In situ crystallization of non-cotectic and foliated igneous rocks on a magma chamber floor

Abstract

Layered mafic intrusions commonly contain non-cotectic, foliated igneous rocks that are traditionally attributed to processes involving settling, transport, and redeposition of crystals. Here we examine the chemistry of magnetitite layers of the Bushveld Complex using a portable XRF spectrometer on drill core and dissolution ICP-MS analysis on pure magnetite separates. While magnetitites contain foliated plagioclase grains in non-cotectic proportions, the magnetite is characterized by a regular upwards-depletion of Cr which is best explained by in situ crystallization. We suggest that plagioclase nucleation in thin residual compositional boundary layers atop a solidification front causes in situ growth of plagioclase in proportions much lower (<10%) than those expected from cotectic crystallization (±85%). Crystallization in such a boundary layer also favours lateral growth of the plagioclase, producing the foliation. We suggest that some non-cotectic, foliated rocks that are commonly interpreted to arise from gravity-induced sedimentary processes may instead be produced by in situ crystallization.