Xenocryst and Antecryst compositional influence in basic to ultrabasic alkaline magmas

Abstract

This research projetc aims to study the xenocryst/antecryst compositional influence in basic to ultrabasic alkaline magmas. This question is specially important in the study of lamprophyres (mesocratic to melanocratic porphyritic rocks), of mafic-ultramafic cumulate bodies and of kimberlitic/kamafugitic pipes related to the Cretaceous alkaline magmatism in the Brazilian Platform. All these magmatic types are representative of complex plumbing systems and present distinct crystal populations with different genetic significance. This project will contribute in specific topics based on new concepts not yet explored for the Brazilian alkaline occurrences, such as 1) the identification and characterization of different crystal populations in heterogeneous and hybrid rocks (e.g., phenocrysts vs. antecrysts, cognate crystals vs. xenocrysts), 2) the compositional influence that non-equilibrium crystal cargo (xenocrysts or antecrysts) exerts on the bulk composition of the system and 3) the petrological processes that could be inferred from the understanding of these assemblages. The crystal/melt equilibrium relations will be evaluated based on textural criteria (types of zoning, corrosion and/or resorption features, overgrowths, etc.) as well as major and trace element compositional trends and in situ isotopic ratios. The schedule comprehends petrographic analysis by optical microscopy, scanning electron microscopy, cathodoluminescence, in situ punctual chemical analysis by electron microprobe (WDS) and by LA ICP-MS, as well as in situ isotopic ratios, mainly 87Sr/86Sr, by LA-MC-ICP-MS. Combining textural, compositional and isotopic criteria between crystals or stages of mineral crystallization, we will attribute the genetic significance of the crystal populations that these lithotypes present and will understand better the complex magmatic processes involved in their evolution. Based on this new understanding, new and more detailed petrological models will be proposed. (AU)