Geochronological systematics of the Huayna Potosí, Zongo and Taquesi plutons, Cordillera Real of Bolivia, by the K/Ar, Rb/Sr and U/Pb methods

Abstract

The Huayna Potosi, Zongo and Taquesi are Triassic plutons located at the core of the Real Cordillera of Bolivia. In this paper, several Rb-Sr and K-Ar ages obtained in the past at the São Paulo Geochronology Laboratory, yet unpublished, will be presented, along with newer U-Pb Sensitive High-Resolution Ion Microprobe (SHRIMP) determinations made in the same laboratory, allowing us to redefine the geologic history of this part of the Central Andes. Rb/Sr analyses of some low grade metapelitic country rocks of the early Paleozoic (Amutara and Cancañiri Formations) yielded a Rb-Sr isochron age of 344 ± 38 Ma, indicating the action of an early Gondwanide regional event. A five-point Rb-Sr isochron from a granite outcrop of the Huayna Potosi pluton yielded an age of 224 ± 28 Ma. In addition, an important Ar loss in micas was detected in the Zongo granitoids and their country rocks, recording a thermal event that opened this isotopic system in the Oligocene. Newer U-Pb SHRIMP zircon ages of ca. 221 Ma were obtained in two other granitic outcrops of the Huayna Potosi granite. They confirmed its Triassic crystallization age, and a similar U-Pb SHRIMP age of 221.9 ± 1.5 Ma was obtained for one sample of the Taquesi pluton. For the Zongo pluton, many of the zircon grains obtained from one sample of its Kuticucho facies yielded extremely high uranium content, which produced reverse discordant apparent ages. However, due to the fair alignment of the analytical points in the Concordia diagram, possibly corresponding to a linear correlation, we made a regression calculation and the interception of the Concordia curve resulted in a rather imprecise age of 220 ± 20 Ma. Our conclusion was that the final magmatic crystallization and the intrusion of plutons in the central part of the Cordillera Real of Bolivia have occurred close to 221.5 ± 2.0 Ma, in late Triassic times. Finally, the U-Pb SHRIMP ages obtained in inherited zircon xenocrysts from the four available granitic rocks yielded very different ages, and many of them are related to previous magmatic episodes of the Andean Tectonic System. A few other age measurements indicated sources related to much older Proterozoic magmatic events associated with rocks from the Andean basement.

Trace element composition and U-Pb ages of cassiterite from the Bolivian tin belt

“…Our U-Pb cassiterite date for Kellhuani (218.3 ± 5.5 Ma) agrees, within the limits of the analytical error, with the K/Ar muscovite date at 210 ± 6 Ma reported by McBride et al (1983) for the greisen at the Chacaltaya porphyry stock, which is genetically connected to the mineralization in the Milluni-Kellhuani district (Lehmann 1985). In addition, this date agrees with the U-Pb zircon concordia dates of 222.3 ± 2.4 and 220.8 ± 1.9 Ma reported by Cordani et al (2019) for two granite samples from the neighboring Huayna Potosí pluton.…”

“…The Bolivian tin belt is genetically connected to two major periods of magmatism related to the Andean orogeny: the first is linked to intrusions of granitic plutons of Triassic to Jurassic ages and the second is related to plutonic, sub-volcanic and volcanic eruptive complexes of Oligocene to Miocene ages (Turneaure 1971;Sillitoe et al 1975;Evernden et al 1977;Lancelot et al 1978;Grant et al 1979;Clark et al, 1983Clark et al, , 1990Clark et al, , 2000McBride et al 1983;Kontak et al 1987Kontak et al , 1990Mlynarczyk and Williams-Jones 2005;Maffione et al 2009;Mi kovi et al 2009;Reitsma 2012;Betkowski et al 2017;Cordani et al 2019). The Triassic granite intrusions were interpreted as typical of rift-related magmatism in a back-arc setting (McBride et al 1983;Kontak et al 1984Kontak et al , 1990Lehmann et al 1990;Miskovic et al 2009;Reitsma 2012).…”

“…The Milluni-Kellhuani Zn-Cu-W-Bi district is located in the northernmost part of the La Paz department and sits at the northern part of the Cordillera Real, ~ 5 km south of the Triassic Huayna-Potosí granite (Gillis et al 2006;Cordani et al 2019). Vein and stratabound mineralization are hosted in Silurian quartzites of the Catavi Formation.…”

Geochronology of the Cordillera Real granitoids, the inner magmatic arc of Bolivia

“…2). Some bear two micas (Huato, Illampu, Huayna Potosí and Zongo) and are metaluminous to peraluminous in composition (Ávila, 1990;Jiménez and López-Velásquez, 2008;Cordani et al, 2019). They commonly bear 0.1-0.5 m rounded mafic enclaves of quartz-diorite.…”

“…ranges varying from 217 to 273 Ma indicating the necessity of better constraint on the emplacement age of the granitoids. Cordani et al (2019) indicated the difficulty of obtaining reliable crystallization ages for the Huayna Potosí and Zongo plutons. Using new U-Pb zircon dates measured in the SHRIMP II laboratory of the University of São Paulo (USP), we set the following two objectives: a) to constrain the crystallization ages of the other granitic plutons of the Real Cordillera to put the crystallization age of this "inner magmatic arc" in a regional context within the Central Andes and b) to obtain additional robust geochronological data for the Huayna Potosí and Zongo plutons to understand better the evolution of their crystallization.…”