Pegmatite emplacement during compressional deformation of the Guacha Corral shear zone, Córdoba, Argentina

DEMARTIS, MANUEL; PINOTTI, LUCIO; D'ERAMO, FERNANDO; CONIGLIO, JORGE; PETRELLI

Asociación Geológica Argentina, Serie D, Publicación Especial Nº14

Asociación Geológica Argentina

Lugar: Mendoza; Año: 2011 p. 71 - 74

0328-2767

INTRODUCTION In the Sierra de Comechingones, Córdoba province, Argentina, a great number of Be–Nb–Ta–U-rich pegmatites were grouped by Galliski (1994; 1999) in the Comechingones Pegmatitic Field (CPF), southeastern Pampean Pegmatitic Province (figure 1a). These pegmatites have a LCT signature and belong principally to the rare-element class, beryl–columbite–phosphate type, and some of them grade to the muscovite class (Galliski 1994). In the Sierra de Comechingones, Córdoba province, Argentina, a great number of Be–Nb–Ta–U-rich pegmatites were grouped by Galliski (1994; 1999) in the Comechingones Pegmatitic Field (CPF), southeastern Pampean Pegmatitic Province (figure 1a). These pegmatites have a LCT signature and belong principally to the rare-element class, beryl–columbite–phosphate type, and some of them grade to the muscovite class (Galliski 1994). These pegmatites were emplaced at pressures of approximately 500 MPa, synkinematically with the crustal scale Guacha Corral shear zone (GCSZ) development (Demartis 2010). At first sight, the transfer of magma through such a ductile, middle pressure continental crust seems to require a huge amount of energy. However, the GCSZ deformation provided open spaces for the migration and ascent of pegmatite magmas, and the high volatile content of the pegmatites enhanced significantly the buoyant force of these pegmatite magmas. These two points suggest a mechanism of easy-intrusion for the pegmatite magmas through the ductile continental crust. In this contribution, we present field and microstructural data from the southern part of the CPF (figure 1b) that evidences syntectonic pegmatite emplacement during the compressional deformation of the GCSZ. We also propose two main deformation related mechanisms for space generation that triggered magma ascent and emplacement. Geological setting Geological setting The GCSZ deformation took place during the Ordovician to Devonian Famatinian orogeny, (Rapela et al. 1998; Otamendi et al. 2004) and it reworked previous gneissic and migmatitic fabrics developed during Neoproterozoic to Cambrian Pampean orogeny. This reworking generated a pervasive submeridional striking moderately dipping mylonitic foliation. The GCSZ deformation took place during the Ordovician to Devonian Famatinian orogeny, (Rapela et al. 1998; Otamendi et al. 2004) and it reworked previous gneissic and migmatitic fabrics developed during Neoproterozoic to Cambrian Pampean orogeny. This reworking generated a pervasive submeridional striking moderately dipping mylonitic foliation. According to deformation evidences, two domains with contrasting finite deformation intensity were defined in the studied area of the GCSZ: “high strain domain” (HSD; figure 1c), dominated by mylonites and ultramylonites; and “low strain domain” (LSD; figure 1d), where gneisses and migmatites predominate. A transition zone between these two end-member domains also occurs. Thus, a westward increasing of strain rate can be established. Granitic pegmatites of the CPF occur as countless bodies with lens or sheet geometry of relatively small size (<50 m thick and 200 m long), that often come into contact, forming larger composite dykes with more than 1 km long. Internal zonation can be usually recognized individually in each lenticular pegmatitic body. Border and wall zones are composed of fine- to medium-grained quartz + muscovite ± microcline ± albite. Coarse- to very coarse-grained intermediate zones are composed of graphic microcline + quartz + muscovite ± albite ± garnet; phosphates and uranium minerals occur in these zones. Core is composed of milky quartz with subordinated microcline and muscovite, with accessory crystals of beryl and columbite. Highly fractionated replacement units occur only locally in some pegmatites (Demartis et al. 2011, this volume). P–T crystallization conditions of the pegmatites from southern CPF are 500 MPa and 600–700 ºC (Demartis 2010). STRUCTURAL ANALYSIS STRUCTURAL ANALYSIS Field and microstructural analysis was carried out both in country rocks and pegmatites from HSD and LSD. Mylonitic, gneissic and migmatitic foliations, as well as mineral stretching lineation and kinematic indicators, were measured in country rocks. Orientation, internal foliations, stretching lineations and kinematic indicators were also measured in pegmatites. Field and microstructural analysis was carried out both in country rocks and pegmatites from HSD and LSD. Mylonitic, gneissic and migmatitic foliations, as well as mineral stretching lineation and kinematic indicators, were measured in country rocks. Orientation, internal foliations, stretching lineations