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40Ar/39Ar Ages from Blueschists of the Jambaló Region, Central Cordillera of Colombia: Implications on the Styles of Accretion in the Northern Andes

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40Ar/39Ar Ages from Blueschists of the Jambaló Region, Central Cordillera of Colombia: Implications on the Styles of Accretion in the Northern Andes Geologica Acta, Vol.9, N o s 3-4, September-December 2011, 351-362 DOI: 10.1344/105.000001697 Available online at www.geologica-acta.com 40Ar/39Ar ages from blueschists of the Jambaló region, Central Cordillera of Colombia: implications on the styles of accretion in the Northern Andes 1 1 2 2 A. BUSTAMANTE C. JULIANI C.M. HALL E.J. ESSENE † 1 Universidade de São Paulo, Instituto de Geociências Rua do Lago 562, CEP 05508-080, São Paulo, SP, Brasil 2 University of Michigan, Department of Geological Sciences 2534 CC Little Building, 1100 North University Ave, Ann Arbor, MI 48109-1005, USA † Deceased May 20, 2010 ABS TRACT This paper presents the first argon dating of blueschists from the Jambaló area (Cauca Department) in the Central Cordillera of the Colombian Andes. Step-heating 40Ar/39Ar spectra were obtained for mica from several lenses of blueschists including greenschist facies rocks. The blueschists are mainly constituted of preserved lenticular cores in strongly mylonitic rocks, which resulted from retrometamorphic processes that affected the high pressure rocks during their exhumation. The majority of 40Ar/39Ar data points to metamorphic ages close to 63±3Ma, but some ages are older than 71Ma. These Maastritchtian–Danian ages correspond to the timing of exhumation of the blueschists near metamorphic peak conditions, because the dated paragonite and phengite crystallized during development of the mylonitic foliation. The continuous exhumation of this blueschist belt between 71–63Ma reflects the flow on an accretionary system/subduction channel environment that was interrupted by the collision of an intra-oceanic arc with the continental margin. Regional geological correlations suggest that this arc–continent collision also took place in Ecuador. This collisional event, although synchronous with other arc–continent collisions in the Northern Andes, was apparently not related to the formation of the great Caribbean arc, but to an arc built in the southeastern margin of the Caribbean plate. KEYWORDS Blueschists. Colombian Andes. Jambaló area. 40Ar/39Ar geochronology. INTRODUCTION blueschists may reveal the type of subduction and the characteristics of the collisional and exhumation regimes Because blueschist facies rocks are associated with (Ernst, 1988; Smith et al., 1999). Usually these high-pressure convergent limits of tectonic plates (Bowes, 1989), and low-temperature rocks record the P–T variations metamorphic and deformational events registered in the with time, which can reflect the complex exhumation 351 A. BUSTAMANTE et al. Age of blueschist and accretion in the Northern Andes, Colombia CARIBBEAN SEA km processes associated with either subduction or collisional 0 50 events or with later transpressive regimes (Cloos, 1982). 0° SOUT H Consequently, the identification and characterization of AMERICA 20°S 8ºN IC ault blueschists in the geological record can provide insights F N r OCEA N PACI FIC e on the nature and timing of the convergence zones that AT LOCEA ANT 40°S gu a 80°W 60°W 40°W characterize the terranes in which they occur (Draper and lm -A Lewis, 1991; Avé-Lallemant, 1996). ca u C a SEA The Meso-Cenozoic orogeny in the Northern Andes, CARIBBEAN especially in Ecuador, Colombia and Caribbean, is characterized by a series of island arc and plateau 8º collisions against the South American continental margin, 1 6º accompanied by subduction and tectonic imbrication of 3 slices of oceanic crust, either in the continental margin or intra-oceanic domains (Kerr et al., 1997, 2002; Luzieux 4º PACIFIC OCEAN 2 et al., 2006). These processes resulted in obduction of Medellín 2º ophiolitic complexes, Barrovian metamorphic belts, high- º 6 77º 7 1. Western Cordillera 2. Central Cordillera 6ºN pressure metamorphic rock units, and in the amalgamation 3. Eastern Cordillera ca- Almaguer Fault au and strong interaction of several tectonic terranes (Ramos, C 1999; Ramos and Aleman, 2000; Kerr et al., 1997, 2002; San Jerónimo Fault Giunta et al., 2002). Some suture zones, such as those recognized in Colombia (González, 1977; Chicangana, 2005), are marked by the occurrence of tectonic slices of blueschist-facies metamorphic rocks, which have been Manizales considered as late Cretaceous in age (Orrego et al., 1980a; Silvia-Pijao De Souza et al., 1984). The origin of these blueschists has Fault been related to oceanic subduction, accretion or collision of an oceanic plate coming from northwest and reaching Cali-Patia Fault the South American plate (Feininger, 1980; Aspden and PACIFIC OCEAN McCourt, 1986; Mégard, 1987; Bourgois et al., 1987; Aspden et al., 1995). In order to test such possibilities, the detailed petrotectonic evolution and the ages of the different 4ºN Mesozoic Barrovian and high-pressure metamorphic belts in western Colombia should be established by means of San Jerónimo Fault detailed geochronologic dating. In this contribution new Cali Ar–Ar data are presented for white mica of the blueschists of the Jambaló region, in the western flank of the Central Cordillera of the Colombian Andes. Cauca-Almaguer Fault JAMBALÓ GEOLOGICAL SETTING AREA Silvia-Pijao Fault In Colombia, the Andean Cordillera is divided into three mountain ranges, the Eastern, Central and Western Cordilleras (Fig. 1), which underwent different tectonic and geologic histories. The Eastern Cordillera is composed 2ºN mainly of a Paleozoic to Tertiary thick sedimentary sequence (Restrepo and Toussaint, 1982; Forero-Suárez, 1991). 