Abstract

PINOTTI, Lucio P. et al. Magmatic structures in granites. Rev. Asoc. Geol. Argent. [online]. 2010, vol.67, n.4, pp.562-572. ISSN 0004-4822.

Structural aspects of  granites related to mechanisms for structures development and their timing of  formation are still a subject of  lively debate. Many authors suggest that structures generated during magmatic stages  (i.e. from early magmatic  flow with mineral grains  suspended  in melt  to  late magmatic grain-supported  flow)  are only hardly  preserved  in  granitic  bodies.  In  this  contribution,  several  magmatic  structures  in  granitoid  plutons  from  Sierras Pampeanas are showed, where small plutons and batholithic-sized bodies are both analysed. We discuss: a) map-scale lithological variations, such as principal magmatic units, where magma dynamics are evidenced by mixing processes within magmatic chamber; b) metre to decametre-scale magmatic layering that may be isomodal or modally-graded; c) overprinting submagmatic structures indicating interplay between plutons or pluton internal units according to regional deformation; d) composite magmatic layering and schlieren commonly associated to mafic microgranular enclaves, locally within synmagmatic shear zones. These structural features can be ascribed to three main processes: a) assembly of  differentiated magmatic pulses; b) local hydrodynamic sorting related to density currents in a mush; c) mechanical disruption and chemical hybridization of  mafic magmas during ascent and emplacement; among others. The identification of  preserved structures in granitoids, their timing and mechanisms of  formation, as well as the structural dynamic of  the surrounding crust, play a prominent role in proposing emplacement models for granitic plutons.

Keywords : Structure; Granite; Rheology of  magma; Sierras Pampeanas.

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