LAUSR

Abstract The Ramadas Volcanic Center on the eastern margin of the central Andean Puna plateau along the Olacapato-El Toro lineament in Argentina erupted a rare strongly peraluminous Mn-rich garnet-bearing rhyolitic tuff in the late Miocene. The voluminous ashes from this eruption, which are distinctive in having euhedral spessartine almandine garnets (Alm 70–72 Sps 22–26 Grs 2–4 Prp 0.5–1 ) as their only phenocryst, are widely dispersed in the Andean foreland. Among these tuffs are those in the Guanaco Formation foreland basin sediments along the Xibi-Xibi and Los Alisos rivers in the Rio Grande de Jujuy basin and the Metan Valley, some 100–200 km east of the Ramada Volcanic Center. The co-occurrence of tubular to cellular pumice fragments and blocky glass shards in an ash matrix in these tuffs is interpreted as indicating that they erupted in an initial vent-opening event with pulsating pyroclastic surges at the initiation of the strong Plinian eruption of the Ramada Volcanic Center. New Ar/Ar ages from the Guanaco Fm. glass shards agree with fossil ages in placing the eruption at ~6.3 ± 0.3 Ma. A number of distinctive chemical, isotopic and mineralogical features including Mg-rich biotite and Mg-hastingsite xenocrysts of the Guanaco Formation and Ramadas Volcanic Center tuffs are consistent with the melt having been derived by extensive crystallization of a mantle-derived mafic shoshonitic series magma contaminated by assimilation/dehydration melts of metapelitic sediment and the Puna crust. Distinctive chemical features include whole rock SiO 2 contents of ~75–76% wt%; A/CNK indices >1.2; low Ca, Mg, Ti, and Fe concentrations; steep REE patterns with extreme negative Eu anomalies; low Ba, Sr, LREE and high Cs, Rb, U concentrations; and recalculated initial ratios of 87 Sr/ 86 Sr at ~0.7119 and 143 Nd/ 144 Nd of ~0.5123 at 6.3 Ma. The erupted magma has a transitional chemical character between those of the ~11 Ma Mn-rich garnet-bearing Coyaguayma ignimbrites to the north and the ~6 Ma Cerro Galan ignimbrites to the south. Unlike these crystal-rich ignimbrites, the Ramadas tuff records the extraction of an extensively fractioned melt from a plagioclase, K-feldspar, quartz and biotite-bearing mush with accessory titanomagnetite and apatite. In line with existing experimental studies on Mn-rich garnets and comparisons with the Coyaguayma ignimbrite, pre-eruption crystallization of the rhyolite segregated from the mush likely occurred at ~800° to 720 °C at a depth of no 2 O content increased from ~4–5% to ~7.5%. Mn-rich garnet was the only phase to be crystallized in the melt extracted from the mush before the eruption, whose rapid rise was facilitated by extension along the Olacapato-El Toro lineament.