In the geologic record, the quartz c-axis patterns are widely adopted in the investigation of crystallographicpreferred orientations (CPO) of naturally deformed rocks. To this aim, in the present work, four differentmethods for measuring quartz c-axis orientations in naturally sheared rocks were applied and compared:the classical universal stage technique, the computer-integrated polarization microscopy method (CIP), thetime-of-flight (TOF) neutron diffraction analysis , and the electron backscatter diffraction (EBSD).Microstructural analysis and CPO patterns of quartz, together with the ones obtained for feldspars andmicas in mylonitic granitoid rocks, have been then considered to solve structural and geological questionsrelated to the Montalto crustal scale shear zone (Calabria, southern Italy). Results obtained by applying thedifferent techniques are discussed, and the advantages as well as limitations of each method arehighlighted. Important, our findings suggest that patterns obtained by means of different techniques arequite similar. In particular, for such mylonites, a subsimple shear (40% simple shear vs 60% pure shear) byshape analysis of porphyroclasts was inferred. A general tendency of an asymmetric c-maximum near tothe Z direction (normal to foliation) suggesting dominant basal slip, consistent with fabric patterns relatedto dynamically recrystallization under greenschist facies, is recognized. Rhombohedral slip was likelyactive as documented by pole figures of positive and negative rhombs (TOF), which reveal also potentialmechanical Dauphiné twinning. Results showed that the most complete CPO characterization on deformedrocks is given by the TOF (from which also other quartz crystallographic axes can be obtained as well asvarious mineral phases may be investigated). However, this use is restricted by the fact that (a) there arevery few TOF facilities around the world and (b) there is loss of any domainal reference, since TOF is abulk type analysis. EBSD is a widely used technique, which allows an excellent microstructural control ofthe user covering a good amount of investigated grains. CIP and US are not expensive techniques withrespect the other kind of investigations and even if they might be considered obsolete and/or timeconsuming,they have the advantage to provide a fine and grain by grain “first round” inspection on theinvestigated rock fabric.

Quartz preferred orientation in naturally deformed mylonitic rocks (Montalto Shear Zone - Italy): a comparison of results by different techniques, their advantages and limitations

FAZIO, EUGENIO
Writing – Original Draft Preparation
;
PUNTURO, Rosalda;CIRRINCIONE, ROSOLINO;PEZZINO, Antonino;
2017

Abstract

In the geologic record, the quartz c-axis patterns are widely adopted in the investigation of crystallographicpreferred orientations (CPO) of naturally deformed rocks. To this aim, in the present work, four differentmethods for measuring quartz c-axis orientations in naturally sheared rocks were applied and compared:the classical universal stage technique, the computer-integrated polarization microscopy method (CIP), thetime-of-flight (TOF) neutron diffraction analysis , and the electron backscatter diffraction (EBSD).Microstructural analysis and CPO patterns of quartz, together with the ones obtained for feldspars andmicas in mylonitic granitoid rocks, have been then considered to solve structural and geological questionsrelated to the Montalto crustal scale shear zone (Calabria, southern Italy). Results obtained by applying thedifferent techniques are discussed, and the advantages as well as limitations of each method arehighlighted. Important, our findings suggest that patterns obtained by means of different techniques arequite similar. In particular, for such mylonites, a subsimple shear (40% simple shear vs 60% pure shear) byshape analysis of porphyroclasts was inferred. A general tendency of an asymmetric c-maximum near tothe Z direction (normal to foliation) suggesting dominant basal slip, consistent with fabric patterns relatedto dynamically recrystallization under greenschist facies, is recognized. Rhombohedral slip was likelyactive as documented by pole figures of positive and negative rhombs (TOF), which reveal also potentialmechanical Dauphiné twinning. Results showed that the most complete CPO characterization on deformedrocks is given by the TOF (from which also other quartz crystallographic axes can be obtained as well asvarious mineral phases may be investigated). However, this use is restricted by the fact that (a) there arevery few TOF facilities around the world and (b) there is loss of any domainal reference, since TOF is abulk type analysis. EBSD is a widely used technique, which allows an excellent microstructural control ofthe user covering a good amount of investigated grains. CIP and US are not expensive techniques withrespect the other kind of investigations and even if they might be considered obsolete and/or timeconsuming,they have the advantage to provide a fine and grain by grain “first round” inspection on theinvestigated rock fabric.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.11769/20273
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