U-Pb LA-ICP-MS GEOCHRONOLOGY

Boletín de Geología Vol. 32, N° 2, julio-diciembre de 2010 U-Pb LA-ICP-MS GEOCHRONOLOGY AND REGIONAL CORRELATION OF MIDDLE JURASSIC INTRUSIVE ROCKS FROM THE GARZON MASSIF, UPPER MAGDALENA VALLEY AND CENTRAL CORDILLERA, SOUTHERN COLOMBIA Camilo Bustamante1, Agustín Cardona1, Germán Bayona2, Andrés Mora3, Víctor Valencia4, George Gehrels5, Jeff Vervoort6 ABSTRACT New U-Pb zircon geochronology from four granitic units sampled along a southeast-northwest transect between the Garzón Massif and the Serranía de las Minas (Central Cordillera), records a Middle Jurassic magmatic activity with two different spatio-temporal domains at ca. 189 Ma and 180-173 Ma. Reconnaissance data suggest that the four granitoids are characterized by mineralogical and geochemical characteristics akin to a continental magmatic arc setting. The new results suggest that the southern Colombian continental margin includes remnants of tectonomagmatic elements formed by the subduction of the Farallon plate under the South American continental margin. This Middle Jurassic arc magmatism is part of the broader Andean scale arc province, and is significant for understanding the tectonic and paleogeographic scenario that characterized the Mesozoic tectonic evolution of the Northern Andes. Key words: U-Pb geochronology, Colombia, Jurassic, Intrusive rocks, Garzón Massif, Central Cordillera. GEOCRONOLOGÍA U-Pb LA-ICP-MS Y CORRELACIÓN REGIONAL DE LAS ROCAS INTRUSIVAS DEL JURÁSICO MEDIO DEL MACIZO DE GARZÓN, VALLE SUPERIOR DEL MAGDALENA Y LA CORDILLERA CENTRAL, SUR DE COLOMBIA RESUMEN Se presentan nuevas edades U-Pb en circones de cuatro unidades graníticas muestreadas a lo largo de una transecta SE-NW entre el Macizo de Garzón y la Serranía de las Minas (Cordillera Central), las cuales registran una actividad magmática en el Jurásico Medio en dos dominios espaciales y temporales diferentes: 189 Ma y 180 a 173 Ma. Estos cuatro granitoides tiene características mineralógicas y geoquímicas afines con un ambiente de arco magmático continental. Los nuevos resultados sugieren que la margen continental al sur de Colombia incluye remanentes de elementos tectono-magmáticos formados por la subducción de la placa Farallón bajo la margen continental suramericana. Este magmatismo de arco del Jurásico medio es parte de la provincia de arco que se presenta a lo largo de los Andes, y es importante para el entendimiento de la dinámica tectónica y la paleogeografía que caracterizó el Mesozoico de los Andes del Norte. Palabras clave: Geocronología U-Pb, Colombia, Jurásico, Rocas intrusivas, Macizo de Garzón, Cordillera Central. 1 Smithsonian Tropical Research Institute, Panamá, Panamá. [email protected], [email protected]; Cll 32D # 76-40 - Apto 402, Medellin, Colombia 2 Corporación Geológica Ares, Bogotá, Colombia. [email protected] 3 Instituto Colombiano del Petróleo-ECOPETROL, Piedecuesta, Colombia. [email protected] 4 Valencia Geoservices, Tucson AZ, USA. [email protected] 5 University of Arizona, Department of Geosciences, Tucson AZ, USA. [email protected] 6 Washington State University, School of Earth and Environmental Sciences, Pullman WA, USA. [email protected] 93 U-Pb LA-ICP-MS geochronology and regional correlation of Middle Jurassic intrusive rocks from the garzon massif, upper Magdalena Valley and Central Cordillera, Southern Colombia. <ul style="display: flex;"><li style="flex:1">INTRODUCTION </li><li style="flex:1">GEOLOGICAL SETTING </li></ul>Early to Late Mesozoic tectonic evolution of the western The Colombian Andes consist of three mountain ranges margin of South America is related to successions of (Figure 1) built as a result of Paleozoic and Mesodifferent tectonic regimes that recorded the initial effects Cenozoic phases of terrane relate accretion interspersed of Pangea break-up, Pacific subduction, and in northern several periods of continuous subduction (Restrepo and South America the formation of the proto-Caribbean Toussaint, 1988; Toussaint, 1995; Cediel et al., 2003; ocean (Pindell, 1985; Jaillard et al., 1990; Toussaint, Pindell et al., 2005; Ordoñez et al., 2006). 