76ºW The Western Cordillera is made up of oceanic volcanic 77ºW Miocene volcanism Blueschists and eclogites(?) sequences accreted since Cretaceous times, whereas the Quebradagrande Complex Ultramafic rocks Central Cordillera comprises a polymetamorphic complex Mesozoic oceanic volcanism Arquía Complex Mesozoic magmatism High grade metamorphic rocks associated with syn-tectonic granitoids, with K-Ar ages Cajamarca Complex Orthogneisses varying between Paleozoic to Cretaceous (Restrepo and Toussaint, 1982, 1988; Restrepo, 1986). Well defined FIGURE 1 Lithodemic units of the Central Cordillera of Colombia, after blueschist occurrences have been recognized in the Maya and González, (1995). Geologica Acta, 9(3-4), 351-362 (2011) 352 DOI: 10.1344/105.000001697 A. BUSTAMANTE et al. Age of blueschist and accretion in the Northern Andes, Colombia western flank of the Central Cordillera in Jambaló and metabasalt and subordinated metasedimentary rocks, all Barragán. Another occurrence of blueschist rocks has been of them metamorphosed to the amphibolite facies; described in the Pijao area (Núñez and Murillo, 1978), but 2) La Mina Greenschist, which includes rocks similar to the geology, petrography and ages have not been clearly those of the San Antonio Unit, but metamorphosed to the defined yet. greenschist facies, and 3) Glaucophanic Schist, formed by glaucophanic, chloritic, amphibolitic and micaceous The Jambaló blueschists are limited to the east by the schists associated with foliated quartzites, marbles polymetamorphic rocks of the Cajamarca Complex (Maya and metaperidotites (serpentinites). This 25km-long and González, 1995), whereas to the west the contact blueschists unit was considered to be a continuous body, is tectonic with arc-related volcanic rocks, which are elongated north–northeast and parallel to the regional regionally grouped in the Quebradagrande Complex. This Andean trend (Orrego et al., 1980b; Feininger, 1982). Our complex includes units of different ages, probably formed field observations, however, do not support the existence in oceanic settings (Maya and González, 1995; Nivia et of a continuous belt, but of a series of discontinuous lenses al., 2006). The regional geological setting of the Jambaló blueschists is illustrated in Figure 1, and the geologic map of the study area is summarized in Figure 2. El Palo 76°13’W 76°20’W River To iver El P al o r Previous studies have considered the age of the Jambaló El Palo TACUEYÓ blueschist as Early Cretaceous (Orrego et al., 1980a; De 121 81º Souza et al., 1984). This interpretation will be discussed 40 39 considering new Ar– Ar isotopic data obtained for 3ºN Sa La Calera n D paragonite and phengite from the blueschists. ie go or El B arri 123 81º a l 41º Vi chiquí 40Ar/39Ar METHODOLOGY TORIBÍ O Micas from six samples of blueschist facies rocks were Elassi o 40 39 concentrated and analyzed by the Ar/ Ar method. Rarer 59º phengite from only one sample was analyzed. 59ºChiquito Rí o Greenschists Fault Mineral concentrates were prepared by manual Blueschists City Keratophyres River twice Metaultramafics separation of grains present in the 0.5–1mm fraction. Two ver Road ri Flay Epidosites 40 39 129 Ar/ Ar samples ó or three grains from each sample were analyzed, although s co Mylonitized Sampling point ranci Quartzites for some samples only one grain was analyzed. n F Marbles Control Point Sa r La Primicia Quartz- ive Structural data monzonite 49º ó r 40Ar/39Ar analyses were performed at the Argon Jambal Quinamayó CORDILLERAS Geochronology Laboratory of the University of Michigan 1 - Western,2 - Central, and3 - Eastern using a continuous laser for step-heating and a VG 1200S Ba N noble gas mass spectrometer equipped with a Daly detector rand illo r iver E l operated in analog mode using the methods outlined in As io Panama Streepey et al. (2000) and Keane et al. (2006). Samples were packed within pure Al foil packs and irradiated for Venezuela 1 2 10.83hr at location 5C at the McMaster Nuclear reactor. 2°50’N 3 Quoted ages are calculated relative to an age of 520.4Ma COLOMBIA for hornblende standard MMHb–1. After irradiation two or Pacific Ocean three crystals were chosen from each sample for extraction and purification of argon by the step-heating method. Ecuador Studied 53º 129 Area Muñoz 52º Chamiza s Brazil Cans C Peru HIGH PRESSURE METAMORPHIC ROCKS FROM THE a B J AMBAL Ó obos 124 JAMBALÓ AREA 49º Cala mbaz 0 1km Orrego et al. (1980b) divided the metamorphic To Pitayó rocks of the Jambaló area in three units: 1) San Antonio Amphibolite, composed of metagabbro, metadiabase, FIGURE 2 Simplified geologic map of the Jambaló area.
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