Revista de la Asociación Geológica Argentina 72 (3): 385 - 403 (2015) 385 GENESIS OF DIOCTAHEDRAL K- AND NA-BEARING MICAS AT THE STRONGLY ALTERED LATE PROTEROZOIC UNCONFORMITY OF THE TANDILIA BELT, CENTRAL- EASTERN ARGENTINA Juan C. MARTÍNEZ1, Hans J. MASSONNE2, Jorge A. DRISTAS3 y María C. FRISICALE1 1 CONICET-INGEOSUR and Departamento de Geología, San Juan 670, UNS, Bahía Blanca, 8000, Argentina*. 2 Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstrasse 18, D-70174, Germany. 3 CIC, INGEOSUR and Departamento de Geología, San Juan 670, UNS, Bahía Blanca, 8000, Argentina. * Corresponding author Dr. Juan Cruz Martínez: [email protected] ABSTRACT The unconformity-related phyllosilicate mineral assemblages of the most pervasively altered basement of the Tandilia belt, central Buenos Aires Province, were studied from two ~50 km distant areas, Barker and San Manuel. The secondary diocta- hedral K-white micas show a compositional spread probably by a solid-solution towards pyrophyllite. In addition, electron mi- croprobe analyses of micas yielded widely variable Na* [Na/(Na+K+Ca+Ba)] ratios, which can be related to a compositional range between nearly pure muscovite and intermediate micas with maximum paragonite contents corresponding to musco- vite55paragonite45 and muscovite40 paragonite60 for the Barker and San Manuel areas, respectively. Scanning electron images suggest the local occurrence of paragonite/intermediate K-Na white micas, finely intergrown within secondary K-white mi- 3+ ca and pyrophyllite. P-T pseudosections (H2O saturated) in the Na-K-Fe-Mg-Mn-Al-Si-Ti-O-H system for different XFe +3 (Fe /Fetot) constrain temperatures of formation of the secondary mineral assemblages around 280 - 340 ºC, whereas such mi- neral assemblages are almost insensitive to pressure between 0.5 and 5 kbar. At these P-T conditions, the most likely mineral assemblages include K-white mica, chloritoid, rutile, and hematite (± pyrophyllite ± paragonite). An additional phase is either quartz, which was observed as relic of the protolith, or diaspore. A 40Ar/39Ar step-heating age of 630 ± 30 Ma on a K-white mi- ca rich sample linked the hydrothermal alteration event to the Brasiliano orogenic cycle. Keywords: K-Na bearing micas, altered basement, Tandilia Late Proterozoic Unconformity, 40Ar/39Ar step-heating thermochronolog y, thermodyna- mic modeling RESUMEN Genesis de micas dioctahédricas con Na y K del basamento intensamente alterado de la discordancia del Proterozoico Tardío de Tandilia, centro-este de Argentina Las asociaciones de minerales filosilicáticos de la discordancia del basamento más intensamente alterado de Tandilia, centro de la provincia de Buenos Aires, fueron estudiadas y comparadas en dos áreas distantes por ~50 km, Barker y San Manuel. Las micas dioctahédricas potásicas de la alteración tienen una variación composicional debida probablemente a solución sólida ha- cia pirofilita. Además, los análisis de dichas micas tienen valores de Na* [Na/(Na+K+Ca+Ba)] ampliamente variables, los cua- les se pueden vincular a un rango de composiciones entre muscovita pura y micas intermedias, con un máximo contenido de paragonita correspondiente a: muscovita55paragonita45 y muscovita40 paragonita60, respectivamente para las áreas de Barker y San Manuel. Las imágenes de escaneado electrónico sugieren la ocurrencia local de paragonita/micas intermedias con K-Na finamente intercrecidas dentro de las micas potásicas y pirofilitas secundarias. Las pseudosecciones de presión - temperatura 3+ +3 (con H2O saturada) generadas para el sistema Na-K-Fe-Mg-Mn-Al-Si-Ti-O-H, con diferentes XFe (Fe /Fetot), permiten definir temperaturas mínimas de formación para las paragénesis de minerales de alteración entre 280 - 340 ºC, mientras que dichas paragénesis son prácticamente insensibles a los cambios de presión entre 0,5 y 5 kbar. En estas condiciones, las paragé- nesis de minerales de alteración más probables que surgen del modelado incluyen mica potásica, cloritoide, rutilo y hematita (± pirofilita± paragonita). Como fases adicionales se hallan cuarzo, relíctico del protolito, o diasporo. Una edad de 630 ± 30 Ma, determinada por calentamiento escalonado Ar40/A r 39 en una muestra rica en mica potásica secundaria, vincula el evento de al- teración hidrotermal al ciclo orogénico Brasiliano. Palabras clave: Micas con K y Na, basamento alterado, discordancia del Proterozoico Tardío de Tandilia, termocronología Ar40/Ar 39, modelado ter- modinámico 386 J.C. MARTÍNEZ, H.J. MASSONNE, J.A. DRISTAS AND M.C. FRISICALE Figure 1: Overview geologic map of the Tandilia belt with localities of interest (modified from Iñíguez-Rodríguez et al. 1989; Poiré and Spalletti 2005). INTRODUCTION ly deduced by thermodynamic model- neous-metamorphic