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Ultra-high-temperature metamorphism in Central Brazil: the Barro Alto complex.
- Source :
- Journal of Metamorphic Geology; Jul2000, Vol. 18 Issue 4, p345-358, 14p
- Publication Year :
- 2000
-
Abstract
- The Barro Alto complex, central Brazil, is a layered mafic–ultramafic intrusion, which was subjected to granulite facies metamorphism during the Neoproterozoic. Ultra-high-temperature conditions are recorded by parageneses that occur in some lenses of quartz-bearing rock (metagranite, metapelite and impure quartzite). The peak paragenesis consists of spinel+quartz±cordierite±leucosome (recording the former presence of melt with quartz in excess), which is replaced by either orthopyroxene+sillimanite or garnet+sillimanite. Quartz+biotite±sillimanite±garnet symplectites are ubiquitous and indicate reactions between Fe–Mg phases and melt. Late kyanite porphyroblasts have overgrown these symplectites. The direct replacement of spinel+quartz±cordierite by orthopyroxene+sillimanite or garnet+sillimanite occurred around the [Sa] invariant point, which appears only in a petrogenetic grid with inverted topology. The topology inversion occurs at conditions of high oxygen fugacity or due to the presence of ZnO-bearing spinel. Minimum peak conditions of ultra-high-temperature metamorphism were calculated as c. 980 °C and c. 7.9 kbar. The succession of observed mineral textures can be explained by a near-isobaric cooling P–T path, with a cooling stage occurring between c. 980 and 750 °C. [ABSTRACT FROM AUTHOR]
- Subjects :
- FACIES
METAMORPHISM (Geology)
Subjects
Details
- Language :
- English
- ISSN :
- 02634929
- Volume :
- 18
- Issue :
- 4
- Database :
- Complementary Index
- Journal :
- Journal of Metamorphic Geology
- Publication Type :
- Academic Journal
- Accession number :
- 5519223
- Full Text :
- https://doi.org/10.1046/j.1525-1314.2000.00262.x