Antecristais e xenocristais máficos em diques ultrabá¡sicos de tendência alcalina da Serra da Mantiqueira (SP)

Abstract

Lopes, J.C.S., 2018, Mafic antecrysts and xenocrysts in alkaline ultrabasic dykes of the Mantiqueira range (SP) [Master's Thesis], São Paulo, Instituto de Geociências, Universidade de São Paulo, 186p. Weakly to strongly silica-undersaturated mafic alkaline dike series occur nearby Santo Antonio do Pinhal (SP) and Campos do Jordão (SP) in a restricted area of the Mantiqueira range, SE Brazil. These are intrusive in the Precambrian crystalline basement (south of the Brasília orogen) and in the Ponte Nova mafic-ultramafic alkaline massif. These are Cretaceous rocks and are grouped as part of the Serra do Mar alkaline province. In the majority, the dikes are classified as alkaline basalts and alkaline lamprophyres. Are porphyritic, with the macrocryst population having clinopyroxene and olivine as main phases. Subordinatedly, kaersutite, and occasionally, phlogopite, enstatite, magnetite, spinel and plagioclase were found. These macrocrysts present desequilibrium textures, such as sieve and embayment, combined with complex zoning styles, which indicate a open-system magmatic evolution. The study involves: (1) detailed petrographic analyses by optical microscopy and scanning electron microscopy, (2) quantitative punctual analyses of major and minor elements by electron microprobe and trace element by laser ablation -inductively coupled plasma mass espectrometry (LA-ICP-MS) in different horizons of the main macrocrystic phases, and (3) whole-rock geochemistry by X-ray fluorescence and ICP-MS. These determinations were collected focusing in the determination of the genesis of each population of zoned crystals. From the obtained compositions, mass balance calculations were done to exclude the macrocryst influence in the bulk compositions of the bodies and to have a compositional estimative of the melts, represented by the fine-grained to aphanitic matrices of these rocks. The crystalline zones of macrocrysts in equilibrium with the calculated melts were defined using the Mg-Fe exchange equilibrium constant between minerals and liquid, -.The equilibrium relationships allowed the designation genetic meanings to the the crystalline zones, being these classified as xenocrysts, antecrysts and phenocrysts. The first two types (in textural and chemical disequilibrium with the matrices) were captured and/or recycled from an early magmatic environment, allowing to access the former magmatic processes during the