Néstor Vegas

Coalescence of lateral spreading magma ascending through dykes: a mechanism to form a granite canopy (El Hongo pluton, Sierras Pampeanas, Argentina)

This study deals with the structure of the El Hongo trondhjemite, a ENE–WSW elongate pluton belonging to the Famatinian magmatic arc that developed from Late Cambrian to Silurian times in the Sierras Pampeanas (Argentina). Anisotropy of magnetic susceptibility (AMS) measurements combined with structural and microstructural data permit the correlation of magmatic and magnetic fabrics within this ferromagnetic pluton. Magnetic foliations strike north–south and dip steeply west but in the marginal facies the foliation becomes parallel to the ENE–WSW pluton contacts. Magnetic lineations mostly trend north–south and plunge SE. Gravity data reveal that the pluton is in the form of a very thin horizontal sheet resting on two north–south-trending root zones detected below the central portion of the pluton. These root zones represent feeder dykes that exploited tension fractures. The emplacement of the El Hongo pluton took place during the action of a buried sinistral shear zone, parallel to the elongation of the pluton, corresponding to a transfer fault associated with the late extensional exhumation of the Famatinian domain. The emplacement of other Famatinian granites of the Sierras Pampeanas could be related to similar late shear zones.

The Sphene-Centered Ocellar Texture: An Effect of Grain-Supported Flow and Melt Migration in a Hyperdense Magma Mush

The sphene-centered ocellar texture consists of leucocratic ocelli with sphene (titanite) crystals at the center, enclosed in a biotite-rich matrix. This texture has been recognized worldwide in hybrid intermediate rocks. On the basis of structural, petrological, and geochronological data from selected outcrops of the Variscan Ribadelago pluton (NW Iberian Massif), we propose that the ocelli were formed by migration and accumulation of a residual melt through a plagioclase- and biotite-dominated crystalline framework. At the late stage of crystallization, the magma acted as a hyperdense suspension and reacted to the pressure gradient caused by the regional stress field, entering the domain of grain-supported flow. Microstructures reveal that aligned crystal domains arose in the crystal framework from the shearing and compaction of the crystal mush and behaved as magmatic microshears. Relative displacement of adjacent crystal clusters along these microshears corresponded to the onset of Reynolds dilatancy that generated an expansion of the crystal mush, involving melt migration and pore aperture. The mineralogy of the ocelli, dominated by andesine and sphene, represents the composition of the migrating melt. The chemistry of this late, Ti-rich melt stems from the incongruent melting of biotite. Magmatic sphene from the ocelli yields a U-Pb age of Ma, which represents the final crystallization of the hybridized magmatic system. Moreover, this texture offers an opportunity to better understand the rheological behavior of highly crystallized magmas.

Anisotropy of magnetic susceptibility of the Pyrenean granites

ABSTRACT In this paper, we report on a compilation of more than 2200 sites (more than 10,000 individual measurements) where anisotropy of magnetic susceptibility (AMS) was studied in granites from the Variscan Pyrenees. The standardization and homogenization of this information has allowed us to produce three Main Maps that synthesize all the information related with the AMS of the Pyrenean granites. We also describe the problems found during the construction of the database (variable geo-positioning, different published information, etc.). The information derived from 21 granite bodies, the database, and the synthesis maps (magnetic susceptibility, Km, and the orientation of the magnetic foliation, plane perpendicular to k3, and of the magnetic lineation, k1) allow us to see for the first time a complete image of this important kinematic and petrographic indicator.