Horizontal and vertical zoning in the Peruvian Coastal Batholith

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Abstract
Plutons of the Coastal Batholith of Peru show both horizontal and vertical zoning in mineralogy and chemistry. The horizontal variation is well seen in the areally extensive early ‘tonalite’ superunits, in which the sequence: marginal quartz diorite, tonalite adamellite, granite is often seen. This is the result of inward crystallization from the periphery of a relatively slowly cooling magma. Boundary layer convection combined with internal cell convection was effective in producing a markedly differentiated ‘rest’ liquid. The mineral assemblages are the result of incomplete separation of the evolved liquidus precipitate and the ‘rest’ liquid. In the smaller ring complex intrusions vertical variation is common and precipitation on the floor of the magma chamber resulted in the final crystallization of water rich aplites at the top. In these later intrusions magma mixing and hybridization may also be important. In both types of magma chamber stratification by boundary layer convection and diffusion seems likely to have complemented the conventional fractionation model of liquid evolution.