Grupo de Investigación en Geofísica y Geología (PANGEA), Programa de Geología, Universidad de Pamplona, Pamplona, Colombia - PDF

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First report and significance of the staurolite metabasites associated to a sequence of calc-silicate rocks from the Silgará Formation at the central Santander Massif, Colombia Carlos A. Ríos 1, *, Oscar

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First report and significance of the staurolite metabasites associated to a sequence of calc-silicate rocks from the Silgará Formation at the central Santander Massif, Colombia Carlos A. Ríos 1, *, Oscar M. Castellanos 2 Ciencias de la tierra 1 Grupo de Investigación en Geología Básica y Aplicada (GIGBA), Escuela de Geología, Universidad Industrial de Santander, Bucaramanga, Colombia 2 Grupo de Investigación en Geofísica y Geología (PANGEA), Programa de Geología, Universidad de Pamplona, Pamplona, Colombia Abstract The Silgará Formation metamorphic rocks have been affected by a Barrovian-type of metamorphism, which has occurred under medium-pressure and high-temperature conditions. Scarce intercalations of metabasites from millimeter up to centimeter scale occur in reaction bands observed in the gradational contact between garnet-bearing pelitic and calc-silicate rocks. In this study, we report for the first time the presence of staurolite metabasites in the Santander Massif (Colombian Andes), which is of particular interest since it is an unusual occurrence, taking into account that staurolite is most commonly regarded as an index mineral in metapelites and is not very well known from other bulk compositions and pressure and temperature conditions. Staurolite metabasites contain plagioclase, hornblende and staurolite, suggesting a history of prograde metamorphism up to amphibolite facies conditions. The origin of staurolite can be associated to aluminium-rich metabasites and, therefore, it is strongly affected by bulk rock chemistry. Taking into account mineral assemblages and geothermobarometric calculations in pelitic rocks, we suggest that the staurolite + hornblende association can be formed at least at 400 to 600 C and 6 kbar at the peak of prograde metamorphism. Retrograde reactions suggest that these rocks experienced nearly isobaric cooling accompanied by retrograde metamorphism. Key words: Staurolite, amphibolite, Silgará Formation, metamorphism, central Santander Massif. Primer reporte y significado de las metabasitas con presencia de estaurolita asociadas a una secuencia de rocas calcosilicatadas en la Formación Silgará de la región central del Macizo de Santander, Colombia Resumen Las rocas metamórficas de la Formación Silgará fueron afectadas por un metamorfismo tipo barroviense en condiciones de presión media y alta temperatura. Las intercalaciones de anfibolitas en escala milimétrica a centimétrica son escasas en las bandas de reacción del contacto gradacional entre rocas pelíticas con granate y rocas calcosilicatadas. En el presente trabajo se reporta por primera vez la presencia de metabasitas con estaurolita en el Macizo de Santander (Andes colombianos), lo cual es de particular interés por lo inusual de su ocurrencia y porque la estaurolita comúnmente se considera más como un mineral índice en metapelitas y no se conoce muy bien a partir de otras rocas de diferente composición y condiciones de presión y temperatura. Las metabasitas con estaurolita contienen plagioclasa, hornblende y estaurolita, lo que sugiere una historia que abarca desde el metamorfismo prógrado hasta las condiciones de la facies anfibolita. El origen de la estaurolita puede asociarse a metabasitas ricas en aluminio, por lo cual está fuertemente afectada por el quimismo de la roca. Teniendo en cuenta las paragénesis minerales y los cálculos geotermobarométricos en rocas pelíticas, los autores proponen que la asociación estaurolita + hornblenda puede formarse al menos a C y 6 kbar en el pico de metamorfismo prógrado. Las reacciones retrógradas sugieren que estas rocas experimentaron un enfriamiento casi isobárico acompañado de metamorfismo retrógrado. Palabras clave: estaurolita, anfibolita, Formación Silgará, metamorfismo, región central del Macizo de Santander. Introduction Staurolite occurs almost exclusively as a typical product of regional metamorphism in rocks of pelitic composition; however, it has been recorded as a rare constituent in metamorphic rocks of mafic composition (Selverstone, et a1., 1984). The occurrence of staurolite in metabasites has been reported by several authors: Miyashiro (1973), in metabasites of the Sambagawa metamorphic belt, Japan; Jan, et al. (1971), in amphibolite of the Timurgara ultramafic complex, Pakistan; 418 Metabasites at the central Santander Massif, Colombia Demange (1976), in epidote amphibolite of the Ovala Sequence, Gabon; Gibson (1979), in sheets of interlayered amphibolite and hornblendite in the metamorphosed gabbroic anorthosite of the Upper Seaforth River, Central Fiordland, New Zealand; Selverstone, et al. (1984), in amphibolites from