Mineral chemistry and deformation in a temperature gradient in the Sierras Pampeanas of Córdoba (Argentina): The Chicamtoltina Tonalite–Trondhjemite Orthogneiss

Abstract

On the eastern margin of the Sierra Chica, in a gneissic host rocks metamorphosed under medium to high amphibolite facies (‘Sil-in’ isograd) during the Neoproterozoic–Cambrian M2-D2 event, a body of tonalitic-trondhjemitic composition was discordantly emplaced, cutting the S2 metamorphic foliation and the ‘Sil-in’ isograd. A strong deformation, with along the temperature gradient of the host rocks, transformed the igneous rocks into an orthogneiss during a M3-D3 event. Based on textural, microstructural and mineral chemistry changes along the mentioned metamorphic transition and age determination, the orthogneissification process is related to the deformational and thermal history in the Sierras Pampeanas of Córdoba. The M3-D3 event would have reworked the S2 foliation, generating an S3 foliation associated with folding and boudinage in the gneissic host rocks, forming a major intrafoliar fold in the tonalitic-trondhjemitic body, with its flanks on both sides of the ‘Sil-in’ isograd. The M3-D3 event is recorded in the textural and mineral chemistry variations, following a medium-high temperature gradient during deformation, from the southeast to the northwest of the body. In that direction, the ‘core and mantle’ texture in plagioclase increased the proportion of the recrystallized mantle, which was also enriched in anorthite content. Quartz textures evolved from polycrystalline ribbons, with internal deformation bands, to dismembered ones, with internal chessboard pattern. Recrystallization of the entire plagioclase-quartz aggregate ended in an interlobate to amoeboid texture, implying high-temperature conditions (~650-750 °C) along the mentioned temperature gradient during the M3-D3 event. Regarding micas, the primary biotite progressively increased in Mg and decreased in Fe towards the northwest, without appreciable textural changes. Concomitantly, the primary Ti-rich ferrimuscovite increases its content in Mg and becomes poor in Ti, at the same time that it developed symplectitic quartz-plagioclase intergrowths. During a final D4 deformational event, also under high ductility conditions, asymmetric intrafolial folds (drag folds, with geometrical S4 axial planes) were developed, associated with imbrication and folding of boudins within the S3 (// S2) foliation. The D2, D3, and D4 deformation events are part of a continuum (progressive deformation) in a combined simple shear and flattening regimen (transcompression = convergent oblique shear). The contrasting rheology between the stiffer Chicamtoltina igneous body and its gneissic host rocks explain the different behavior during high ductility deformation. Internally, this small igneous body, with plagioclase as stress-supporting mineral framework (plagioclase-quartz ratio 2:1), mirrored the rheological behavior of the middle to lower crust deformation. A concordia age of 535.81 ± 0.79 Ma (U–Pb LA-ICP-MS on zircon) obtained for the Chicamtoltina orthogneiss is interpreted as the crystallization age of the tonalitic-trondhjemitic protolith (igneous event Ig1). By correlation with the tonalitic orthogneisses from the Sierra Chica and Sierra Norte of Córdoba, this protholith is linked to the Pampean magmatic arc, with an age range of 541–515 Ma and a peak of magmatic activity at 530–535 Ma.