Phreatomagmatism related to trachyte dome explosions

HALLER MJ; NULLO FE; RISSO C

Auckland

Conferencia; 4th International Maar Conference; 2012

Geoscience Society of New Zealand

An important trachyte volcanism developed on the periphery of the Somuncura basalt plateau In Patagonia, Argentina, during Miocene time. Two calderas and their volcanic products are the records of this explosive episode. Talagapa Chico caldera is 3.25 x 3.12 km, with a cavity volume of 2.1 km3. Talagapa Grande caldera is 4 x 5.4 km in size, with a cavity volume of ca. 5.4 km3. Field evidence shows that a basal trachyte lava flow is covered by a thick pyroclastic succession. An intercalated non consanguineous basalt flow yielded isotopic (K-Ar) ages from 19 to 20 Ma and helps to date the trachytic event. The pyroclasts are covered by a 15 m thick trachyte lava flow. A good exposure of tephra located half way between the two Talagapa calderas gives a good insight to understand the development of one of the several episodes of Talagapa complex eruptive history. The succession of light gray tephra layers 98 m thick can be divided into three rock units on the base of the dominant facies, bedding style and clast size. The presence of abundant bomb sags, accretionary lapilli, scour channels, undulatory bedding, and iron alteration of lapilli indicates the participation of water in the eruptive system. On the other hand, trachyte vitrophyre indicates a quickly cooled rock which may be consistent with a rapidly quenched magma plug in a water-rich environment. Such viscous lava would form a dome in the crater. The presence of common large trachyte vitrophyre bal-listic blocks and boulders in the upper unit of the pyroclastic succession is interpreted to result from a destructive explosion of the dome accompanied by emission of phreatomagmatically-fragmented material. The base ignimbrite could be formed by a low energy volcanic column. The intermediate unit was formed by base surges indicated by the scour channels and alternations of ash-fall deposits with wet and dry surges, as suggested by the accretionary lapilli, truncated contact surfaces and clast-supported levels. In the upper unit wet surges and ash-fall occurred during the deposition of the lower levels while hyperconcentrated flows took an important role during the deposition of the upper beds, as indicated by the well bedded fine-poor lapilli flat beds. The pyroclastic succession is capped by a non-consanguineous basalt lava. The fact that erupted products were dominantly trachyte throughout the eruption and the abundance and large size of trachyte vitrophyre ballistic block suggests that the pyroclastic deposits are related to dome explosions in the nearby trachyte calderas.