Rodingites in serpentinized ultramafic rocks of the Abitibi greenstone belt, Ontario

Abstract

This is the first mineralogical and geochemical study of rodingites that occur within serpentinized ultramafic rocks of the Abitibi greenstone belt in Ontario. Remnants of the original texture and mineralogy are locally preserved; the mineralogical and textural changes associated with the rodingitization process can thus be traced. Rodingite protoliths include diabase and lamprophyre dykes, mafic lithic inclusions, serpentinites and intermediate volcanic rocks. Clinozoisite and epidote represent the early stage of alteration, and are followed by the appearance of hydrogrossular and prehnite, which form at the expense of epidote. The most advanced stage of alteration is represented by a completely recrystallized, fine-grained, diopside-bearing rock. Mass-balance calculations, using data from a larnprophyte for the frst two stages, suggest an isovolume replacement during which Ca, Al and Fe were added, whereas Na, K and Mg were removed. Crystallization temperatures of 270-330'C at pressures of l-2 kbars were obtained from primary fluid inclusions in late-stage diopside. The fluid had a low salinity (-2.5 equiv. wt.9o NaCl). The rodingitization process was not different in Archean times than in the Phanerozoic, so that the difference in tectonic settings is not an important factor. Thus, deformation is not an integral part of'the process, although it certainly affects the final texture and distribution of the rocks.