HR: 1330h
AN: V22B-0583    [PDF]
TI: MODELLING THE FORMATION OF ELLIPTICAL GARNETS DURING HIGH STRAIN IN A PARTIALLY MELTED METAPELITE
AU: * Alvarez-Valero, A
EM: alvarez@dmp.unipd.it
AF: University of Padova Department of Mineralogy and Petrology, Corso Garibaldi 37, Padova, Ita 35137 Italy
AU: Cesare, B
EM: bernardo.cesare@unipd.it
AF: University of Padova Department of Mineralogy and Petrology, Corso Garibaldi 37, Padova, Ita 35137 Italy
AB: Some metapelitic xenoliths in the NVP (Neogene Volcanic Province) of SE Spain, display the development of high strain zones during partial melting. These rocks are composed of biotite (XMg= 0.29-0.35, TiO2= 4-7 wt %), sillimanite, plagioclase (An%= 45-55), garnet, hercynite (XMg = 0.15-0.20), cordierite (XMg = 0.45), graphite and melt. Melt of granitic composition occurs as inclusions in minerals and as intergranular pockets. In the high strain zones, the foliation is outlined by layers of oriented fibrolite, biotite, graphite and melt, which wrap around crystals of garnet ranging in shape from elliptical to sigmoidal. Elliptical garnets may have aspect ratio up to 4:1. Both from microstructural and chemical ground, the garnets are characterized by well distinguishable core and mantle. The core typically contains primary inclusions of biotite and melt, trapped during garnet growth. The concentric pattern of inclusions of garnet core is often truncated at strain caps. This part of the garnet is chemically homogeneous, with a composition of Alm76-Pyp08-Sps14-Grs03. The cores are surrounded by a thin (~100 mm) mantle, which is irregular in shape and appears to overgrow the foliated matrix around garnet. Along the strain caps these mantles are rich of oriented fibrolite inclusions, whereas they are intergrown with biotite at strain shadows. In places, the overgrowths form skeletal elongated arms which extend parallel to the foliation. Compared with the cores of garnets, the overgrowths have composition poorer in Mn, but maintain the same XMg= 0.85. These elliptical garnets might be interpreted as a result of: a) crystal plastic deformation b) dissolution (and redeposition) c) constrained growth Investigation by means of orientation contrast imaging and electron backscattered diffraction, leads us to rule out crystal plasticity as a possible mechanism. Based on the observed intracrystalline microstructures and chemical zoning of garnet we can model the formation of elliptical garnets in multistage sequence. After growth of idiomorphic garnet cores, rich of melt inclusions and high in Mn, a process of pressure-solution - redeposition occurred during foliation development, with dissolution at strain caps and growth of garnet poorer in Mn in the strain shadows. In a later episode, after the strain event ceased, a mantle poorer in Mn continued to grow all around garnet. The elliptical shape and the skeletal arms were constrained by the adjacent foliation, and garnet included trails of oriented matrix phases, mainly biotite and fibrolite. Grt-Bt thermometry of the elliptical garnets provides high temperatures, in the range 800-950$\deg$C, and in agreement with the Grt-Crd thermometer. There are no systematic differences in T among the different microstructural domains (core, strain shadow, strain cap, skeletal arm) of elliptical garnets. This suggests that high temperature conditions were present throughout the whole garnet development, which is in accordance with the presence of melt both within and outside garnet.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting