Banca de DEFESA: Rodrigo Antonio de Freitas Rodrigues

Uma banca de DEFESA de MESTRADO foi cadastrada pelo programa.
STUDENT : Rodrigo Antonio de Freitas Rodrigues
DATE: 18/10/2022
TIME: 14:00
LOCAL: Sala de videoconferência do IGD
TITLE:

Refertilization of the lithospheric mantle beneath the southwestern part of the São Francisco Craton, Brazil

KEY WORDS:

mantle xenoliths, mineral chemistry, metasomatism, refertilization, São Francisco Craton

PAGES: 222
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

In the Southwestern border of the São Francisco Craton, central-western Brazil, a suite of kimberlite-hosted mantle xenoliths provides valuable insights into the nature and evolution of the continental lithosphere. These peridotites vary from spinel lherzolites (876-915 °C; 1.5 GPa) and clinopyroxene-bearing harzburgite (914 °C; 5.28 GPa) to garnet-phlogopite wehrlites (1066-1101 °C; 5.03-5.23 GPa). The chemical compositionof these peridotites reveals a stratified subcontinental lithospheric mantle beneath the São Francisco Craton. The deeper mantle (~170 km) has experienced multiple metasomatic events, but the shallower mantle is a product of partial melting and shows only little evidence of metasomatic reaction. Samples from the deeper mantle underwent ~10 % of partial melting and recorded metasomatism caused by alkaline-rich fluids (K2O, H2O, CO2, F, ± Cl), carbonatite, and proto-kimberlite melt. The low-Ti-Cr phlogopite (Phl1) that occurs associated with Cr-spinel around garnet in the garnet-phlogopite wehrlites is formed by metasomatic reactions with alkaline-rich fluids. This phlogopite has similar composition compared to phlogopites from metasomatized xenoliths worldwide and phlogopite xenocrysts present in kimberlites. The reaction between the alkaline-rich fluids and peridotites generates this low-Ti-Cr phlogopite and also Cr-spinel and carbonate at expense of garnet. The high-Cr low-Al clinopyroxenes present in garnet-phlogopite wehrlites and harzburgite have high Mg#, high Ca/Al, La/YbN, Zr/Hf ratios, and high Sr contents, coupled with low to intermediate Ti/Eu and Ti/Nb ratios, suggesting that they were formed by carbonatite melts. Olivines present in the garnet-phlogopite wehrlites and harzburgite contain carbonate inclusions identified by Raman spectroscopy as magnesite (Raman shifts at 213, 330, 738, and 1095 cm-1) and dolomite (Raman shifts at 175, 300, 723, and 1097 cm-1) reinforcing the carbonate-rich melt as the metasomatic agent. The high-Ti-Cr phlogopite (Phl2) occurs as rims around Phl1 as well as crystals around garnet and in the groundmass. The composition of this phlogopite is similar to phlogopites from polymict mantle breccias, often thought to be crystallized from failed kimberlite melts in the deep mantle. Conversely, the shallower mantle (~50 km) represented by the spinel lherzolites mostly records partial melting processes and slight enrichment in incompatible elements. These samples underwent a lower degree of partial melting, varying from 2-4 %. Some high-Al low-Cr clinopyroxene crystals of these samples are slightly enriched LILE, LREE, and Ti. They also display low Ca/Al ratios at high Mg#, low La/YbN, Zr/Hf ratios, and Sr contents, coupled with high Ti/Eu and Ti/Nb and strong positive correlations between Ti versus HFSE and LREE, suggesting certain degree of interaction with a subalkaline silicate melt depleted in incompatible elements. The chemical compositions together with geothermobarometric data and partial melting estimates indicate that the subcontinental lithospheric mantle beneath the Catalão region is stratified. The refertilization in the deeper parts of the São Francisco Craton is attributed to fluids and deep-seated low-degree melts enriched in incompatible elements that reacted with the depleted peridotites typically present in the root of the cratonic lithosphere ofArchean ages. This process modifies the lithosphere chemically and mineralogically, which may have a great impact on the tectonic stability of the craton.

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