The Marifil Volcanic Complex at Arroyo Ventana: evidence of long-lived volcanic episode in the North Patagonian Massif

One of the most voluminous episodes of continental volcanism on Earth during the Phanerozoic, corresponds to the widespread Jurassic magmatism developed in Patagonia, South Africa, Antarctica, India, Australia and New Zealand, associated with the Gondwana Supercontinent break-up.

From the Antarctic Peninsula to North Patagonia, this volcanism is known as the Chon Aike volcanic Province with ages from 186 to 155 Ma approx. Apparently it was concentrated in three discrete stages of activity, according with Pankhurst et al., (2000): V1 (188–178 Ma), mainly represented in the North Patagonian Massif, V2 (172–162 Ma) in the Deseado Massif, and V3 (157–152 Ma) in the Patagonian Cordillera (Santa Cruz province).

Recent contribution by Pavón Pivetta et al., (2019) recognizer the V0 stage of volcanism (195–190 Ma) for the east part of the North Patagonian Region.

The outcrops of the Arroyo Ventana area (66°21´53´´W - 41°35´56´´S), comprise four facies association representated by efussive porphyritic lavas of rhyolitic composition, efussive trachitic lavas related to subaerial dacitic domes, epiclastic deposits and explosive widespread high-welded tuff of rhyolitic composition, that includes deformed fragments of rhyolites and dacites lavas. According to Strazzere and Gregori (2019), the SiO₂ content varies from 65 to 75 wt.%. In discrimination diagrams samples fall in the rhyolite and rhyodacite fields.

Rhyolitic and trachytic lavas are enriched in LILE and show light negative anomalies of Sr, P y Ti. Samples from the epiclastic deposits and high-welded tuff show strong Sr, P y Ti anomalies. The REE content show that lavas are enriched in LREE without negative Eu anomaly absent. Differently, the high-welded tuff shows strong negative Eu anomaly. This point to an older event with a subduction-type chemical signature, and a younger with intraplate-magmatism chemical signature.

In order to improve the age of the Marifil Volcanic Complex in this area, rocks from the high-welded tuff were dated using LA-ICP-MS U-Pb in zircon at the Multilab Laboratory, Universidade do Estado do Río de Janeiro, Brazil. To obtain a precise U-Pb age, only zircons that fall in a Wetherill Concordia curve were used.

As a result, three groups were identified: the older one, composed by 10 zircons yielded an age of 183.7±2.5 Ma, with a MSDW of 0.0034. The second group, formed by 8 zircons yielded an age of 171.9±3.3 Ma, with a MSDW of 0.18. The last group include 7 zircons with an age of 161.2±3.9 Ma and a MSDW of 0.00016. This age is considered as the crystallization and older ones are considered as inherited zircons from the rhyolitic lavas and dacitic domes.

When compared with the data of Pankhurst et al., (2000) and Pavón Pivetta et al., (2019), the V0 event seems to be not present in this area. V1 is represented by the older age, also recorgized in Arroyo Verde and Sierra de Pailemán areas, 100 km east. According the first authors this event is typical of the North Patagonian Region. The V2 event, mainly developed in the Deseado Massif, is represented here by the age of 171.9±3.3 Ma. This age shows the volcanism continued, without mayor changes by more than 10 Ma. The younger V3 volcanic event is widespread in the Patagonian Cordillera and is related to a volcanic arc, at the edge of the continent. Our younger age, 161.2±3.9 Ma, is related to a change in the tectonic setting, from subduction-related to intraplate-related magmatism.

Our data indicate that in the Arroyo Ventana area, and in the eastern North Patagonian Region, the Marifil Volcanic Complex lasted for more than 30 Ma, consistent with a long-lived volcanic episode, that recorded the three volcanic events indicated by Pankhurst et al., (2000) in separated areas of Patagonia.