RHEOLOGICAL CONTROL OF THE NATURE OF MAGMATIC STRUCTURES

PINOTTI, LUCIO PEDRO; D'ERAMO, FERNANDO JAVIER; WEINBERG, ROBERTO; ZAMBRONI, NICOLÁS EGIDIO; DEMARTIS, MANUEL; TUBÍA MARTÍNEZ, JOSÉ MARÍA; ARAGÓN, EUGENIO; CONIGLIO, JORGE ENRIQUE

Florianópolis

Simposio; 8th Hutton Symposium on Granites and related rocks; 2015

Sociedade Brasileira de Geologia

Most structuralstudies concerning emplacement mechanisms of granitoid rocks generally resultin regional maps at the pluton (or batholithic) scale, where the structuralpattern of the granitoid rocks are reconstructed. However, this mapping cannotbe achieved without exhaustive field and laboratory works, involving outcropdescription and measuring, combined with microstructural observations and anisotropyof magnetic susceptibility (AMS) studies. Thus, a large amount of structuralinformation like foliations, lineations and microstructural data of the igneousbodies, is generated, but only in a few cases the meaning and origin of thesestructures are addressed. The present paper mostly deals with the significanceand genetic aspects of these structures. The study of magmatic structures iscrucial for the understanding the growth and differentiation of silicic magmachamber. We performed field observations and measurements of meter-scale ofmagmatic structures complemented by AMS studies, to identify and illustratedifferent structures analyzed from some plutons of the Sierras Pampeanas,central Argentina. We will discuss the rheological state of the magma duringtheir formation and the emplacement of the granitic body. Granitic bodies ofthe Sierra Pampeanas preserve, in excellent exposures, a series of magmaticstructures that formed as magmas stepped through its different rheological stateson the way to becoming a solid. These structures include hydrodynamic alignmentof crystals and enclaves, interaction between liquids with differentcomposition, large scale flow folds, magmatic banding and shearing, ladderdykes, and pipes, among others. Furthermore, the type of magmatic structuresdeveloped depends on their intrusion mode. Plutons that have grown bymulti-pulse emplacement, comprising a large number of small magma batches,usually preserve the magmatic structures formed during their growth. Unlike multi-pulseemplacement, plutons crystallized from a few batches of voluminous magma thatconstitute large batholiths only occasionally preserve the structures developedduring their emplacement.