Abstract
AbstractReplacive symplectites (vermicular intergrowths of two or more minerals) are an important feature of layered igneous intrusions, recording evidence of late-stage reactions between interstitial liquid and crystals. They are common throughout the Layered Series of the 564 Ma Sept Iles layered intrusion in Quebec, Canada, and fall into three types: oxy-symplectites, ‘Type I’ symplectites, and ‘Type II’ symplectites. Oxy-symplectites are comprised of magnetite and orthopyroxene, nucleate on olivine primocrysts, and form via the reaction Olivine + O2 → Orthopyroxene + Magnetite; Type I symplectites (of which there are 3 distinct categories) are comprised of anorthitic plagioclase with pyroxene, amphibole, or olivine vermicules, grow from primocryst oxide grains, and replace primocryst plagioclase; and Type II symplectites (of which there are 2 distinct categories) are comprised of anorthitic plagioclase with orthopyroxene ± amphibole vermicules, grow from primocryst olivine grains, and replace primocryst plagioclase. Rare symplectites composed of biotite and plagioclase are also present. Symplectite growth occurred at 700–1030°C with pressure constraints of 1–2 kbar. We propose that Type I symplectites, and some Type II symplectites, formed from the interaction of primocrysts with residual Fe-rich liquid as a consequence of differential loss of an immiscible Si-rich liquid conjugate from the crystal mush. However, redistribution and concentration of hydrous fluids in incompletely solidified rock, or an increase in water activity of the interstitial melt, may be more plausible processes responsible for the formation of replacive symplectites comprising abundant hydrous mineral assemblages.
Highlights
Detailed studies of volcanic rocks worldwide show that crystals found in lavas are often in disequilibrium with their carrier melts (Passmore et al, 2012)
A similar stratigraphic distribution was found in the Skaergaard intrusion by Holness et al (2011), who interpreted their formation as the result of an increase in oxygen fugacity in the residual liquid during the last stages of solidification, triggering the following reaction: Olivine + O2 Orthopyroxene + Magnetite. This reaction would form the mineral assemblages observed in oxy-symplectites from the Sept Iles intrusion, with textural and compositional relationships indicating the symplectites here formed in the same fashion as those at Skaergaard
The blue patches probably represent relics of primocrysts crystallized from the main magma body, whereas green areas are of anorthitic plagioclase that has grown via a replacement process similar to that which formed the symplectites
Summary
Detailed studies of volcanic rocks worldwide show that crystals found in lavas are often in disequilibrium with their carrier melts (Passmore et al, 2012). Type I-c symplectites are composed of olivine (which is locally replaced by clinopyroxene at the outer margins) and anorthitic plagioclase, and grow from olivine rims surrounding Fe-Ti oxide grains (Fig. 3c) They are found only in the region of stratigraphy covering the upper part of pomica-C cumulates at the top of MCU II, near the top of the S-9 drill core, and into the base of MCU III (Fig. 5). There is no cumulus orthopyroxene in the Sept Iles intrusion, except perhaps at the top of MCU II, there are many orthopyroxene rims surrounding primocryst grains of olivine and FeTi oxides These rims have a narrow compositional range, with Mg# of 69 to 74, whereas that of the symplectite orthopyroxene ranges from the same value as the rim down to Mg# 59 (Fig. 9c). The highest temperatures (comprising the majority of the data) are from biotite laths rooted to Fe-Ti oxide grains
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
