Trace element mapping of clinopyroxene and apatite, and in situ 87Sr/86Sr isotope analysis in apatite from the Pariquera-Açu K-alkaline complex: Study of magma mixing and crustal assimilation processes in a fossilised shallow magma chamber

Abstract

The doctoral project aims to investigate the processes of magma mixing and crustal assimilation in mafic to felsic rocks with alkaline character, emplaced within a shallow crust. The research area selected for this investigation is the Lower Cretaceous Pariquera-Açu K-alkaline complex located in southern part of the São Paulo, Brazil. This complex comprises a variety of rock types, including shonkinites, malignites, monzonites, and nepheline syenite rocks, which display notable textural and mineralogical variability. These rocks also exhibit unique characteristics that suggest the operation of an open system at both macroscopic and petrographic scales. Examples of this include the presence of felsic enclaves into mafic rocks and the occurrence of disequilibrium textures and complex, reverse and oscillatory compositional zoning, manly in clinopyroxene crystals. The methods applied in this project include petrographic characterization (textural and mineralogical), as well as geochemical and isotopic analysis. Petrographic analyses were conducted using optical microscopy and scanning electron microscopy. Major element analyses were performed on clinopyroxene, olivine, feldspars, biotite, kaersutite, magnetite, ilmenite, and apatite via electron microprobe (EPMA). Trace element in situ analyses on clinopyroxene, olivine, and apatite were performed via inductively coupled plasma mass spectrometry with laser ablation (LA-ICP-MS).In order to provide comprehensive support for the project, we propose conducting (1) trace element mapping and (2) in situ 87Sr/86Sr isotopic analysis on the minerals found in different textural contexts. These analytical techniques would be crucial for identifying and characterizing magmatic processes within open systems. Therefore, we suggest implementing the BEPE project abroad, which involves carry out compositional mapping of trace elements specially in selected clinopyroxene and apatite crystals, as well as in situ 86Sr/87Sr isotope analysis in apatite. These analyses will be conducted at the laboratories of the University of Queensland, under the supervision of Professor Teresa Ubide, a worldwide specialist in this subject. In these laboratories, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) will be employed for compositional mapping of trace elements in clinopyroxene and apatite. Furthermore, laser ablation multi-collector mass spectrometry (LA-MC-ICP-MS) will be utilized for in situ Sr-isotope analysis of apatite. Identifying trace element variation during crystal growth will provide key information on crystallization conditions, and the acquisition of high-resolution isotopic data will enable the evaluation of different source regions of the melts involved. This chemical and isotopic approach will facilitate the identification, characterization, and quantification of magma mixing and crustal assimilation processes, linked to the observed textural characteristics. This project is coherent with the main philosophy of FAPESP and USP, related to the establishment of relevant collaboration research projects with well-productive and academic distinctive international groups in the area of interest. (AU)