Wollastonita-escapolita-clinopiroxênio mármore do Complexo Anápolis-Itauçu, Goiás: evidência adicional de metamorfismo de temperatura ultra-alta

Impure marble in a quarry near Goianira in the Anapolis-Itaucu Complex, central Brazil, contains blocks of contrasting calc-silicate rocks with different bulk compositions, producing a range of mineral assemblages during granulite facies metamorphism. The subject of this paper is the common host impure marble from the quarry outcrop, which is composed of calcite, clinopyroxene, quartz, ternary feldspar, titanite, wollastonite and scapolite. Wollastonite and scapolite only occur together in layers of appropriate bulk composition, and the coexistence of these minerals is diagnostic of ultrahigh-temperature (UHT) metamorphism (T >900°C). In thin section, although rare, wollastonite and scapolite are observed in mutual contact. More commonly, wollastonite exhibits a corona of calcite+quartz, but where wollastonite is close to scapolite, these minerals are separated by a corona of grossular with plagioclase+calcite+quartz. The coexistence of wollastonite+scapolite in contact is inferred to record the metamorphic peak and the coronae are interpreted as an early post-peak retrograde feature. Another indication of UHT metamorphism is the presence of ternary feldspar, in which exsolved andesine occurs in an orthoclase host. Titanite yields a TIMS U-Pb age of 632 ± 6 Ma, which we interpret as a post-peak cooling age during regional UHT metamorphism. Zircon from intrusive rocks nearby the marble yields a SIMS U-Pb age of 626 ± 2 Ma. Since these two ages overlap within uncertainty, we cannot rule out the possibility that contact metamorphism was the heat source for UHT metamorphism.