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An early metamorphic stage for the Variscan Ulten Zone gneiss (NE Italy): evidence from mineral inclusions in kyanite

Published online by Cambridge University Press:  05 July 2018

R. Braga ,
H.-J. Massonne  and
L. Morten
R. Braga*
Affiliation:
Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy
H.-J. Massonne
Affiliation:
Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy
L. Morten
Affiliation:
Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy
*
E-mail: r.braga@unibo.it
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Abstract

The early P-T evolution of a garnet-kyanite gneiss from the Variscan Ulten Zone has been defined by detailed petrographic observations leading to the detection of chlorite-epidote- and staurolite-bearing assemblages enclosed in kyanite porphyroblasts. Calculations of P-T pseudosections in the system NaCaKFeMgAlSiHO allowed us to constrain the evolution of these relics to the earliest metamorphic stages. The overall path shows a P-T increase to a peak of 11–12 kbar and 600–650°C followed by decompressional heating to 720°C and 9–10 kbar and final cooling at 7 kbar, 550–600°C. This clockwise P-T path reflects crustal thickening and subsequent thermal decay related to the continental collision of the Variscan orogeny 330–340 Ma ago. Our study demonstrates that large kyanite porphyroblasts may preserve assemblages related to prograde metamorphic stages. As a result, the detection of mineral inclusions in kyanite can complement many similar studies on mineral suites hosted in garnet and zircon.

Keywords

porphyroblastinclusionsP-TpathgarnetkyaniteVariscan Ulten Zoneelectron microprobe
Type
Research Article
Information
Mineralogical Magazine , Volume 71 , Issue 6 , December 2007 , pp. 691 - 702
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2007

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Footnotes

†

Deceased, 18th of November, 2006

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