Universität zu Köln

Tholen, Soren ORCID: 0000-0001-5577-9582, Linckens, Jolien, Heckel, Catharina ORCID: 0000-0002-0890-7876 and Kemperle, Marina (2022). Reaction-induced phase mixing and the formation of ultramylonitic bands. Tectonophysics, 827. AMSTERDAM: ELSEVIER. ISSN 1879-3266

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Abstract

As for most upper mantle shear zones, mylonitic and ultramylonitic peridotites exposed in the northern Lanzo shear zone display thoroughly mixed, fine-grained (< 25 mu m) bands. The weak crystallographic preferred orientations (CPO) of all their phases are indicative for deformation by diffusion creep. Although interpreted as significantly decreasing the strength of the upper mantle if interconnected, the formation process of fine-grained phase mixtures is still debated. Microstructural analysis of gradually higher strained peridotites of the northern Lanzo shear zone revealed the formation of fine-grained (< 25 mu m) polymineralic tails adjacent to clino- and orthopyroxene porphyroclasts in mylonitic samples. Phase mixtures (av. 65% phase boundaries) are dominated by neoblasts derived from the porphyroclast (Opx/Cpx) and olivine, with minor abundances (< 10 area-%) of plagioclase, spinel +/- amphibole. Neoblast microstructures and mixing of phases directly at the border of the pyroxenes, indicate a reaction of pymxene porphyroclasts to neoblast phase mixtures. Neoblast temperature estimations (similar to 860 degrees C), their systematic change in composition, and plagioclase/spinel abundances suggest that continuous net-transfer reactions, enhanced by the spinel lherzolite to plagioclase lherzolite transition are likely the main driving forces for the formation of ultramylonitic phase mixtures in the Lanzo shear zone. These reactions were potentially enhanced by the presence of fluid as evidenced by pargasitic amphibole crystallization. Comparing these results to other upper mantle shear zones and deformed xenoliths, the importance of reactions (melt-rock, fluid-rock and/or metamorphic) for the formation of polymineralic, ultramylonitic bands is confirmed.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Tholen, Soren UNSPECIFIED orcid.org/0000-0001-5577-9582 UNSPECIFIED Linckens, Jolien UNSPECIFIED UNSPECIFIED UNSPECIFIED Heckel, Catharina UNSPECIFIED orcid.org/0000-0002-0890-7876 UNSPECIFIED Kemperle, Marina UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-664142
DOI:	10.1016/j.tecto.2022.229230
Journal or Publication Title:	Tectonophysics
Volume:	827
Date:	2022
Publisher:	ELSEVIER
Place of Publication:	AMSTERDAM
ISSN:	1879-3266
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language UPPER-MANTLE PERIDOTITES; STRAIN LOCALIZATION; SHEAR ZONE; DEFORMATION MECHANISMS; DIFFUSION CREEP; ORTHO-PYROXENE; MASSIF; PLAGIOCLASE; IHERZOLITE; MYLONITES Multiple languages Geochemistry & Geophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/66414