Palaeoproterozoic ing. For instance, this method was ap- basement and an overlying mainly silici- K-Na-bearing micas such as paragonite, plied to naturally occurring mineral clastic Neoproterozoic sedimentary su- brammallite (Na-illite), metastable inter- assemblages with paragonite from Al- ccession. These assemblages have been mediate K-Na white micas, intergrowth pine and Variscan low-T metamorphic studied for more than three decades in or mixed-layered K-white mica and pa- terrains (Árkai et al. 2008). In previous the Barker and San Manuel areas (Dris- ragonite are known to occur in very low studies of K-Na-bearing micas, such as tas and Frisicale 1984; Regalía 1987; Zalba grade metamorphic sequences (e.g., Frey those mentioned above, the recogni- et al. 1992; Frisicale and Dristas 1993; Zal- 1969; Kisch 1983; Frey 1987; Li et al.. tion of textures and the way such fine- ba and Andreis 1998; Dristas et al. 2003; 1994; Livi et al. 1997; Merriman and Pea- grained layer-silicates appear in mineral Dristas and Martínez 2007; Martínez cor 1999; Merriman and Frey 1999; Mer- assemblages were systematically tackled and Dristas 2007; Martínez et al. 2010; riman 2002; Árkai et al. 2003; Árkai et by means of electron microscopic tech- Fernández et al. 2010). However, the find- al. 2008). In addition, the formation of niques: transmission electron microsco- ing of locally occurring dioctahedral K- such dioctahedral phyllosilicates has also py (TEM), scanning electron microsco- Na-bearing micas was only possible by a been reported from alteration of precur- py (SEM), electron microprobe (EMP) systematic XRD study of samples from sor minerals in hydrothermal environ- analyses, analytical electron microsco- several alteration profiles combined with ments (e.g., Shau et al. 1991; Jiang and Pea- py (AEM); and by the most traditionally EMP analyses and analyses of bulk-rock cor 1993; Monecke et al. 2001; Giorgetti used technique of X-ray powder diffrac- mass changes (Martínez et al. 2010). et al. 2003). tometry (XRD). This work is aimed at explaining the gen- Conditions of pressure (P) and temper- In the Tandilia belt (Fig. 1) phyllosilicate esis of the local occurrence of dioctahe- ature (T) of the formation of the above assemblages of alterations are restricted dral K-Na-bearing micas in secondary dioctahedral micas were hitherto rare- to the unconformity zone between an ig- mineral assemblages, which developed Genesis of dioctahedral K-Na-bearing micas of the Tandilia Belt 387 over the basement in a restricted zone (1 - nos Aires Complex (BAC, Marchese and 2a), the basement is composed of band- 15 m: below the unconformity) in two dis- Di Paola 1975). The BAC is largely com- ed migmatites with interbedded amphib- tant areas (~50 km between Barker and posed of granitoids, granitic to tonalitic olite lenses which are less than 1.5 m wide San Manuel). For doing this, three main gneisses and migmatites. Subordinated and up to 20 m long. These rocks show topics were considered. The first one was rocks of this igneous-metamorphic com- a regional subvertical foliation striking related to the characterization of the che- plex are amphibolites, ultramafic rocks, west-north-west (N70ºW to N60ºW). mistry and arrangement of these micas. calc-silicate rocks and scarce schists. The The leucosomes of these migmatites are The usage of XRD technique was limit- formation and main tectonic evolution of dominated by plagioclase (andesine) and ed by the very low amount of paragonite the BAC is attributed to a continent-con- orthoclase with scarce microcline, where- and/or any other Na-bearing mica in the tinent collision during the Trans-Ama- as the melanosomes are mainly composed studied samples. Thus, EMP analyses and zonian orogenic cycle, spanning main- of biotite. Accessory minerals include backscattered SEM imagery were carried ly 2.2-1.8 Ga (Cingolani and Dalla Salda garnet, apatite, zircon, allanite, mona- out in order to chemically characterize K- 2000; Cingolani et al. 2002; Hartmann zite and Ti-Fe oxide minerals. At the un- white micas and adjacent pyrophyllite and et al. 2002; Pankhurst et al. 2003; Cingo- conformity, in the east of Cuchilla de Las to texturally recognize different mineral lani et al. 2005; Massonne et al. 2012). Two Aguilas - La Tolva (Fig. 2a), the intense- phases. The second topic referred to the suites of calc-alkaline (andesitic to rhyo- ly altered migmatite consists of granoblas- physical conditions, pressure and tempe- litic) and tholeiitic (basaltic) dykes intrud- tic quartz relicts, with marked dissolution rature, of formation of the mineral assem- ed the BAC during the Early Proterozoic embayments, floating in a groundmass blages with these phyllosilicates. Thermo- (2.0 Ga.) and Middle Proterozoic