Xenocrysts and cognate crystals in kimberlites from the Alto Paranaíba Alkaline Province (MG-GO, Brazil): petrologic contribution from elemental and isotopic in situ micro-analyses

Abstract

The present research proposal will cover specific topics that have not yet been explored for Brazilian kimberlite occurrences, such as 1) identification and characterization of the different crystal populations presented in these heterogeneous hybrid kimberlite rocks (e.g., cognate minerals vs. xenocrysts), based on the new recommended criteria focused on the micro-scale detailed study combined with elemental and isotopic cryptic variations registered in zones of singular crystals; and 2) petrologic processes (e.g., crystal-liquid fractionation processes, mixtures of kimberlitic magma pulses, assimilation of partial melting of crustal wallrocks, degassification of magmas) and of mantle dynamics that can be inferred and/or evidenced from the compositional signatures of such populations. The equilibrium relationships between melts/magmas and crystal populations will be evaluated by means of textural criteria (types of zonings, corrosion and/or resorption textures, overgrowths, etc.) as well as compositional tendencies, with major and trace elements, as well as isotopic signatures obtained by in situ analyses. The research proposal is for a period of 6 months, starting in May 2018. The choice of the supervisor, Dr. D. Graham Pearson, who holds the position of Canada Excellence Research Chair at the University of Alberta, is due to his prominence and leading edge in the kimberlite, mantle geochemistry and diamond researches, as well as their wide experience with in-situ mineral analyzes of both trace elements and isotopic ratios in the renowned Arctic Resources - Geochemistry Facility, also linked to the University of Alberta. This proposal is directly linked to the Research Support Project "The compositional influence of xenocrysts and antecristais in basic magmas to alkaline ultrabasics" (FAPESP 2017/03768-8) and to the Thematic Project "The Paraná-Etendeka Magmatic Province in Brazil: temporal and petrologic relationships between the tholeiitic and alkaline magmatism and geodynamic implications" (FAPESP 2012/06082-6), and the analyses to be performed abroad were already considered in these projects. Kimberlite bodies related to the Cretaceous alkaline magmatism in the Brazilian Platform, more specifically, linked to the Alto Paranaíba Alkaline Province, were selected for this study. The analytical programming of this research includes mainly in-situ trace element mineral analyzes by mass spectrometry LA ICP-MS (to be performed in the main mineralogical phases: olivine, perovskite, garnet, monticellite, clinopyroxene, spinel, ilmenite and phlogopite), as well as isotopic analyzes, mainly on the 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb, and U/Pb geochronology by mass spectrometry LA-MC-ICP-MS (to be performed mainly in perovskite crystals). Combining these elements, it will be possible to attribute genetic meanings to different crystal populations (xenocrysts and cognate crystals) present in a given intrusion, to better understand the intrinsic magmatic processes and to develop more detailed petrological models. This is a original proposal based on the main discussion focuses, on the work scale, and on the selected type of micro-analyses, all those not previously explored in detail to the study of kimberlite magma of this province.