PERMO-TRIASSIC LEUCORHYOLITIC IGNIMBRITES AT SIERRA DE LIHUE CALEL, LA PAMPA PROVINCE, ARGENTINA

The Lihue-Calel ignimbritic plateau in central Argentina represents the easternmost extension of the Permo-Triassic Choiyoi rhyolitic province. The rocks of this pampean plateau are chiefly pyroclastic, with minor intrusives and lavas. Rhyolites are a high-silica, high-potassium, slightly peraluminous type depleted in compatible elements (Sr, Ba, Eu) and variably enriched in incompatible elements (Rb, Cs, U, Th, Y, Nb, Ta). REE patterns are flat (La/Yb = 0-10), showing conspicuous negative Eu anomalies. The geochemistry of the ignimbrites reflects two processes: (1) fractional crystallization leading to a very high degree of differentiation, and (2) the activity of a residual F-enriched fluid phase during the last cooling stages. In this respect, they resemble the topaz rhyolites of the western United States. In both cases, the silicic magmas were erupted in an extensional tectonic setting, subsequent to a previous compressive episode of crustal thickening and uplifting. Thus, the silicic rocks of Lihue-Calel exhibit particular geochemical features that distinguish them from subduction-related calc-alkaline types and from the peralkaline rocks of anorogenic intraplate regions.