The U-Pb and Hf isotope evidence of detrital zircons of the Ordovician Ollantaytambo Formation, southern Peru, and the Ordovician provenance and paleogeography of southern Peru and northern Bolivia

摘要

The Ordovician Ollantaytambo Formation represents one of only two known occurrences of Lower Paleozoic volcanic rocks in southern Peru and northern Bolivia. Its lower part consists of mafic lapilli tuffs, shales and mature sandstones form the upper part. We present LA-ICP-MS U-Pb ages and Hf isotope data of detrital zircons from one of the upper member sandstones in order to determine both the duration of volcanism and the provenance of the mature detritus, and to use the data to further define the paleogeography of the Ordovician basin in the northern Central Andes. The detrital zircon ages of the Ollantaytambo Formation range from 2013 Ma to 445 Ma. They are grouped mainly between 1400 and 1100 Ma (35%), 1100 and 900 Ma (14%), 770 and 650 Ma (14%), and from 500 Ma to 440 Ma (30%). Within these groups the main peaks are at 1249 Ma, 1052 Ma, 741 Ma and 459 Ma. The older groups correspond to major orogenic cycles recorded on the southwestern Amazonia craton, the Rondonia-San Ignacio, Sunsas, and Brasiliano orogenies. The younger one reflects the activity of the Early Paleozoic Famatinian magmatic arc known mainly from the southern Central Andes, but also recognized on the Arequipa Massif and in northern Peru. The provenance of the grains with ages between 770 Ma and 650 Ma is enigmatic as there are no known sources in southwestern Amazonia or the Central Andes. The εHf(t) values of selected Ollantaytambo Formation zircons are between -22 and +3 and considered to be moderately juvenile to evolved. Truely juvenile zircons with a composition similar to the depleted mantle were not identified. Together with additional literature data from Ordovician formations in southern Peru, the Hf-isotope data indicate production of juvenile crust mainly in the Mesoproterozoic, and increasing recycling of this crust during the Neoproterozoic and Early Paleozoic orogenic events.