the Mara Rosa volcano-sedimentary sequence, central Brazil; Purttscheller & Mogessie (1984), in garnet amphibolite from Sölden, Ötztal Old Crystalline Basement, Austria; Helms, et al. (1987), in amphibolites of the Laurel Greece mafic-ultramafic complex, northeastern Georgia Blue Ridge, U.S.A.; Enami & Zang (1988), in metabasic eclogites from Jiangsu Province, East China; Moeen (1991), in amphibolites in the Vinjamum area of the Nellore granite-greenstone terrain of India; Soto & Azañón (1993), in amphibotites from the Beltic Cordillera, Spain; Kuyumjian (1998), in ortho-amphibolites from the Chapada region, Goiás, central Brazil; Tsujimori & Liou (2004), in epidote-amphibolites from the Early Palaeozoic Oeyama belt, SW Japan; Faryad & Hoinkes (2006), in Alrich metabasites from the Speik Complex in the Eastern Alps. However, the literature contains little reference to staurolite metabasites of igneous origin. In this study, we report and discuss data concerning these unusual staurolite metabasites at the central Santander Massif (CSM) region with the aim of determining whether these rocks resulted from unusual bulk rock composition or from unusual physical conditions. Geological setting Several studies have been published on the geology of the Santander Massif since the first work undertaken by Julivert (1958), which was followed by those by Ward, et al. (1969a, 1969b, 1970, 1973). Structural geologic studies have been carried out by Julivert (1970), Forero (1990) and Kammer (1993). Ward, et al. (1973) divide the pre-devonian crystalline basement of the Santander Massif into the following three deformed and metamorphosed rocks: Bucaramanga Gneiss Complex, Silgará Formation and Orthogneiss, all of which are cut by Paleozoic Jurassic intrusive bodies (Goldsmith, et al., 1971; Banks, et al., 1985; Boinet, et al., 1985; Dörr, et al., 1995; Restrepo-Pace, 1995; Ordoñez, 2003; Ordóñez & Mantilla, 2004) and smaller Cretaceous intrusive bodies. However, Mantilla, et al. (2009) reported U-Pb ages in zircons of ±0.2 Ma from riodacite porphyry bodies in the central part of the Santander Massif, which evidences a magmatic phase during the Late Miocene (Tortonian) that took place during the Andean Orogeny. New evidences on this Miocece magmatism have been recently reported by Mantilla, et al. (2011), who determined U-Pb ages in zircons of 10.9±0.2 Ma (from porphyritic andesite) *Corresponding author: Carlos A. Ríos, Recibido: 24 de julio de 2014 Aceptado: 9 de diciembre de 2014 and 10.1±0.2 Ma (from porphyritic granodiorite). Wellexposed sections of the Silgará Formation crop out at the Santander Massif, which has been recognized as a classic area for the study of rock metamorphism and deformation caused by continental collision during the Caledonian orogeny (Ríos, et al., 2008a). This metamorphic unit has been studied by Ríos, et al. (Ríos, 1999, 2001, 2005; Ríos & Takasu, 1999; Ríos & García, 2001; Castellanos, 2001; Ríos, et al., 2003a, 2003b, 2008a, 2008b, 2010; García, et al., 2005; Castellanos, et al., 2004, 2008), mostly focusing their research during the last two decades on the estimation of metamorphic conditions, taking into account that the CSM represents a natural laboratory to understand the geotectonic evolution of the northwestern margin of South America. The Lower Paleozoic Silgará Formation at the CSM crops out into two N-S trending strips, locally interrupted by the presence of dykes and sills of orthoamphibolites with banded to gabbroic structures (Figure 1). It is mainly composed by metapelitic rocks with minor intercalated psammitic, semipelitic, metabasic and metacarbonate rocks, which were affected by a metamorphism to upper amphibolite facies regional grade during the Caledonian orogeny, and reveals a very complex tectonic and metamorphic history. Ríos, et al. (2008b) described in detail the metacarbonate and associated rocks that occur in the contact between marble and pelite layers, displaying a broad spectrum of physical conditions varying from greenschist facies to amphibolite facies; a non-economic mineralization reaction calcic exoskarn (except by the exploitation of marble) for the metacarbonate and related rocks that form part of the metamorphic sequence of the Silgará Formation at the CSM has been suggested by these authors based on the composition and texture of the resulting skarn, as well as on the available terminology for these rocks, among other aspects. The rocks of interest in this study correspond to the staurolite metabasites of the Silgará Formation, which were not reported by these authors. Field sampling and analytical methods A research team from Universidad Industrial de Santander carried out reconnaissance fieldwork in the Santander Massif, primarily focused on localities presenting amphibolites in the reaction bands observed in the gradational contact between garnet-bearing pelitic and calc-silicates rocks. The team took samples containing reaction bands close to marbles from several outcrops. The metabasites for the study were collected in one outcrop close to the Curpaga