1995; Ramos and Aleman, 2000; Ramos, 2009; 2010; <ul style="display: flex;"><li style="flex:1">Pindell and Keenan, 2010). </li><li style="flex:1">The Western Cordillera includes volcanic rocks with </li></ul>intercalated marine sediments of Cretaceous age These events are responsible for overimposed tectonic formed in an oceanic plateau environment (reviews in scenarios of passive margin, rift and/or arc setting in the Kerr et al., 1997), whose accretion took place during Colombian Andes, and potential along strike and lateral the Late Cretaceous, linked to the advance of the segmentation of the different geological environments allocthonous Caribbean Plate (Toussaint, 1996; Kerr (Aspden et al., 1987; Toussaint, 1995; Ramos and and Tarney, 2005; Pindell et al., 2005). The Central Aleman, 2000; Cediel et al., 2003; Sarmiento-Rojas et Cordillera comprises a pre–Mesozoic polymetamorphic al., 2006; Vásquez et al., 2010). basement intruded by several Meso-Cenozoic plutonic rocks (Toussaint, 1993; Ordóñez-Carmona et al., 2006; Additional complexities include along strike translation Vinasco et al., 2006). Its major tectonic record reflect and juxtaposition of a variety of Mesozoic continental several collisional and subduction events between 290- magmatic related fragments (Bayona et al., 2006; 2010), 230 Ma link to the agglutination of Pangea (Ordóñez- that seems to be linked to the existence of an oblique Carmona and Pimentel, 2002; Vinasco et al., 2006; subduction configuration with the Colombian margin Cardona et al., 2010). Albian to Aptian rocks are as the final stop for the redistribution of continental discontinuously and limited exposed (Toussaint, 1996). para-authocthonous type terranes (Jaillard et al., 1990; Restrepo and Toussaint, 1988; Toussaint, 1993; Bayona Finally, the Eastern Cordillera includes Precambrian and <ul style="display: flex;"><li style="flex:1">et al., 2006; 2010; Keenan and Pindell, 2010). </li><li style="flex:1">Paleozoic metamorphic rocks, with overlain deformed </li></ul>Paleozoic sediments (Toussaint, 1993; Restrepo-Pace InthiscontributionwepresentnewU-PbLA-ICP-MSzircon et al., 1997; Cediel et al., 2003; Cordani et al., 2005; geochronology and reconnaissance geochemistry from four Ordóñez-Carmona et al., 2006). Meso-Cenozoic plutonic bodies sampled along an east-west transect between sedimentary marine and continental successions are the Garzón Massif and the westernmost limit of the Upper registered within this cordillera, and record the changing Magdalena Valley in order to establish more precise time tectonic conditions from passive to active margin that constrains on the Jurassic plutonism in Southern Colombia end in the Andean orogeny (reviews in Cediel et al., <ul style="display: flex;"><li style="flex:1">and contribute to the discussion of its tectonic scenario. </li><li style="flex:1">2003; Mora et al., 2006; Bayona et al., 2008). </li></ul>Geochronological data for these rocks has not been published so far, and their precise interpretation has been built on lithological correlations and geochronological data from plutonic rocks at the northern extension of this batholiths, which have shown mainly cooling ages that varies between ca. 177-136 Ma (Vesga and Barrero, 1978; Sillitoe et al., 1982; Brook, 1984; Aspden et al., 1987; Altenberger and Concha, 2005). JURASSIC MAGMATISM Jurassic volcanic and plutonic rocks are widespread along the Colombian Andes (Figure 1), and can be related