Role of general melt convection in the development of cryptic layering of intrusive complexes at cratons

Abstract

The role of melt movement is discussed with reference to the development of phase and cryptic magmatic layering. Differences in the evolution of unmoving and moving melt are controlled by the corresponding types of local equilibria: mosaic and volumetric, respectively. Unmoving melt crystallizes within its marginal gradient zone containing suspension of crystals, and this process gradually extends to the whole melt volume in the chamber. Mineral grains nucleate in the homogeneous melt ahead of the crystallization front. Each crystallization center is immediately surrounded by a locally equilibrated isolated diffusion cell, and hence, only complementary gravitational fractionation of the solid phase from a melt of unchanging composition can take place in unmoving melt. This is the reason for the absence of cryptic layering in the crystallization products. Phase layering is generated by the overlap of batch fractionation with the participation of phase convection, which disturbs the even character of the former, on the continuous process of melt equilibrium crystallization within isolated diffusion cells. Moving melt undergoes in-situ fractional crystallization within a narrow marginal gradient layer. Its composition and temperature systematically change in the course of its consolidation, and this results in the cryptic layering of the crystallization products. The phase layering is controlled by the crystallization sequence of phases on the liquidus at decreasing temperature and by the self-sustained oscillatory process of nucleation in the undercooled melt in the course of cotectic crystallization. It is demonstrated that the origin of layered rock series of the Moncha Tundra and Monchegorsk massifs and one of the world’s largest Stillwater Complex was formed by the crystallization of unmoving melt. Intrusive massifs in the Imandra gabbroid complex in the Monchegorsk district and the Pilgujarvi gabbro-wehrlite complex in the Pechenga district, Kola Peninsula, the Burakovka Massif in southern Karelia, and the Bushveld Complex in South Africa were formed with the participation of overall melt convection. The compositional variations of bodies filling fractures in the root zone of the Monchegorsk Pluton were controlled by the fractional crystallization of mantle melt at the walls of magma conduits during melt ascent to upper crustal levels and provide evidence of variations in the melt velocity.