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Formation of garnet clusters during polyphase metamorphism
Author(s) -
Floess David,
Baumgartner Lukas
Publication year - 2013
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1111/ter.12018
Subject(s) - metamorphic rock , metamorphism , geology , overprinting , nucleation , geochemistry , mineralogy , chemistry , organic chemistry
Pre‐ A lpine garnets of V ariscan age from metapelitic basement units in N orthern I taly were strongly retrogressed at near‐surface conditions prior to A lpine contact metamorphism. The replacement by sheet silicates caused a significant volume increase during retrogression, producing pervasive fracturing. Up to several hundreds of angular fragments formed from each crystal. Electron backscatter diffraction analysis documents a maximum misorientation of ~22° of some fragments as a result of local rotation during fracturing. New garnet growth is observed on the garnet fragments during contact metamorphic overprinting, resulting in garnet clusters. Fragments can be identified due to calcium‐rich domains. Fragment orientations were inherited, and only minor new nucleation occurred. These garnets develop features typically associated with multiple nucleation models, but here they reflect multiple metamorphic events. We propose that clusters can be indicative of multiple metamorphic events, which were separated by a period of intense retrograde alteration.