to a broader magmatic province that affect the entire western margin of South America (Aspden et al., 1987; Lucassen, et al., 1996; Noble, et al., 1997; Jaillard et al., 2000; Kramer et al., 2005; Oliveros et al., 2006; 2007; Mpodozis and Ramos, 2008). Therefore, the precise timing and tectonic implication of this Jurassic plutons provide insights on the Middle Jurassic tectonomagmatic regime of northwestern South American and serve as piercing point for understanding the variable Middle Mesozoic tectonic events (Ramos and Aleman; 2000; Sarmiento-Rojas et al., 2006; Bayona et al., 2006, 2010; Vásquez et al., 2010). Magmatic activity is recorded by an extensive series of elongated batholithic bodies distributed along the eastern margin of the Central Cordillera, the margins of the Upper Magdalena Valley and the Garzón and Santander Massifs. Similar elements are also found within several more isolated 94 Camilo Bustamante, Agustín Cardona, Germán Bayona, Andrés Mora, Víctor Valencia, George Gehrels, Jeff Vervoort massifs in northern Colombian, including the San Lucas and Dewey, 1982; Maze, 1984; McCourt et al., 1984; Serrania and the Caribbean massifs such as the Sierra Nevada Aspden et al., 1987; Ross and Scotese, 1988; Pindell de Santa Marta, Perijá and Guajira Serranias (Aspden et al., and Erikson, 1993; Bayona et al., 1994; Toussaint 1995; 1987;Alvarez, 1967;Tschanz et al., 1974;Toussaint, 1995 and Pindell and Tabutt, 1995; Meschede and Frisch, 1998; references therein). Available geochronological data mostly Cediel et al., 2003; Vásquez et al., 2006). However due obtained by the K-Ar method reveals the existence of at least to the changing nature of the Mesozoic tectonic regimes three major magmatic peaks of magmatic activity between c.a. in northern South America (reviews in Toussiant, 195-180 Ma, 167-160 Ma and 151-142 Ma (review inAspden 1995; Pindell and Keenan, 2010; Ramos, 2010), the et al., 1987). Most of these rocks are spatially associated with apparent existence of significantly displaced crustal volcanic rocks of effusive and explosive character, which segments (Bayona et al., 2006; 2010) and the paucity of suggest a shallow level of emplacement and a protracted geochronological and geochemical data, a distinction on <ul style="display: flex;"><li style="flex:1">tectono-magmatic evolution. </li><li style="flex:1">the timing and evolving nature of the tectonic regimes </li></ul>or the along strike and lateral tectonic variation of the Tectonic models related to the formation and evolution margin is not clear (Toussaint, 1995; Cediel et al., 2003; of this magmatism, include variable rift and arc to Bayona et al., 2006; Vásquez et al., 2006; Sarmientoback-arc related settings (Tschanz et al., 1974; Pindell Rojas et al., 2006). FIGURE 1. Major Mesozoic and Cenozoic plutons from the Colombian Andes (Modified from Aspden et al., 1987). WC: Western Cordillera; CC: Central Cordillera EC: Eastern Cordillera. 95 U-Pb LA-ICP-MS geochronology and regional correlation of Middle Jurassic intrusive rocks from the garzon massif, upper Magdalena Valley and Central Cordillera, Southern Colombia. and the Central Cordillera. In Southern Colombia is JURASSIC PLUTONISM BETWEEN THE GARZÓN MASSIF AND THE UPPER MAGDALENA VALLEY limited at the east by La Plata Fault and the Chusma Fault system which juxtaposes it with Jurassic and Tertiary sedimentary rocks. Along its western contact also intrudes metamorphic rocks of the Las Minas Migmatites. Compositionally it is made predominantly of tonalite and granodiorite. In southern Colombia, along the Central Cordillera, Upper Magdalena Valley and Garzón Massif, several magmatic units are exposed (FIGURE 2). Based on regional correlations several Jurassic age plutonic and volcanic rocks have been recognized