Turkish Journal of Earth SciencesCrystallization Processes and Role of Compositional Convection in the Macrolayer Formation in a Small Layered Complex
Laboratoire de Pétrologie-Volcanologie, B.504, Université de Paris-Sud, 91405 Orsay-FRANCE
Abstract: The Peloso complex is a 1000m thick gabbroic body composed of two main layered zones (ZA and ZB) that are made up of cyclic macrolayers of olivine gabbro, leucogabbronorite and anorthosite cumulates, and a third zone of unlayered quartz-gabbro (ZC) which may represent a residual liquid. Within a representative macrolayer, cryptic evolution of minerals can not be neglegted, showing that the crystallizing magma batch had a limited volume and was renewed between the formation of two successive macrolayers. The comparison between calculated parental magma density and densities of successive cumulates shows that the residual liquid was probably extracted by compositional convection at the crystallization front, or by intercumulus compositional convection within the cumulate pile. The calculated convective velocity at the bottom of the layer decreased from 30 cm/year to 10 cm/year when the leucogabbronorite crystalized. The minimum value also gives the order for the crystallization velocity of a macrolayer and a time scale for the reservoir activity. The residual liquid corresponding to the anorthositic top of a macrolayer is denser than the parent liquid. A strongly unstable density gradient therefore existed with the underlying gabbronoritic residual liquid, and the liquid extraction was prior controlled by the intercumulus convection. Liquids moving through the cumulate, may have induced adcumulus growth of plagioclase and resorption of mafic phases, leading to anorthosite formation at the top of the macrolayer. Mean crystal sizes, calculated trapped liquid fractions and fractionation density of cumulus associations are linked. Crystal size grading in cumulates is not only controlled by the variations in nucleation rate and crystal growth prevailing at the crystallization front. It is also strongly dependent upon variations concerning amounts, compositions and motions of residual liquids. Finally, in this small layered intrusion, the great majority of cumulates are orthocumulates. Restricted amounts of light residual liquid could have been extracted from them and become mixed with the overlying magma batch, resulting in an overall density decrease. Assuming that the magma reservoir was stratified, the residual magma batch, becoming gravitationally unstable, would be periodically removed. However, the macrolayer could also represent succesive influxes of new magma in the chamber, it is supported by isotopic studies where the R-F.C. (recharge, fractional crystallization) process is also clearly evidenced in this small magmatic chamber.
Turkish J. Earth Sci., 7, (1998), 167-184.
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Other articles published in the same issue: Turkish J. Earth Sci.,vol.7,iss.3.