Dual origin of Fe–Ti–P gabbros by immiscibility and fractional crystallization of evolved tholeiitic basalts in the Sept Iles layered intrusion

Abstract

We present a detailed study of two ca. 200 m-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).

Trace element compositions of apatite from the middle zone of the Panzhihua layered intrusion, SW China: Insights into the differentiation of a P- and Si-rich melt

“…However, modal apatite in layered intrusions is usually much lower than that of plagioclase so that the D Sr bulk is mainly related to modal plagioclase and partition coefficient of Sr in plagioclase. In the Sept Iles intrusion, rock layers below the apatiterich unit contain 60-70% plagioclase with An contents of 68-60 mol%, whereas the apatite-rich unit contains 40-50% plagioclase with An contents ranging from 52 to 34 mol% (Namur et al, 2010(Namur et al, , 2012. Namur et al (2010) considered that the rock layers below the apatiterich unit may have formed by crystallization of~70% plagioclase in a mineral assemblage of plagioclase, clinopyroxene, Fe-Ti oxides and olivine and they calculated the D Sr bulk to be close to unity (1.05) by assuming a D Sr plagioclase of 1.5.…”

“…If the MZb3 is assumed to be the product of mixing magma of the MZb1 melt and syenitic magmas, 50% syenitic magma is required to yield a Eu/Eu* ratio of 1.14 for the mixed magmas. It is considered that normal magmas would have 54-59 wt.% SiO 2 when apatite reaches saturation (c.f., Namur et al, 2012;Tegner et al, 2006). Si-rich melt inclusions trapped within the apatite at the top of the MZb3 have SiO 2 contents ranging from 56.3 to 76.5 wt.% (Wang et al, 2013), whereas the syenitic rocks from the Daheishan pluton havẽ 60 wt.% SiO 2 (Zhong et al, 2009).…”

“…Such a "delayed" apatite saturation may be related to the high-temperature immiscibility of basaltic magmas. Experimental results indicate that P is preferentially incorporated into immiscible Fe-rich melt rather than Si-rich melt (Lester et al, 2013;Shearer et al, 2001;Veksler et al, 2006), however, the P 2 O 5 content at the saturation of apatite in the Fe-rich melt is closely related to temperature as shown in the equation in liquid and T is temperature in K. If the initial magma of the MZb is assumed to have 55 wt.% SiO 2 and 6.5 wt.% CaO when cumulus apatite begins to crystallize like other layered intrusions (Namur et al, 2012;Tegner et al, 2006), 3.4 wt.% P 2 O 5 in magma is required for apatite saturation at 1050°C (Fig. 8a).…”

“…Occasionally, nelsonite layers that are composed of two-thirds Fe-Ti oxides, onethird apatite and minor olivine and plagioclase can form and interlayer with Fe-Ti-P-rich gabbro in the upper parts of layered intrusions, such as those in the Bushveld Complex in South Africa (Reynolds, 1985;Tegner et al, 2006) and the Sept Iles intrusion in Canada (Tollari et al, 2008). On the other hand, apatite-rich units often occur stratigraphically above major Fe-Ti oxide-rich layers in some mafic-ultramafic layered intrusions, such as those in the Skaergaard intrusion in Greenland Thy et al, 2009), the Bushveld Complex (Tegner et al, 2006;Von Gruenewaldt et al, 1985) and the Sept Iles intrusion (Namur et al, 2010(Namur et al, , 2012. Therefore, factors that control the crystallization sequence of Fe-Ti oxides and apatite from magmas are enigmatic and the role of P in the formation of major Fe-Ti oxide-rich layers of layered intrusions needs to be examined.…”

Immiscible iron- and silica-rich liquids in the Upper Zone of the Bushveld Complex

“…Philpotts, 1982;Charlier et al, 2013) and in plutonic environments (e.g. Jakobsen et al, 2005;Namur et al, 2012;Kamenetsky et al, 2013;Veksler and Charlier, 2015). In the case of the Bushveld Complex, South Africa (Fig.…”