Crystallization Sequence and Magma Chamber Processes in the Ferrobasaltic Sept Iles Layered Intrusion, Canada
Olivier NamurBernard CharlierMichael J. ToplisMichael D. HigginsJean-Paul LiégeoisJacqueline Vander Auwera
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Abstract:
The Sept Iles layered intrusion (Quebec, Canada; 564 Ma) is a large plutonic body with a diameter of 80 km and a thickness of 6 km made up from its base to top of a layered series with troctolite and gabbro, and an upper border series with anorthosite, capped by cupolas of A-type granite. Chilled margin compositions suggest a ferrobasaltic parental magma close to that of the Skaergaard intrusion, but much richer in iron and titanium. Samples from drill-cores and surface sampling of the 4·7 km thick layered series reveal a succession of massive troctolites and layered gabbros that contain 24 Fe–Ti oxide layers cm- to m-thick and many anorthositic autolithic blocks. The sequence of crystallization in the layered series is: plagioclase (An72–34) and olivine (Fo75–21) followed by magnetite and ilmenite, then Ca-rich pyroxene and finally apatite. An olivine gap is observed between Fo66 and Fo59. The saturation of Fe–Ti oxides before Ca-rich pyroxene is interpreted to be the result of the high FeOt and TiO2 contents and the low CaO content of the parental magma. Contamination by old continental crust has occurred during crystallization of the layered series, as indicated by Sr isotopic compositions (87Sr/86Sr564 = 0·70360–0·70497). The differentiation trend of the intrusion is interrupted by two large and many small reversals to more primitive compositions of cumulus phases, Cr content of magnetite and lower Sr isotope ratios. These reversals and the intermittent disappearance of some phases are interpreted as resulting from magma chamber replenishments by undifferentiated primitive basaltic magma and mixing with the resident magma.Keywords:
Layered intrusion
Anorthosite
Magma chamber
Norite
Pyroxene
Ilmenite
Fractional crystallization (geology)
Pigeonite
Massif
Norite
Anorthosite
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The study area is located at the western boundary of the Nain Plutonic Suite and focuses on the Pearly Gates anorthosite pluton (PGA) and the adjacent composite body of mangerite and jotunite called the Tessiarsuyungoakh intrusion (TI). The contact between these intrusions is deformed and original relationships are obscured. The PGA consists of an inner zone of massive anorthosite and an outer zone of foliated, partially recrystallized layers of anorthosite and norite. Undeformed norite bodies intruded the inner and outer zone of the PGA. The contact-parallel fabric in the TI is pervasive through most of the unit, except along the western contact with the older Tasiuyak paragneiss. Where the fabric is weakest, jotunite can be seen to have intruded into the mangerite and the contact is irregular and cuspate. Field evidence, petrography and new U-Pb zircon TIMS geochronology ages have been combined to propose a mechanism for mid-crustal emplacement of these intrusions. Based on zircon crystallization ages obtained from six major rock units with well-constrained fi eld relationships, there appear to be 4 temporally distinct events. 1) The growth of prismatic zircons at 1370 ± 5 Ma in a sample from the PGA. In thin section these zircons were observed as inclusions in plagioclase and are thought to pre-date the intrusion of the PGA and be related to the development of a plagioclase crystal mush at depth. 2) The intrusion and crystallization of the TI composite body at ca. 1360 Ma. 3) The intrusion of anorthosite at 1355 ± 1 Ma into the TI. However, it is unknown whether this anorthosite is part of the PGA. 4) Norite intruded the PGA at ca. 1340 Ma and post-dates the deformation of the PGA and TI. Interstitial, fragmental zircon from a sample of the PGA provides a crystallization age of 1335 +7/-3 Ma, which places it within error of the crystallization age of the norite. The model suggested here for the emplacement of this suite of rocks is as follows. Reactivation of Paleoproterozoic structures in the Tasiuyak paragneiss provided conduits for the ascent of mangeritic and jotunitic magmas of the TI. An older plagioclase crystal mush that had formed at depth later followed the same conduit system in intermittent pulses. The margins of the PGA and TI were deformed and recrystallized and then intruded by norite.
Anorthosite
Norite
Layered intrusion
Charnockite
Porphyritic
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Anorthosite
Pigeonite
Pyroxene
Norite
Layered intrusion
Ilmenite
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Anorthosite
Layered intrusion
Pigeonite
Norite
Pyroxene
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Abstract Bushveld anorthosites commonly contain the so-called “mottles” comprising irregular, typically centimetric domains of oikocrystic pyroxene or olivine enclosing small, embayed plagioclase grains. The mottles were traditionally interpreted to result from solidification of trapped intercumulus liquid or via in situ crystallisation at the top of the crystal mush. Here, we present microtextural and compositional data of a mottle to place further constraints on the formation of anorthosite layers. Element maps generated by scanning electron microscopy reveal that plagioclase within and around the mottle has markedly elevated An contents (up to An 95 ) relative to the host anorthosite and is strongly reversely zoned. Other unusual features, some of which were reported previously, include a halo of sub-vertically oriented, acicular phlogopite around the mottle, elevated contents of disseminated sulfides, and relatively evolved yet Ni-rich olivine (Fo 71–75 , 3000 ppm Ni). These features are interpreted to result from reactive porous flow of hot, acidic fluid enriched in nickel and sulfur through proto norite. The fluids dissolved mafic minerals and leached alkalis from the outer rims of plagioclase grains. Reconnaissance studies suggest that reversed zoning of plagioclase is a common feature in Bushveld norite and anorthosite. This implies that reactive porous flow could have been far more pervasive than currently realised and that Bushveld anorthosite layers formed through recrystallisation of norites.
Anorthosite
Norite
Layered intrusion
Pyroxene
Phlogopite
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Anorthosite
Norite
Sill
Layered intrusion
Magma chamber
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Anorthosite
Pyroxene
Pigeonite
Layered intrusion
Norite
Ilmenite
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Anorthosite
Norite
Layered intrusion
Magma chamber
Fractional crystallization (geology)
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