The influence of tectonic strain on the diagenetic degree and illitization process of mixed-layers illite-smectite at shallow crustal conditions was studied. For this purpose, the modal composition of clay fraction and illite FWHM parameters of argillites deformed by a regional-scale fault zone were studied in detail by XRD, chemical analyses and by SEM observations. Analyses were performed on deformed samples of the fault rock and compared with the non-deformed rocks off the fault zone. In addition, this paper reports a detailed comparative analysis of deformed (shear surfaces and cleavage domains) and non-deformed domains (lithon cores) of a scaly fabric in the fault rock. A systematic increase in illite concentration, a decrease of Kubler index and FWHM((002)) values, and an enrichment of K(+) ions were observed in cleavage domains with respect to the non-deformed sediments off the fault zone and the lithon cores within the fault rock. Migration of K(+)-rich fluids along scaly cleavage domains causes progressive conversion of smectite-rich I-S to illite-rich I-S and thickening of illite crystallites along the c-direction. Changes in mineralogical and crystallographic parameters, therefore, seem to be strongly controlled by shear plane development in highly sheared rocks.

The role of tectonic shear strain on the illitization mechanism of mixed-layers illite–smectite. A case study from a fault zone in the Northern Apennines, Italy

PINI, GIAN ANDREA;
2008-01-01

Abstract

The influence of tectonic strain on the diagenetic degree and illitization process of mixed-layers illite-smectite at shallow crustal conditions was studied. For this purpose, the modal composition of clay fraction and illite FWHM parameters of argillites deformed by a regional-scale fault zone were studied in detail by XRD, chemical analyses and by SEM observations. Analyses were performed on deformed samples of the fault rock and compared with the non-deformed rocks off the fault zone. In addition, this paper reports a detailed comparative analysis of deformed (shear surfaces and cleavage domains) and non-deformed domains (lithon cores) of a scaly fabric in the fault rock. A systematic increase in illite concentration, a decrease of Kubler index and FWHM((002)) values, and an enrichment of K(+) ions were observed in cleavage domains with respect to the non-deformed sediments off the fault zone and the lithon cores within the fault rock. Migration of K(+)-rich fluids along scaly cleavage domains causes progressive conversion of smectite-rich I-S to illite-rich I-S and thickening of illite crystallites along the c-direction. Changes in mineralogical and crystallographic parameters, therefore, seem to be strongly controlled by shear plane development in highly sheared rocks.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11368/2695829
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