in these regions (Velandia et al., 2001a, 2001b). Within the Garzón Massif the plutonic bodies intrude Precambrian high grade metamorphic rocks whereas in the Upper Magdalena Valley, plutonic rocks intrude migmatites of unknown age (Álvarez, 1981; Kroonemberg, 1982; Álvarez and Linares, 1983; Velandia et al., 2001a, 2001b; Jiménez et al., 2006). ANALYTICAL TECHNIQUES Whole rock Geochemistry Bulk whole rock chemical analysis of 4 samples was determined by inductively coupled plasma-mass spectrometry (ICP-MS) atAcmeAnalytical Laboratories Ltd. in Vancouver, Canada. A 0.2 g aliquot is weighed into a graphite crucible and mixed with 1.5 g of LiBO2 flux. The crucibles are placed in an oven and heated to 1050° C for 15 minutes. The molten sample is dissolved in 5% HNO3.Calibration standards and reagent blanks are added to the sample sequence. Sample solutions are aspirated into an ICP emission spectrograph (Jarrel Ash Atom Comb 975) for determining major oxides and certain trace elements (Ba, Nb, Ni, Sr, Sc, Y & Zr), while the sample solutions are aspirated into an ICP- MS (Perkins-Elmer Elan 6000) for determination of the trace elements, including rare earth elements. Four plutonic units were sampled in an east-west transect between the Garzón Massif and Serranía de las Minas in the western limit of the Upper Magdalena Valley (Figure 2). Most of the geological descriptions of these rocks are from the 1:100.000 regional mapping of the Colombian Geological Survey and presented below (Velandia et al., 2001a, b). Garzón Granite: is an elongated intrusive body that intrudes Precambrian high grade metamorphic rocks from the Garzón Massif. The Garzón – Algeciras thrust fault juxtaposed this intrusive with Tertiary sedimentary rocks. The composition of this pluton ranges from granite to monzodiorite. U/PB LA-MC-ICP-MS U-Pb-Th geochronology was conducted in two sessions at the University of Arizona and Washington State University. Operating procedures and parameters are described in Valencia et al. (2005) and Chang et al. (2006), respectively. Altamira Monzogranite: this intrusive exposed in the eastern limit of the Upper Magdalena Valley has a faulted contact with Jurassic volcano-sedimentary rocks related to the Saldaña Formation. The Suaza Fault separates the Garzón massif from the Jurassic plutonic rocks. Its composition includes mainly monzogranites and is intruded in turn by different andesitic dikes. Zircon crystals were analyzed in polished epoxy grain mounts with a Micromass Isoprobe multicollector ICP- MS equipped with nine Faraday collectors, an axial Daly collector, and four ion-counting channels. Las Minas Monzodiorite: this intrusive outcrop in the central segment of the Upper Magdalena Valley and overthrusts Tertiary sedimentary rocks. At its western segment it presents intrusive contacts with metamorphic rocks included in the pre-Jurassic Las Minas migmatites. Its composition ranges from diorite to monzonite with gabbros at its border. U-Pb zircon crystallization ages were estimated and plot using Isoplot 3.0 (Ludwig, 2003) and Arizona LaserChron Excel macro age pick program. Two uncertainties are reported on these plots. The smaller uncertainty (labeled mean) is based on the scatter and precision of the set of 206Pb/238U or 206Pb/207Pb ages, weighted according to their measurement errors (shown at 1-sigma). The larger uncertainty (labeled age), which is the reported uncertainty of the age, is determined as the quadratic sum of the weighted Ibagué Batholith: This is one of the largest Jurassic batholiths in Colombia and consists of several intrusions of different ages that span from ca. 151 to 142 Ma (Aspden et al., 1987; Altenberger and Concha, 2005). It extends both along the Upper Magdalena Valley 96 Camilo Bustamante, Agustín Cardona, Germán Bayona, Andrés Mora, Víctor Valencia, George Gehrels, Jeff Vervoort mean error plus the total systematic error for the set calibration standard, composition of common Pb, and of analyses. The systematic error, which includes U decay constants, is generally ~1-2% (2-sigma). contributions from the standard calibration, age of the FIGURE 2. Geologic map of the studied area including samples location (Modified from Velandia et al., 2001a). 