marble quarry, and belong to the staurolite-kyanite metamorphic zone. The thin section for microscopic analysis was performed at the Sample Preparation Laboratory of the School of Geology; the mineralogical and petrographic analysis of the sample was performed in a Nikon (Labophot2-POL) transmitted light microscope with trinocular viewing to establish the modal 419 Ríos CA, Castellanos OM Figure 1. Above, location of the Santander Massif and its corresponding geologic sketch map (modified after Ward, et al., 1973), showing the CSM and the distribution of its basement metamorphic and igneous rocks. Below, geologic map of the CSM (modified after Ward, et al., 1970), showing the distribution of metamorphic isograds of García, et al. (2005). The black star indicates the location of the staurolite metabasites. percentage of mineral constituents and mineral assemblages, with emphasis on textural relationships between mineral phases; photographs were taken with a NIKON AFX-DX microphotographic system at the Research Group in Basic and Applied Geology of the School of Geology. Mineral abbreviations are after Kretz (1983). SEM-BSE/EDS imaging and analysis were carried out by environmental scanning electron microscopy (FEI Quanta 650 FEG) to examine textures and cross-cutting relationships in the mineral phases in the staurolite metabasites under the following analytical conditions: magnification = x, WD = 9,9 mm, HV = 20 kv, signal = Z CONT, detector = BSED. 420 Field occurrence Metacarbonate and associated rocks occur as scarce intercalations of variable morphology (with sharp contacts) and thickness, developing discontinuous bands and lenticular bodies within the metamorphic sequence of the Silgará Formation at the CSM. According to Ríos, et al. (2008b), marbles show a transition into carbonate-silicate rocks, which, in turn, pass into calc-silicate and carbonate-bearing silicate rocks; finally, when carbonate tends to disappear in calc-silicate and carbonate-bearing silicate rocks, they pass into metapelitic and metamafic rocks. These rocks show a very complex mineralogy and appear most commonly as green reaction zones along the contact between marbles or carbonate-silicate rocks and pelitic layers of millimeter to centimeter scale, and their regional proportion is difficult to assess due to exposure limitations. The banding is characterized by the alternation of carbonate-rich layers with pelitic Metabasites at the central Santander Massif, Colombia and/or calc-silicate layers. The reaction zones are parallel to the main foliation and in many cases have been folded with it. Gradational contacts between garnet-bearing pelitic and calc-silicate rocks were also observed, which are especially abundant in strongly deformed rocks where calcsilicate zones may have a very irregular shape and variable thickness. The outcrop of interest in this study reveals the occurrence of scarce layers of staurolite metabasites from millimeter up to centimeter scale belonging to the reaction zones that show a gradational contact from garnet-bearing pelitic rocks to marbles, as reported by Ríos, et al. (2008b). The general features of the staurolite metabasites at field and hand-specimen scale are shown in Figure 2. The outcrop where the staurolite metabasites occurred was found close to a marble quarry characterized by an abrupt topography (Figure 2a). Figure 2b displays the occurrence of interbedded marble (light color) and staurolite metabasite (dark color) bands. A close-up of the staurolite metabasites (dark color) Figure 2. Field and hand-specimen photographs of the staurolite metabasites of the Silgará Formation at the Central Santander Massif 421 Ríos CA, Castellanos OM is shown in Figure 2c, where folded calcite veins concordant with the regional foliation quartz veins are observed. Figure 2d shows a hand-specimen of these metabasites, displaying the banding and mineral alignment. The main foliation of the rock is defined by the preferred orientation of staurolite and hornblende. Figures 2e-f illustrate close-ups of the mineral phase relationships and texture features in the staurolite metabasites. The reddish-brown mineral is staurolite, which displays typical elongate and six sided crystals. Petrography The contact zone between garnet-staurolite pelitic schists and staurolite metabasites reveals interesting features, which are illustrated in Figure 3. A typical garnet-staurolite pelitic schist with garnet porphyroblasts displaying a sigmoidal pattern of inclusions due to rotation in a matrix, and mainly composed of muscovite, biotite and quartz, is illustrated in Figures 3a-b. Staurolite porphyroblasts in garnet-staurolite pelitic schists commonly display a pattern of inclusions of quartz and ilmenite, which is discordant with the main foliation of the rock defined by biotite flakes (Figures 3c-d). Garnet amphibolites can be found close to the contact zone (Figures 3e-f). Figures 3g-h illustrate the contact between a staurolite-bearing biotite schist in the top and a staurolite metabasite in the bottom. A detail of the occurrence