Extensive Eocena felsic volcanism in the Cordilleran Series of northern Patagonia

MAZZONI, M.; RAPELA, C.W.

Denver

Congreso; Geological Society of America, Centennial Meeting; 1988

Geological Society of America

The Huitrera formation is the most voluminous and widespread silicic unit of the Paleocene-Eocene Pilcaniyeu Volcanic Belt in north-central Patagonia. These silcic volcanic have been erupted from large calderas during a period of oblique subduction along the Pacific margin. Well-studied sections include the type section at Cerro Huitrera, 10 km. east of Rio Chico (41º 43´S. 70º 21´W) and nearby Cerro Mesa. These sections define a sequence of primary and reworked pyroclastics overlain by lavas. The uppermost part is a thick lava flow, grading from a perlitic obsidian at the base to a felsic foliated flow with ramp structures at the top. Chemical analyses show that the lavas are a homogeneous suite of transitional cale-alkaline rhyolites and dacties (68-73% (most 72-73%)SiO2). Slightly higher SiO2 and K2O contents and Rb/Sr ratios and lower CaO and MgO contents in the Cerro Mesa lavas compared to the Cerro Huitrera obsidians are interpreted to result from separate magmatic pulses from a caldera, which in earlier eruptions produced the underlying pyroclastic rocks. Trace element patterns for five samples from different parts of the sequences are similar (La = 44-52 ppm, La/Ta = 23-25; Ba/La = 15 – 16, La/Yb = 16-19, Eu/Eu* = 0.48 – 0.54). Compared to Plio-Pleistocene to recent arc dacites from Puyehue Volcano, these lavas have a more subdued are signature as indicated by their lower Ba/La and La/Ta ratios and higher trace element content. These geochemical characteristics may be inherited from a mantle that has only been slightly modified by a subduction component consistent with a back-are setting. New K-Ar whole rock ages (45.9+- 2.3 and 20.9 +- 2.5 Ma.) agree with previous ages from the Huitrera formation. Similar ages (50.5 +- 1.8 and 50.9 +- Ma) from equivalent rhyolites in the Rio Chubut complex, 100 km to the SSE of Rio Chico, suggest the same origin for these extensive felsic lavas and dykes.