CRYSTALLIZATION AND RE-EQUILIBRATION OF ZONED CHROMITE IN ULTRAMAFIC CUMULATES, VAMMALA Ni-BELT, SOUTHWESTERN FINLAND

PETRI PELTONEN
Geological Survey of Finland, SF-02150 Espoo, Finland

Abstract

Cotectic proportions of chromite and olivine coprecipitated in tholeiitic island arc-type magmas that intruded the Svecofennian supracrustal rocks of southwestern Finland during the orogeny. Crystal growth of chromite continued until grains became trapped by olivine, or was terminated because of the appearance of clinopyroxene. From grain to grain Cr/(Cr + Al) varies between 0.25-0.80, whereas Mg/(Mg + Fe2+) varies only little for any given Cr#. The large Cr# range has been related to the continuous depletion of Cr in the magma due to chromite crystallization, and to concomitant enrichment o f magma in Al as a response to assimilation of pelitic sediments and accumulation of olivine. Because fractional crystallization took place in feeder conduits at moderate crustal pressures, the possibility remains that some of the Cr# variation arises from polybaric crystallization of chromite during magma ascent. Almost all chromite grains are zoned toward a more Al-rich rim. Such chemical zonation cannot be explained by any postcumulus or subsolidus mechanism, but is interpreted as initial growth-induced zonation, which produces a grain-scale "memory" of the incremental compositional changes that took place during crystal growth. During subsolidus cooling, the grains of chromite re-equilibrated extensively with enclosing silicates by diffusion- controlled Mg-Fe2+ cation exchange. This is not only supported by abnormally low (500-600 C) olivine-spinel blocking temperatures, but also by diffusion profiles preserved in the adjacent olivine. Subsolidus re-equilibration between olivine and spinel proceeded to an unusual extent, indicating a slow cooling rate for the ultramafic cumulates of the Vammala Ni-belt.

Keywords: chromite, cumulates, crystal zoning, fractional crystallization, assimilation, subsolidus re-equilibration, diffusion, Proterozoic, Vammala, Finland.