We collected and investigated mylonitic gabbro nodules from Stromboli’s oldest pyroclastic rocks, and they are reported on here for the fi rst time. The petrography, bulk-rock geochemistry, and mineral chemistry of these ylonitic gabbros show that they are cognates with undeformed cumulate gabbros also found in the same host rocks. Both types of gabbro nodules are inferred to have crystallized within the lower crustal levels of this active volcano. The mylonitic gabbros are characterized by discontinuous layers of plagioclase aggregates, interspersed with amphibole, pyroxene, and olivine crystals, and they are enveloped by a mylonitic foliation consisting of <5- to <100-μm-sized, dynamically recrystallized plagioclase grains. The deformed rims of the plagioclase porphyroclasts disaggregate by fracturing into rectangular grains along the main cleavage systems {010} and {001}. In higher-strain zones, the grain size becomes smaller, and the shape is controlled by grain boundary migration driven by melt-assisted diffusion creep that accommodates grain boundary sliding. This is consistent with the dispersed crystallographic orientations of the mylonitic grains and their complex compositional zoning patterns. We invoke a switch from cataclastic fl ow to melt-assisted, grain boundary sliding as the main deformation mechanisms for these mylonitic gabbro nodules. The cataclastic fl ow was triggered by the hydrostatic pressure of a magma column acting atthe base of the lowermost Stromboli crustal magma chamber (7–11 km). These intrusive processes, which occur along the transtensional shear zones of the Stromboli NESW–striking fault systems, should have fi nally driven the mylonitic deformation in a brittle-ductile transition zone that is inferred to have been active during the evolution of the magmatic crustal roots of the volcano.

Mylonitic gabbro nodules of Stromboli (southern Italy): Microstructural evidence of high-temperature deformation of cumulates during the evolution of the magmatic crustal roots of an active volcano

MATTIOLI, MICHELE;RENZULLI, ALBERTO
2017

Abstract

We collected and investigated mylonitic gabbro nodules from Stromboli’s oldest pyroclastic rocks, and they are reported on here for the fi rst time. The petrography, bulk-rock geochemistry, and mineral chemistry of these ylonitic gabbros show that they are cognates with undeformed cumulate gabbros also found in the same host rocks. Both types of gabbro nodules are inferred to have crystallized within the lower crustal levels of this active volcano. The mylonitic gabbros are characterized by discontinuous layers of plagioclase aggregates, interspersed with amphibole, pyroxene, and olivine crystals, and they are enveloped by a mylonitic foliation consisting of <5- to <100-μm-sized, dynamically recrystallized plagioclase grains. The deformed rims of the plagioclase porphyroclasts disaggregate by fracturing into rectangular grains along the main cleavage systems {010} and {001}. In higher-strain zones, the grain size becomes smaller, and the shape is controlled by grain boundary migration driven by melt-assisted diffusion creep that accommodates grain boundary sliding. This is consistent with the dispersed crystallographic orientations of the mylonitic grains and their complex compositional zoning patterns. We invoke a switch from cataclastic fl ow to melt-assisted, grain boundary sliding as the main deformation mechanisms for these mylonitic gabbro nodules. The cataclastic fl ow was triggered by the hydrostatic pressure of a magma column acting atthe base of the lowermost Stromboli crustal magma chamber (7–11 km). These intrusive processes, which occur along the transtensional shear zones of the Stromboli NESW–striking fault systems, should have fi nally driven the mylonitic deformation in a brittle-ductile transition zone that is inferred to have been active during the evolution of the magmatic crustal roots of the volcano.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11576/2641636
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