PLEISTOCENE SILICIC FLARE-UPS IN THE NORTHERN ANDES OF ECUADOR

The Northern Andes Volcanic Arc is characterized by Pliocene to Present andesitic to dacitic magmatism. Nested within the row of active andesitic stratovolcanoes of Ecuador’s Eastern Cordillera, is a small rhyolitic province (here called the Eastern Rhyolitic Province - ERP) comprised of several rhyolitic and dacitic caldera complexes no older than 3 Ma. The older Chacana complex extends more than 50 km N-S and is comprised of episodic ignimbrites and lava flows of dacitic, rhyolitic, and andesitic composition whose total outflow volume is about 800 km3. Back-arc volcanism immediately east of the ERP has erupted basic alkaline magmas in recent times.

The ERP’s most recent flare-up was about 200 ka which started with a voluminous 100 km3 rhyolitic pumice flow at the 20 km-wide Chalupas Caldera, south of Chacana. It was associated in time with the 11 km3 Cuyuja andesitic lava flow from Antisana volcano, located between the two calderas. At approx. 175 ka the northern sector of Chacana witnessed rhyolitic lava flows and extensive pumice lapilli falls whose combined volume is close to 100 km3. The Chalupas eruption was followed by dome growth while the Chacana event was a single eruption. Recent studies have identified four other small rhyolitic centers, 40 to 5 ka in age, east of Chalupas. Much earlier, Cotopaxi volcano adjacent to the Chalupas caldera had had extensive rhyolitic activity dated at approx. 450-500 ka. Elsewhere in the Chacana complex, post flare–up magmatism has been modest, decreasing in volume, and mainly andesitic.

The spatial and temporal coincidence of the ERP with an underlying 250 mgal negative gravity anomaly, a conjectured 50-70 km-thick crust, and the alkaline magmatic province directly to the east, which are all aligned with the location and direction of the subducting Carnegie Ridge under central Ecuador, thought to have begun about 5 Ma ago, suggests a cause-and-effect relationship. Quite to the contrary, the subduction of aseismic oceanic ridges is generally thought to result in the cessation of magmatism, not silicic flare-ups.