97 U-Pb LA-ICP-MS geochronology and regional correlation of Middle Jurassic intrusive rocks from the garzon massif, upper Magdalena Valley and Central Cordillera, Southern Colombia. mineral is often replaced by epidote, very fine muscovite and PETROGRAPHY calcite. Microcline is forming a braided pattern due to albite lamellae in the Garzón Granite (Figure 3a). Hornblende is abundant in the Las Minas Monzodiorite (CB0007A) with values from 10 – 15% and contrast with the other rocks where it ranges from 5 to 10%. Las Minas Monzodiorite and the Ibague Batholith presents Augite (Figure 3b) as an accessory with less than 2% and commonly enclosed into biotite crystals (Figure 3c) forming poikilitic textures (Figure 3d). Biotite crystals range from 5% to 10% and is commonly replaced by chlorite. Four representative samples from the different intrusive rocks were analyzed. The rocks described include two samples from the Ibagué Batholith, one from Las Minas Monzodiorite and one from Garzón Granite. The main differences identified rely on the ferromagnesian minerals contents. The sample from the Altamira Monzogranite was extremely weathered and was not possible to review. Mineral abbreviations were taken from Kretz (1983) and Spear (1993). Quartz crystals are xenomorphic and range from 20 to 30% in all samples. Plagioclase is the most abundant mineral and range from 40 – 50%. Crystals are commonly zoned and presents undulatory extinction and deformation twins. This Epidote is also widespread as a secondary mineral, and is found filling veinlets in the Ibagué Batholith (CB0011). Accessory minerals include zircon and opaque with limited contents of titanite. FIGURE 3a. Braided pattern due to albite lamellae in FIGURE 3b. Anhedral augite crystal surrounded by K-feldspar. Kfs: K-feldspar; Pl: Plagioclase. hornblende, biotite and quartz. Aug: Augite; Qtz; Quartz; Bt: Biotite; Hbl: Hornblende. FIGURE 3c. Augite and plagioclase inclusions into a biotite FIGURE 3d. Poikilitic textures were large plagioclase crystals crystal. Aug: Augite; Bt: Biotite; Pl: Plagioclase. are enclosed in biotite. Bt: Biotite; Pl: Plagioclase. 98 Camilo Bustamante, Agustín Cardona, Germán Bayona, Andrés Mora, Víctor Valencia, George Gehrels, Jeff Vervoort TheGarzónGraniteandtheAltamiraMonzograniterepresents GEOCHRONOLOGY the eastern segment yield ages from ca. 179 to 174 Ma (Figures 4a, 4b). Intrusive rocks located at the westernmost part of the profile are the Las Minas Monzodiorite and the Ibagué Batholith, and present ages at least 10 Ma older. These ages ranges from ca. 189 to 187 Ma (Figures 4c, 4d).All these ages are related to the zircon forming event which is a major record of the magmatic crystallization of the plutonic rocks. Analytical results are included in Table 1. U-Pb LA- ICP-MS zircon ages were obtained in the Garzón Granite (CB0001), Altamira Monzogranite (CB0005), Las Minas Monzodiorite (CB0007A) and the southern segment of the Ibagué Batholith (CB0010). TABLE 1. Analytical results of the U-Pb LA-ICP-MS zircon geochronology. <ul style="display: flex;"><li style="flex:1">Isotope ratios </li><li style="flex:1">Apparent ages (Ma) </li></ul>206Pb/ 207Pb ± (%) 207Pb/ 235U ± (%) 206Pb/ 238U ± (%) 206Pb/ 238U ± (Ma) 207Pb/ 235U ± (Ma) 206Pb/

U-Pb LA-ICP-MS GEOCHRONOLOGY

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