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Multi-stage dehydration–decompression in the metagabbros from the lower crustal rocks of the Serre (southern Calabria, Italy)

Published online by Cambridge University Press:  25 February 2008

PASQUALE ACQUAFREDDA ,
ANNAMARIA FORNELLI ,
GIUSEPPE PICCARRETA  and
ANNARITA PASCAZIO
PASQUALE ACQUAFREDDA
Affiliation:
Dipartimento Geomineralogico Bari University via E. Orabona, 4-70124 Bari-Italy
ANNAMARIA FORNELLI*
Affiliation:
Dipartimento Geomineralogico Bari University via E. Orabona, 4-70124 Bari-Italy
GIUSEPPE PICCARRETA
Affiliation:
Dipartimento Geomineralogico Bari University via E. Orabona, 4-70124 Bari-Italy
ANNARITA PASCAZIO
Affiliation:
Dipartimento Geomineralogico Bari University via E. Orabona, 4-70124 Bari-Italy
*
*Author for correspondence: a.fornelli@geomin.uniba.it
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Abstract

Porphyroblastic garnet-bearing metagabbros from the base of the lower crust section of the Serre (southern Italy) exhibit multi-stage dehydration and decompression after the Panafrican emplacement of their protoliths. The first dehydration event produced Am–Opx–Cpx–Pl–Grt as the peak assemblage. Two decompression stages are documented by: (1) coronas of Opx–Pl and Opx–Am, and symplectites of Opx–Am–Pl around clinopyroxene within the porphyroblastic garnet as well as in the matrix and (2) symplectites of Pl–Am–Opx–Grt having different textures around the porphyroblastic garnet. During the second decompression stage, a new local, somewhat intense, dehydration occurred and produced rims of Opx+Pl around the porphyroblastic amphibole, or lenses of Pl–Opx–Am–Spl±Bt between layers of dominant amphibole. A deformation stage separates older from younger reaction textures. The porphyroblastic garnet, its inclusions and the matrix are affected by fractures, which have been overgrown by coronas and symplectites around the porphyroblastic garnet and the amphibole of the matrix. Preferred P–T estimates are: ∼900 °C and ∼1.1 GPa at the metamorphic peak; ∼850 °C and 0.8–0.9 GPa during the formation of corona around clinopyroxene; 750–650 °C and 0.7–0.8 GPa during the formation of corona around garnet. All these textures formed under granulite-facies conditions. The subsequent metamorphic evolution consists of rehydration under amphibolite-facies conditions. The P–T–t path agrees with the path shown by the uppermost migmatites of the Serre section, and the P–T estimates at the top and the bottom of the section are consistent with the thickness (7–8 km) of the lower crustal segment. A contractional regime, which caused a crustal thickening of about 35 km, was followed by an extensional one producing significant crustal thinning; the change of tectonic regime probably occurred about 300 Ma ago when the emplacement of voluminous granitoids and the initial stages of exhumation of the lower crustal section had taken place.

Keywords

metagabbrolower crustgranulite faciesP–T conditionsCalabria Italy
Type
Original Article
Information
Geological Magazine , Volume 145 , Issue 3 , May 2008 , pp. 397 - 411
Copyright
Copyright © Cambridge University Press 2008

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