Synkinematic interplay between felsic dykes and host rock mylonitization: how magmatism assists the formation of ductile narrow shear zones in the Sierra Chica de Córdoba, Argentina

BOFFADOSSI, MARÍA ALEJANDRA; CONIGLIO, JORGE ENRIQUE; MAFFINI, MARÍA NATALIA; PINOTTI, LUCIO PEDRO; RADICE, STEFANÍA; D'ERAMO, FERNANDO JAVIER; DEMARTIS, MANUEL; MURATORI, MARÍA EUGENIA; CONIGLIO, JOAQUÍN

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2021

0895-9811

This study provides new insights about mechanisms of synkinematic interplay betweendyking emplacement and ductile narrow shear zones formation in the Sierra Chica deCórdoba. A detailed analysis of structural, microstructural, geometric-scaling andgeochronologic data is performed in order to demonstrate the important role that minorand meso-scale felsic dykes played in the formation of paired shear zonessymmetrically from dyke contacts. This magmatism and associated deformationconspicuously overprint medium- to high grade metamorphic and igneous rocks ofPampean age. Internally, the shear zones are formed by mylonites with mineralparagenesis in the amphibolite facies and foliation planes totally concordant to dykecontacts. A low rheological contrast between igneous intrusions and host rocks,together with fast rates of magma channeling and emplacement, were responsible fora brittle mechanical regime at moderate to high temperatures (500-700 °C). Melt-filledshear fractures rapidly propagated while the host rock was being mylonitized at ductileconditions. Shear structures controlling dyke construction and mylonite formationadjust to a strike-slip dominated dextral transtensional regime. The predicted axes offinite maximum shortening (Z) and maximum extension (X) have NNE and NNWdirections, respectively. Ar/Ar muscovite cooling ages determined for pegmatite dykes,yielded minimum crystallization ages between 451 ± 1 and 435 ± 3 My. Thesegeochronological data are further supported by field structural relationships and theregional knowledge, pointing to a Famatinian age for magmatism and related narrowshear zones. This paper constitutes a new contribution reporting the occurrence ofmagmatism and strike-slip-dominated dextral transtension deformation in the innerforeland of the Famatinian accretionary orogen, developed in the Middle Paleozoicalong the southwestern Gondwana margin.