Petrological Features and Magnetic Properties of Pillow Lavas from the Thetford Mines Ophiolite (Quebec)

Abstract

A detailed examination was made to evaluate the variations of the remanent magnetization and magnetic susceptibility through four ophiolitic pillow lavas of Cambrian age. These pillowed metabasalts, which are covered by a thin layer of cherty argillite, derive from an olivine tholeiite of probable oceanic origin. They have been metamorphosed in the greenschist facies under a regime of low pressure, moderate temperature, and in the presence of water but absence of significant stress. They still display their primary structural zoning characterized by a thin outer shell, a much wider globulitic intermediate zone, and a porphyritic core. Morphologies of quenched microphenocrysts of olivine and plagioclase are well preserved.The magnetization resides in very fine-grained magnetite and, in spite of a very low remanent magnetization, the primary magnetic imprint appears to be still discernible. Remanent magnetization and Koenigsberger ratio vary from the pillow margin to its center in a fashion similar to the magnetic signature of some recent and fresh oceanic basalts. The magnetic zoning matches the textural, mineralogical, and chemical zoning characteristic of these pillow lavas. We found also that the N.R.M. vector is consistent and relatively stable within the pillow inner part of the intermediate zone and the outer part of the core and therefore that samples from these zones can be used for a regional paleomagnetic study of the ophiolitic complex.