Polymetamorphic History of the Crixás-Açu Gneiss, Central Brazil: Shrimp Upb Evidence From Titanite and Zircon

Abstract

A sample of the Crixás-Açu gneiss in Central Brazil contains protolith and metamorphic zircons, and two generations of metamorphic titanite. SHRIMP U-Pb data of these different mineral generations indicate the following temporal sequence: tonalitic magmatism at 2817 ± 9 M.y derived from an older source region (3050 to 2930 M.y zircon cores); Archaean metamorphism at 2772 ± 6 M.y (from zircon) with cooling to the blocking temperature of titanite (at 2711 ± 34 Ma); followed by Palaeoproterozoic metamorphism and weak fabric development at 2011 ± 15 Ma, and a possible Neoproterozoic metamorphism. The field relations and these age data indicate the polymetamorphic history of the area and demonstrate the value of in situ age determinations on well-characterized rocks.

U–Pb SHRIMP and Sm–Nd geochronology of granite–gneiss complexes and implications for the evolution of the Central Brazil Archean Terrain

“…Pulz (1995) obtained a ca. 2.0 Pb-Pb model age on galena and Queiroz et al (2000) a similar U-Pb SHRIMP age of zircon from a felsic sill, both related to gold occurrences in a shear zone at the western border of the Pilar de Goiás greenstone belt. Additionally, Queiroz (2000) reports 2.15 Ga U-Pb SHRIMP titanite metamorphic age from the Crixás Açu tonalite gneiss.…”

0.3 byr of drainage stability along the Palaeozoic palaeo-Pacific Gondwana margin; a detrital zircon study

“…1; e.g. Pimentel et al 1999;Queiroz et al 2000;Laux et al 2005), because such zircon ages contributed little to the sedimentary rocks in this study; hence, the continent interior probably was shielded by the arc.…”

Low- and high-alumina komatiites of Goiás, Central Brazil

“…Whole-rock Pb-Pb and Sm-Nd yield ages of about 2.8 Ga (Arndt et al, 1989), though these authors argue that REE remobilization after eruption, either by metamorphism or alteration, may suggest the komatiites are as old as 3.0 Ga. Recent Sm-Nd isochron data for the basal komatiites and basalts indicate an age of 3.0G0.07 Ga (Fortes et al, 2003), consistent with the average 2.853-2.817 Ga U-Pb SHRIMP zircon age of the granitoid-gneiss complexes adjacent to the belt (Queiroz, 2000;Queiroz et al, 2000). This age probably is also compatible with latest Archean deformation of the Guarinos and Pilar de Goiás belts, which resulted from the uplift of gneisses ranging in U-Pb SHRIMP ages between 2.844 and 2.707 Ga (Queiroz, 2000).…”