of the staurolite metabasites is shown in Figures 3i-j. Staurolite metabasites are characterized by alternating nematoblastic bands composed of hornblende and staurolite (with penetration twinning) and granoblastic bands composed of quartz and plagioclase (Figures 3k-l). Staurolite metabasites show an inequigranular texture with staurolite and hornblende randomly distributed developing intergrowth. They are mainly composed by staurolite, hornblende and plagioclase, with minor opaque minerals (ilmenite). Accessory minerals are titanite and rutile, whereas chlorite and sericite are the common secondary minerals. Of special interest is the coexistence of staurolite + hornblende + plagioclase, a mineral association not commonly reported in the literature regarding metabasites. The results of the mineralogical and petrographic analysis of this sample are described below. Figure 4 illustrates the main petrographical aspects of the staurolite metabasites under study. Staurolite occurs as large and defined lozenge-shaped porphyroblasts randomly oriented. In some cases it shows a simple interpenetrating twin. It can be partly included in hornblende. Hornblende occurs as prismatic (with a diamond-shaped basal cross section) nematoblasts in various orientations, which can be observed as few inclusions in staurolite. Plagioclase shows Figure 3. Microtextural relations of metamorphic minerals in the contact zone between garnet-staurolite pelitic schists and staurolite metabasites from the Silgará Formation at the CSM. a - b. Sigmoidal pattern of inclusions in garnet porphyroblasts in garnet-staurolite pelitic schist. Note the occurrence of a staurolite around garnet in the right side. c - d. Staurolite porphyroblasts with a pattern of inclusions of quartz and ilmenite discordant to the main foliation. Note the granoblastic domains composed of quartz and plagioclase. Garnet is not shown. e - f. Garnet amphibolites close to the contact zone. g - h. Contact zone between a staurolite-bearing biotite schist and a staurolite metabasite. i - j. Occurrence of the staurolite metabasites, with randomly orientation of hornblende, which has been partially replaced by chlorite. k - l. Alternating nematoblastic bands of hornblende and staurolite and granoblastic bands of quartz and plagioclase in the staurolite metabasites. Note the orientation not only of these bands but also of the ilmenite laths. 422 Metabasites at the central Santander Massif, Colombia Figure 4. Photomicrographs showing representative textural relationships between staurolite and hornblende and associated mineral phases in staurolite metabasites Figure 5. SEM photomicrographs of the staurolite metabasites a tabular or lath-like shape and may appear cloudy due to incipient alteration to sericite. It occurs as a matrix phase and also as inclusions in hornblende and staurolite. Ilmenite laths, usually randomly oriented locally, tend to develop an oriented trend and are observed as inclusions in staurolite, hornblende and plagioclase. Chlorite occurs as an alteration mineral along irregular fractures in staurolite and hornblende. The staurolite metabasites show interesting textural relationships between staurolite and hornblende. Staurolite displays 90 cruciform twins and included in hornblende with incipient alteration to chlorite. A pseudohexagonal staurolite crystal with an inclusion (quartz)-rich core and inclusion-poor rim is surrounded by large hornblende individuals. Ilmenite laths are randomly distributed. Large staurolite individuals with numerous ilmenite inclusions are closely related to hornblende and plagioclase. Staurolite can be included in honblende, whereas plagioclase sometimes is partly included in staurolite. Figure 5 reveals some interesting relationships between staurolite and associated mineral phases in the analyzed sample. Figures 5a and 5b display the relationships between staurolite and honblende, showing incipient alteration to chlorite that usually appears along irregular fractures. 423 Ríos CA, Castellanos OM Both minerals contain numerous inclusions of ilmenite laths randomly oriented. Staurolite also contains inclusions of hornblende and fluorapatite, whereas plagioclase and quartz are included in hornblende. Figure 5c illustrates numerous ilmenite laths which tend to show an orientation across the matrix. Figure 5d shows a detail of the occurrence of chlorite along an irregular fracture in staurolite, which contains an inclusion of hornblende. The SEM image in Figure 6 shows the textural relationships between staurolite and honblende, as well as associated mineral phases with semi-quantitative energy dispersive spectrum (EDS) analysis at different points. Energy Dispersive Spectroscopy (EDS) allowed to identify those particular elements and their relative proportions in the mineral phases that constitute the staurolite metabasites. The EDS spectrum of staurolite (1) revealed that it mainly consists of O, Al, Si and Fe elements. The mass ratios of O:Al:Si:Fe were 31.11:27.96:13.29
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