Granitic Magma generation and migration through continental crust: detailed studies in granites and migmatites from Nazaré Paulista region, SP (SE Brazil)

Author(s)

Lucelene Martins

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2006-02-03

Examining board members

Valdecir de Assis Janasi; Roberto Dall'Agnol; Renato de Moraes; Lauro Valentim Stoll Nardi; Silvio Roberto Farias Vlach

• Advisor: Valdecir de Assis Janasi

Abstract

The conditions of melt generation and migration in the continental crust were investigated through a detailed study of two outcrops of the anatectic Nazaré Paulista granites (Socorro-Guaxupé, Nappe, Atibaia region, SE Brazil), using as main tools the rock and mineral major and trace-element geochemistry and Sr-Nd isotopy. The Nazaré Paulista granite shows a wide compositional variation, and can be grouped into two main types: a garnet leucogranite and a veined grey garnet-biotite granite, the latter dated at ~625 Ma (monazite U-Pb ID-TIMS). These types occur as subconcordant bodies 10-50 m wide and are present in both outcrops; where observed, the contact relationships show that the garnet leucogranites intruded after the grey granites. Two types of leucosome were recognized in migmatitic paragneisses associated to the Nazaré Paulista granites: (1) garnet-biotite tonalite with low Rb/Sr (0.3 ), slightly fractionated REE patterns ((La/Yb)N = 4-5) with negative Eu anomalies and Sr-Nd isotope signature identical to that of the mesosome; and (2) garnet leucogranite, with Rb/Sr ~0.56, slightly fractionated REE patterns ((La/Yb)N ~ 5) with positive Eu anomalies, eNd(t) identical to that of the mesosome, but lower 87Sr/86Sr(t). Both leucosomes are different from the granites, and were interpreted as the products of water-present melting of the paragneiss, respectively before and after the crystallization of K-feldspar in the protolith. Some relevant geochemical contrasts are observed between the two types of Nazaré Paulista granites: compared to the leucogranites, the gray granites show greater mg# and Zr and more fractionated REE patterns; the Rb/Sr ratios of both are low (0.4-0.7). The Sr-Nd isotope signature has important variations (eNd(t) = -16 to -13 and 87Sr/86Sr(t) = 0.716-0.728); only two leucogranite samples have signatures that are coincident with the field of the regional paragneisses. Assuming the paragneisses as the only source of the Nazaré Paulista granites would require that melting occurred under disequilibrium and water-present conditions. The trace-element chemistry of garnet was important to identify the differentiation processes that respond for the observed variety of anatectic granites. Part of the gray granites carries a restitic component, represented by ETRP+Y-rich garnet cores that are similar to the garnets from the country-rock paragneisses. The garnet leucogranite veins that cut the gray granite were probably formed by \"partial remelting\" of the original mush during rapid ascent, their idiomorphic garnet crystals have a distinctive composition (low HREE in the cores, increasing slightly to the rims), interpreted as magmatic. Some of the leucogranites forming isolated bodies may have been generated by fractionation from the gray granite, as indicated by field relations and rock geochemistry. On the other hand, the leucogranite with high 87Sr/86Sr(t) has ETRP+Y-rich garnet cores and its monazite is compositionally similar to that of the paragneisses, features indicative that they were probably produced by direct melting of these rocks. (AU)