The geology, petrology, and geo-archaeology of Sierra Las Navajas, Hidalgo, Mexico
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Description
Sierra Las Navajas (20°16'N, 98°3 'W), an extinct Plio-Pleistocene volcano located on the northern edge of the eastern Mexican Volcanic Belt (MVB), consists of peralkaline comendite lavas interbedded with pyroclastic flow and fall deposits. The volcanic history of Sierra Las Navajas can be divided into four major flow complexes, each of which is composed of one or more lava flows that tapped the same sector of a stratified magma chamber. One of these units bears green obsidian that has been a major source of obsidian to Mesoamerican societies for more than 3,000 years, and has been the subject of focused archaeological study the implications of which are addressed herein. Peralkaline trachytes of Cerro Gordo to the west and mildly alkaline basalts that emanated from numerous cinder cones along the flanks of Las Navajas were erupted in association with the comendites of Sierra Las Navajas A series of pyroclastic eruptions late in the first stage of volcanism caused slope destabilization, which resulted in sector collapse of the northern flank. This collapse generated a catastrophic debris avalanche estimated at 32 +/- 9 km3 volume. The debris avalanche deposit that resulted was later dissected by downcutting of the Rio Grande Tulancingo to the north and is now well exposed in the walls of the canyon. 40Ar/ 39Ar dates of basalts immediately underlying (2.41 +/- 0.08 Ma) and overlying (1.87 +/- 0.03 Ma) the debris avalanche deposit constrict its age to Late Pliocene. Slightly less peralkaline rhyolite flows that subsequently filled the collapse amphitheater define the second major stage of volcanism Several models for magma petrogenesis were tested, including magma mixing, partial melting, and fractional crystallization. Based on major and trace element geochemical signatures, which partially or completely negate the other possibilities, fractional crystallization with volatile transfer appears to be the most viable model for rhyolite genesis at Sierra Las Navajas. Multiple liquid lines of descent are present, indicating a constantly changing fractionating assemblage. The peralkaline trachytes however are better explained by partial melting of a mildly alkalic basalt source. Peralkaline volcanism is limited to this area in the eastern MVB, where it appears to be related to back-arc extensional tectonic forces