Growth mechanism of garnet megaporphyroblasts of the Passos Nappe, Southern Brasilia Orogen, Brazil

Author(s)

Hartung, Rafael F. ^[1] ; Simoes, Luiz S. A. ^[1] ; Trouw, Rudolph A. J. ^[2] ; Silva, Alex J. C. A. ^[3]

Affiliation

^[1] Univ Estadual Paulista UNESP, Inst Geociencias & Ciencias Exatas, Dept Petrol & Metalogenia, Ave 24-A, 1515, BR-13506900 Rio Claro, SP - Brazil

^[2] Univ Fed Rio de Janeiro UFRJ, Inst Geociencias, Dept Geol, Rio De Janeiro, RJ - Brazil

^[3] Univ Fed Vales Jequitinhonha & Mucuri UFVJM, Inst Engn Ciencia & Tecnol, Janauba, MG - Brazil

• Document type: Journal article
• Source: Journal of South American Earth Sciences; v. 102, OCT 2020.
• Web of Science Citations: 0

Abstract

The Passos Nappe is an important tectonic unit of the Southern Brasilia Orogen with inverted metamorphism, grading from greenschist facies at the base to granulite facies in the upper part. In the central part, of amphibolite facies, mica schist layers contain garnet megaporphyroblasts (3-9 cm diameter) that contrast with the common-size garnet (0.2-1.5 cm diameter) present in other mica schist layers above and below. This paper intends to describe this textural feature in detail and to discuss its genesis. The megaporphyroblasts are ellipsoidal, with major axis parallel to the mineral and stretching lineation, associated with the main foliation (S-2). Inclusion trails analysis in forty-six megaporphyroblasts, demonstrate straight internal foliation (S-i = S-1) in the core, progressively crenulated towards the rim by D-2. S-i is discontinuous with the external foliation (S-e = S-2), which wraps around the megaporphyroblasts, indicating early syn-D-2 growth. The angle between the axial planes of the included crenulations and S-2 in the matrix indicates that the D-2 deformation continued after porphyroblast growth. Asymmetric deformation shadows reveal non-coaxial D-2 deformation, associated with the emplacement of the nappe (top-to-the SE). The megaporphyroblasts are almandine-rich (73-82%) and spessartine-poor (0.1-2.5%) exhibiting prograde growth zonation. The common-size garnet porphyroblasts of the adjacent micaschists show similar inclusion patterns and chemical zonation indicating a similar tectonometamorphic evolution. A possible origin of the megaporphyroblasts by introduction of fluids along shear zones is discarded because the megaporphyroblasts are distributed along S-0//S-1 planes that make an angle with S-2. Since the porphyroblasts grew early syn-D-2, these shear zones should be parallel to S-2 which does not match the observations. The chemical data indicate that megaporphyroblast formation is related to a high-Al protolith composition, associated with scarce nucleation sites. (AU)

• FAPESP's process: 17/03663-1 - Study of the Brasilia Belt in the Northwestern Passos Nappe Region
• Grantee: Luiz Sérgio Amarante Simões
• Support Opportunities: Regular Research Grants