Petrografia, litogeoquímica e datação Ar-Ar dos montes submarinos e dos rochedos de Martim Vaz - Cadeia Vitória-Trindade

Abstract

This master's thesis studied the island of Martin Vaz and five seamounts of the Vitoria-Trindade Ridge - Columbia, Dogaressa, Davis, Jaseur and Montague. Martin Vaz is a group of islands formed by the main island - Martin Vaz, two steep and inaccessible islands - the North Island and South Island and several smaller rocks, as the Agulha Rock, around 48 km east of Trindade,making a total area of 0.3 km². Martin Vaz and seamounts belong to the Vitoria-Trindade Ridge,and are embedded in a context where the W-E regional tectonic trend suggests to represent the track of the Trindade mantle plume when the South American Plate passed on it since the Tertiary (Crough et al. 1980; O'Connor & DUNCAN, 1990, Gibson et at. 1997). The petrography of the samples indicates that the basanites in Martin Vaz are partially altered, melanocratic,aphanitic texture, porous, presenting vesicles around 1.0 to 5.0 millimeters. Displays phenocrysts of pyroxene and some translucent olivine ranging from 1.0 to 3.0 millimeters. These basanitics samples are also described in the seamounts studied in this work and they are Columbia, Dogaressa, Davis, Jaseur and Montague seamounts. The main island has also phonolitic dikes and necks with altered oriented matrix featuring white sheet-like mineral (alkali feldspar) and purple (titanoaugite) beyond small opaques. Small aegerine-augite phenocrysts strongly pleocroic, which some have simple twinning and sometimes zoned. They presents extinction varying from X c ∧ α = 23° or 33°. Kaersutite with poiquilitic texture (1.0 mm), tiny crystals of euhedral titanite (rare), and isotropic crystals pseudohexagonais altered nosean and carbonates are also observed The samples used in this study have minimum value of 33.91 wt% SiO2 (TRIM-01D) and a maximum of 52.2wt% (AMM-01) ranging from basic to ultrabasic. Through analysis of the oxides SiO2 and MgO is possible to distinguish two groups of rocks to Martin Vaz: an ultramafic magnesium (<42% SiO2 and> 7% MgO) and one basic (> 45% SiO2) and for seamounts, two groups: an ultramafic magnesium (> 9% MgO <42% SiO2) and one basic (> 45% SiO2 and MgO values around 4%). Ar-Ar Analyses for four samples of Martin Vaz present ages to basanites varying from 320 ± 366 Ka (MVA-10) to 623 ± 127 Ka (MVA-04). A single sample representing the phonolite dike dated is the MVA-05B and obtained age of 649 ± 84 ka,indicating to be contemporary with basanite extrusion.