Abstract

ROLANDO, A.P. et al. SHRIMP U-Pb zircon dates from igneous rocks from the Fontana Lake region, Patagonia: Implications for the age of magmatism, Mesozoic geological evolution and age of basement. Rev. Asoc. Geol. Argent. [online]. 2004, vol.59, n.4, pp.671-684. ISSN 0004-4822.

In the eastern margin of the Patagonian Andes and between 44° 30´S and 45° 30´S (Fontana Lake region), Middle Jurassic to Early Cretaceous volcanic and sedimentary rocks were intruded by granitic bodies during the Cretaceous. The reconstruction of the Jurassic-Cretaceous magmatic evolution in the Fontana Lake region and in the adjacent Patagonian Batholith was made possible by the consideration of the following characteristics: distribution in time and space of several intrusive bodies, retro-arc basin formation and volcanic intensity. U-Pb SHRIMP dating of zircon crystals from an ignimbrite, a dacitic porphyry and two granitoid rocks yielded dates of 148.7 ± 2.3, 144.5 ± 1.6, 117 ± 1.7 and 99.6 ± 2.8 Ma, respectively. The Cerro Bayo Ignimbrite (148.7 ± 2.3 Ma, Late Jurassic) was included in the Lago La Plata Formation; this unit hosts an epithermal ore deposit. The Laguna Escondida dacitic porphyry (144.5 ± 1.6 Ma, Jurassic-Cretaceous boundary) intruded metasedimentary rocks of the Lago La Plata Formation; this sub-volcanic body can chronologically be linked to the Patagonian Batholith. After the Jurassic volcanic events, a retro-arc basin formed in the eastern sector of the Patagonian Range at about 140-115 Ma (Late Berriasian-Barremian) and magmatism ceased during this event. The dating of granitoids (117 ± 1.7 and 99.6 ± 2.8 Ma) in the Fontana Lake region confirms a temporal magmatic continuity with the Patagonian Batholith. These dates also are in agreement with the volcanic rocks of the Divisadero Group and epithermal deposits in the region (La Ferrocarrilera deposit). One of the analyzed granitoids (Dedo Chico, 99.6 ± 2.8 Ma) has inherited zircon crystals of about 2,100 and 3,410 Ma, in agreement with other previous isotopic evidence for the occurrence of an underlying Precambrian basement in the region.

Keywords : Zircon; Magmatism; Lago Fontana; Cordillera Patagónica.

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