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Alteration and deformation microstructures of biotite from plagioclase-rich dykes (Ronda Massif, S. Spain)

1993; Elsevier BV; Volume: 5; Issue: 2 Linguagem: Inglês

10.1127/ejm/5/2/0245

1617-4011

Antonio Sánchez‐Navas, Jesús Galindo-Zaldı́var,

earthquake and tectonic studies

TEM analysis of microstructures and mechanisms related to alteration and deformation in igneous biotites was carried out on plagioclase-rich dykes from the ultrabasic Ronda massif (Betic Cordilleras, Spain). Compositional data for biotite and chlorite are used to establish the mass transfer of the alteration reaction, here considered as Al-conservative. Crystal defects -such as wavy layers and edge dislocation commonly occur during the early stages of chloritization. Deformation microstructures reveal that, at low-temperature and low-pressure, biotite and chlorite behave as multilayer packets, with the (001) planes acting as slip surfaces. Under these conditions, microscale kink-folds originate by flexural-slip folding. Most of the strain in fold limbs corresponds to intracrystalline gliding, but several other shear-strain structures are also observed: low-angle boundaries, voids, as well as ordered and disordered complex polytypes. Although ordered complex polytypes may originate by crystal growth, they probably arise in the studied samples by regular relaxation of low-magnitude shear stress around (001) slip planes. Hinge zones progressively evolve from kink-type through concentric-type with triangular voids, to serrated-type hinge zones. Intracrystalline permeability progressively increases during deformation and favours further alteration and recrystallization processes