Sequential crystal overproduction triggering Mg-Cr-Ti-V-P-MREE- enrichment in a single-pulse tholeiitic mafic sill in the Central Iberian Zone, Spain

Abstract

We study the causes of Mg-Cr-Ti-V-P-MREE enrichment in a 134 m thick mafic sill emplaced in a single magma pulse. Whole-rock chemistry indicates that TiO2 (up to 5.51 wt%) and P2O5 (up to 1.60 wt%) enrichment occurred in a closed system. Immiscibility processes during magmatic crystallization and interaction with late fluids seem to have played little or no role for the Ti-P distribution. Liquid lines of descent for anhydrous and hydrous conditions reveal that the melt did not follow a Fenner trend (anhydrous), which may lead to strong Ti or P enrichment, but a Bowen trend (hydrous), during crystallization at low fO2 (∆FMQ-2) and low water contents (0.5% H2O). The Mg-Cr-Ti-V-P-MREE-rich samples are cumulates and their composition is offset from the liquid line of descent due to postcumulus compaction and compositional convection. Theoretical formulation of compaction and compositional convection demonstrates that classical postcumulus scenarios of strong undercooling (sill <150 m thick) and high accumulation rates (ca. 1 m/y), using uniform cooling and anhydrous viscosity values, properly explain the observed Ti enrichment, but not the P and Cr enrichment. Latent heat released during overproduction of clinopyroxene, plagioclase, ilmenite, and apatite may result in non-uniform cooling with periods of near-isothermal conditions. Non-uniform cooling may locally reduce the accumulation rate, which temporarily enhances the efficiency of compaction and compositional convection and leads to the development of MgCr, TiV, or P-MREE-rich cumulates. The efficiency of postcumulus processes declines once the latent heat does not compensate heat loss to the country rock, leading to cumulates with high intergranular liquid fractions and without Cr, Ti, and P enrichment. High water contents in the upper level of the sill facilitate postcumulus processes and migration of residual liquids and fluids upwards, leading to the formation of abundant pegmatoid pockets and the development of a sandwich horizon that does not follow the liquid line of descent.