and kinematic indicators were also measured in pegmatites. In HSD, mean mylonitic foliation plane strikes submeridionally and dips moderately toward E, but orientation variations throughout this domain are very common and an anastomosed mylonitic foliation pattern is therefore defined. Kinematic analysis using mylonitic foliation and mineral stretching lineation indicates an almost reverse shear sense with an east-to-west tectonic transport, in a simple shear deformation context, where rotational elements are represented by sigma and delta-type porphyroclasts and S/C structures. Pegmatites in this domain generally crop out conformably with mylonitic foliation, even though where its orientation changes significatively, harmonically with anastomosing mylonitic foliation pattern. An internal foliation defined by stretched and reoriented microcline, quartz and muscovite of the intermediate zone also develops in the pegmatites, and a stretching mineral lineation can be locally observed, yielding an analogous sense of shear to that measured in mylonitic country rocks. Contrary to the higher strain domains, pegmatites of the LSD are usually discordant with NNW-striking, moderately E-dipping gneissic and migmatitic foliations. Pegmatite orientations also strike submeridionally and dip eastwards, but two different mean dipping angles were measured: low dipping angles (<30-40º) and high dipping angles (>70º). Both pegmatites from the LSD and the HSD display deformation textures and microstructures developed from submagmatic to low temperature subsolidus state. pegmatitic magma ascent and Emplacement mechanisms pegmatitic magma ascent and Emplacement mechanisms Field and microscopic structural data from the country rocks and pegmatites indicate synkinematical emplacement and crystallization for pegmatitic magmas during compressive deformational event of the GCSZ. Field and microscopic structural data from the country rocks and pegmatites indicate synkinematical emplacement and crystallization for pegmatitic magmas during compressive deformational event of the GCSZ. Two different magma ascent and emplacement mechanisms are proposed for space generation in both different strain domains. Discordant pegmatites cropping out in the LSD are thought to represent magmas emplaced in Riedel fractures that were developed in response to simple shear deformation, unrelated to any previous anisotropy, hereafter called “fracture-controlled” mechanisms (Clemens & Mawer 1992; Weinberg & Regenauer-Lieb 2010). Therefore, low dipping pegmatites are regarded as magmas intruding T extensional fractures, normal to the minimum compressive stress (σ3), while high dipping pegmatites represent magmas ascended through synthetic P fractures that acted as feeding channels, transferring magmas from lower structural levels. Many pegmatites from the HSD outcrop concordantly, even where anastomosed mylonitic foliation changes its orientation. They are regarded to represent pegmatitic magmas emplaced in local spaces generated by “magma pumping” mechanism. This mechanism takes place when spaces begin to open due to anastomosing mylonitic foliation laminae sliding during deformation, originating a local pressure gradient due to the vacuum generated between two adjacent laminae. This pressure gradient, together with enhanced magma buoyancy force, induces the ascent and migration of pegmatitic magmas towards these sites, separating mylonitic foliation laminae and pumping the magma into these spaces. Similar mechanism has been descrip for granitic magmas by (D’Lemos et al. 1992; Weinberg et al. 2009). Relatively high volatile contents (pegmatite mineralogy and geochemical signature indicate relatively high proportions of P, B and H2O in these melts) reduce magma viscosity (Dingwell et al. 1996), thus increasing buoyancy force. Moreover, during transport these volatile-rich magmas also favour mylonitic foliation laminae sliding, enhancing further deformation, generating a feed-back relationship between magma migration and progressive deformation. Fracture-controlled mechanism seems to have dominated magma ascent and emplacement during the initial stages of deformation, allowing magma transfer previously to anastomosed mylonitic foliation development. Once emplaced, with further deformation pegmatites were rotated and sheared by mylonitic flow, parallelising them with mean mylonitic foliation plane and losing their original orientation. Moreover, magma pumping mechanism began to take place when deformation originated anastomosed mylonitic foliation pattern. However, pegmatites emplaced in the LSD, which experienced almost none further rotation, represent the former stages of pegmatite emplacement and allowed reconstructing a general model for magma ascent and emplacement throughout this crustal scale shear zone. This model for mass transfer within ductile crust contributes to resolve energetic and rheological problems for magma ascent from lower to middle crustal zones.