REACTIVE LATE-STAGE MICROSTRUCTURES IN THE SEPT ILES LAYERED INTRUSION, QUEBEC, CANADA

Ferrobasaltic layered intrusions form some of the largest magma chambers on earth and host the world’s richest reserves of platinum group elements, Fe, Cr, Ti and V. Despite significant research on the liquid line of descent of the bulk magma of such intrusions during igneous differentiation, minimal research has been conducted on the evolution of the interstitial liquid within the crystal mushy layer. In order to help tease apart the late-stage magmatic history of the interstitial liquid in layered intrusions, microstructural evidence can be incorporated into existing natural and experimental evidence. Recent work by Holness et al. (2011) on microstructures in the Skaergaard Intrusion of East Greenland has shown that symplectites, “vermicular intergrowths” of two or more minerals that form from the breakdown of unstable phases, may have formed from phase separation of two immiscible liquids within the crystal mush.

Here we present data from symplectites in the Sept Iles layered intrusion, a 564 Ma ferrobasaltic intrusion in Quebec, Canada. Optical microscopy, scanning electron microscopy, cathodoluminescence and electron microprobe analyses were used in order to test the hypothesis that symplectites in layered intrusions require separation of immiscible liquids to form. All symplectites analyzed contain highly anorthitic plagioclase compared to the replaced primocryst, with intergrowths of a mafic phase (orthopyroxene, clinopyroxene, olivine, amphibole or biotite). The pressure of symplectite growth has been previously constrained to 1-2kbar, and temperature constraints determined from hornblende-plagioclase thermometry give crystallization temperatures of 850-950°C.

The high anorthite content of the plagioclase cannot be explained by crystallization of a primitive, high-temperature plagioclase, but must be a consequence of a late-stage process. Liquid immiscibility within the crystal mush may therefore have produced the symplectites of the Sept Iles. However, hydrous partial melting of incompletely solidified rock or late-stage fluid infiltration could also produce such microstructures. As symplectites may be a ubiquitous feature of layered intrusions, further work can provide information on late-stage processes operating during the crystallization of layered mafic intrusions.