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This Special Paper describes the geology and evolution of Iztaccíhuatl (5,286 m), a major Quaternary volcano in the central part of the Trans-Mexican Volcanic Belt, and volcanic rocks, tephra, and epiclastic deposits in adjacent areas of the Sierra Nevada and Valley of Mexico.

The eruptive products of Iztaccíhuatl (450 km3) are calc-alkaline andesites and dacites (58 to 66 wt percent SiO2) comprising viscous flows and flow breccias with minor intercalated pyroclastic material. The principal vents are aligned north-northwest to south-southeast and probably reflect control by basement lineaments. New K-Ar dates for Iztaccíhuatl indicate that cone construction began prior to 0.9 Ma and likely continued until late Pleistocene (Wisconsin) glaciation when eruptive activity ceased.

The evolution of Iztaccíhuatl is divided into two main phases of cone growth represented by rocks of the Older Volcanic Series (>0.6 Ma) and Younger Volcanic Series (<0.6 Ma). The Older Volcanic Series (300 km3) is characterized by augite-hypersthene-phyric lavas and pyroclastic breccias (Older Andesites and Dacites) that form the central edifice of Llano Grande (220 km3), a lava shield crowned by a 4.5-km-diameter caldera, and the cone of Ancestral Pies to the south. Parasitic activity on the northern flank of Llano Grande (Older Flank Activity) produced the hornblende-bearing andesitic to rhyolitic lavas of La Trampa (9 km3) and the Tlacupaso rhyodacite flow (0.3 km3).

The Younger Volcanic Series is predominantly composed of hornblende-phyric rocks (Younger Andesites and Dacites) that are differentiated into lavas and pyroclastic breccias (125 km3) extruded from vents in the summit region of Iztaccíhuatl (Summit Series), and similar rock types farther south (Pies) that buried most of the Ancestral Pies structure. Two plug domes, El Solitario and Los Yautepemes, were emplaced northwest of the summit, and eruptions at Pies culminated with a crater-forming event. Parasitic activity (Younger Flank Activity) on the southern flank of Iztaccíhuatl at 0.27 Ma produced La Joya basaltic to andesitic lavas and scoriae (>0.15 km3), and the northern flank was covered by viscous Teyotl dacite flows (5 km3) at approximately 0.08 Ma.

Other volcanic rocks examined in the Sierra Nevada include preglacial basaltic andesites of Iztaltetlac volcano, postglacial (<0.012 Ma) Papayo dacites (21 km3), Rio Frio pumice deposit, and Buenavista dacite flow (0.2 km3). Andesitic to dacitic monogenetic cones of the Chichináutzin Group in the Valley of Mexico occur near Amecameca in the southwestern corner of the map area.

The sequences of glacial moraines established by White (1962) for the west side of Iztaccíhuatl have been mapped to the east and correlated with radiocarbon-dated moraines of La Malinche (Heine, 1973). At high altitudes in the Sierra Nevada, airfall-ash and -pumice from Volcán Popocatépetl are interbedded with epiclastic detritus, and thick loess and volcanic breccias mantle the lower slopes.

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