Holocene tephrochronology record of large explosive eruptions in the southernmost Patagonian Andes

For regionally widespread Holocene tephra layers in southernmost Patagonia, correlations based on both chemical and chronological data indicate their derivation from five large-volume (> 1 km(3)) explosive eruptions of four different volcanoes in the southernmost Andes. Bulk-tephra and tephra-glass major and trace-element chemistry and Sr isotopic ratios unambiguously distinguish different source volcanoes, and imply that two of the regionally widespread tephra (MB1 and MB2) were derived from Mt. Burney (52S), one (R-1) from Reclus (51S), one (A(1)) from Aguilera (50S) and one (H-1) from Hudson volcano (46S). The H-1 tephra derived from the Hudson volcano, which is located at the southern end of the Andean Southern Volcanic Zone (SVZ; 33-46S), contains distinctive greenish andesitic glass with FeO > 4.5 wt.% and TiO2 > 1.2 wt.%. In contrast, rhyolitic glass in tephra derived from the eruptions of Mt. Burney, Reclus and Aguilera volcanoes, which are located in the Andean Austral Volcanic Zone (AVZ; 49-55S), is clear and transparent and has significantly lower FeO and TiO2. Tephra derived from these three AVZ volcanoes all contain plagioclase, orthopyroxene, minor clinopyroxene and amphibole. Biotite occurs only in the Aguilera A(1) tephra, which also has the highest bulk-tephra and tephra-glass K2O and Rb contents. Averages of new and published C-14 ages determined on organic material in soil and sediment samples above and below these tephra constrain the uncalibrated C-14 age of the R-1 eruption of Reclus volcano to 12,685=260 years BP, the MB1 and MB2 eruptions of Mt. Burney to 8,425=500 and 3,830=390 years BP, the Hudson H-1 eruption to 6,850=160 years BP, and the A(1) eruption of Aguilera volcano to 3,000=100 years BP. The volume of the largest of these eruptions, H-1 of the Hudson volcano, is estimated as > 18 km(3). The volume of the Reclus R-1 eruption is estimated at > 10 km(3), the Aguilera A(1) eruption at between 4 and 9 km(3), and the younger Mt. Burney MB2 eruption at eN2.8 km3. The volume of the older MB1 Mt. Burney eruption is the least well constrained, but must have been larger than the younger MB2 eruption. The data indicate that the frequency of explosive activity of volcanic centers in the AVZ is lower than in the southern SVZ.