charlier, Bernard; Holness, M. B.
2012In Lithos, 154, p. 100-114
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Keywords :
Ferrogabbro; Forward model; Liquid immiscibility; Pigeonite; Silicate liquid; Unmixing
Abstract :
[en] We present a detailed study of two ca. 200m-thick apatite-bearing ferrogabbro horizons of the Sept Iles layered intrusion (Canada). These rocks are the most evolved cumulates of the megacyclic units (MCU) I and II, and mark the transition between basaltic and silicic magmatism. They are made up of plagioclase (An 55-34), olivine (Fo 66-21), clinopyroxene (Mg#75-55), ilmenite, magnetite, apatite, ±pigeonite and are a significant source of Fe-Ti-P ore. Ferrogabbros have relatively uniform bulk-rock compositions in MCU I but are bimodal in MCU II. The liquid lines of descent for major elements in equilibrium with cumulates of MCU I and II have been calculated using a forward model formalism. Both trends evolve towards SiO 2-enrichment and FeO t-depletion after saturation in Fe-Ti oxides. However, because of magma mixing in MCU II, they do not follow the same path. Evolved liquids from MCU II are shown to enter the experimentally-determined two liquid stability field, while MCU I liquids do not. Immiscibility in MCU II and its absence in MCU I are supported by the presence of contrasted reactive symplectites in cumulate rocks. Apatite-bearing ferrogabbros in MCU II have crystallized from distinct immiscible Fe-rich and Si-rich silicate melts which have physically segregated in the slow-cooling magma chamber. Two different types of cumulate rocks are thus produced: leucocratic and melanocratic gabbros. This is consistent with the presence of Si-rich and Fe-rich melt inclusions in apatite. In contrast, homogeneous ferrogabbros from MCU I were produced by simple fractional crystallization of a homogeneous liquid. Our data suggest that immiscibility could also explain the large geochemical variability of ferrogabbros in the Upper Zone of the Bushveld Complex (South Africa). © 2012 Elsevier B.V.
Disciplines :
Earth sciences & physical geography
Author, co-author :
Namur, O.;  Department of Earth Sciences, University of Cambridge, CB2 3EQ Cambridge, United Kingdom
Charlier, Bernard  ;  Université de Liège - ULiège > Département de géologie > Pétrologie, géochimie endogènes et pétrophysique
Holness, M. B.;  Department of Earth Sciences, University of Cambridge, CB2 3EQ Cambridge, United Kingdom
Title :
Dual origin of Fe-Ti-P gabbros by immiscibility and fractional crystallization of evolved tholeiitic basalts in the Sept Iles layered intrusion
Publication date :
2012
Journal title :
Lithos
ISSN :
0024-4937
Publisher :
Elsevier, Netherlands
Volume :
154
Pages :
100-114
Peer reviewed :
Peer Reviewed verified by ORBi
Available on ORBi :
since 12 November 2014

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