THE CRITICAL ROLE OF MELT IN REACTION EVOLUTION OF UHT METAMORPHIC ROCKS: EXAMPLES FROM CENTRAL BRAZIL

Melting occurs along clockwise P-T-t paths, and melt is present in deep crust for the high-T evolution. Melt flows from sites of generation to sinks of lower pressure. Melt loss changes composition of the equilibration volume, and declining T decreases size of the equilibration volume. Since the proportion of melt to residue in the equilibration volume may change with decreasing size, composition also changes with declining T. These factors are critical in reaction evolution of UHT-granulites during exhumation.

In the Anápolis-Itauçu Complex, part of the Neoproterozoic Brasília Fold Belt (metamorphic age 633 ± 28 Ma), central Brazil, the mineral assemblage in felsic granulite is quartz-orthopyroxene-feldspars-garnet-biotite+/-sillimanite-cordierite. However, at two localities north of Goiânia, sapphirine-bearing assemblages occur, which establishes minimum P-T of 10kbar and 1050oC. At this temperature it is expected that peak assemblages coexist with melt.

At the first locality, sapphirine-bearing assemblages occur in two bulk compositions - quartz-present and quartz-absent. Using reaction textures we infer that orthopyroxene-garnet-sillimanite reacted to sapphirine-cordierite-spinel, then garnet broke down to orthopyroxene(2)-sapphirine(2)+/-cordierite(2)-spinel(2) symplectites (melt-present, quartz-absent) and sapphirine-orthopyroxene-garnet reacted to cordierite-spinel-sillimanite (melt-present, quartz-present) along an isothermal-decompression P-T path. Once biotite-in is crossed during cooling, UHT mineral assemblages react during crystallization to biotite-bearing assemblages. Thus, cordierite(3)-spinel(3)-biotite intergrowths formed at the expense of orthopyroxene(2)-sapphirine(2) symplectites (quartz-absent) and garnet reacted with melt to form a corona of orthopyroxene(2)-cordierite(2) (quartz-present). At a later stage, orthopyroxene(2)-cordierite(2) coronae were replaced by biotite due to reaction with melt. At the second locality, the only reaction texture observed is orthopyroxene-sillimanite coronae separating sapphirine from garnet and quartz.

We conclude that where melt is lost, metamorphic peak assemblages are preserved and P-T paths are poorly constrained, whereas with partial melt loss some reaction occurs, which enables a more complete P-T path to be established. Where melt remains throughout the evolution, the assemblage records only final stages of granulite facies conditions; in this case peak UHT conditions and the high-T segment of the P-T path are unrecorded.