Timing the evolution of a monogenetic volcanic field: Sierra Chichinautzin, Central Mexico

Abstract

The unique nature of monogenetic volcanism has always raised questions about its origin, longevity and spatial distribution. Detailed temporal and spatial boundaries resulted from a morphometric study, mapping, relative dating, twenty-four new 40Ar/39Ar dates, and chemical analyses for the Sierra Chichinautzin, Central Mexico. Based on these results the monogenetic cones were divided into four groups: (1) Peñón Monogenetic Volcanic Group (PMVG); (2) Older Chichinautzin Monogenetic Volcanic Group (Older CMVG); (3) Younger Chichinautzin Monogenetic Volcanic Group (Younger CMVG) and (4) Sierra Santa Catarina Monogenetic Volcanic Group (SSC).The PMVG cover the largest area and marks the northern and southern boundaries of this field. The oldest monogenetic volcanism (PMVG; 1294±36 to 765±30ka) started in the northern part of the area and the last eruption of this group occurred in the south. These basaltic-andesite cones are widely spaced and are aligned NE-SW (N60°E). After this activity, monogenetic volcanism stopped for ~527ka.Monogenetic volcanism was reactivated with the birth of the Tezoyuca 1 Volcano, marking the beginning of the second volcanic group (Older CMVG; 238±51 to 95±12ka) in the southern part of the area. These andesitic to basaltic andesite cones plot into two groups, one with high MgO and Nb, and the other with low MgO and Nb, suggesting diverse magma sources. The eruption of the Older CMVG ended with the eruption of Malacatepec volcano and then monogenetic volcanism stopped again for ~60ka.At ~35ka, monogenetic volcanism started again, this time in the eastern part of the area, close to Popocatépetl volcano, forming the Younger CMVG (<35±4ka). These cones are aligned in an E-W direction. Geochemical composition of eruptive products of measured samples varies from basalts to dacites with low and high MgO. The Younger CMVG is considered still active since the last eruptions took place <2ka.The SSC (132±70 to 2±56ka) is located in the northern part of the area, in the old Chalco Lake and is separated by faults from the rest of the volcanic groups as a different range. The SSC formed closely spaced basaltic andesites to andesitic cones oriented NE-SW (N70°E). The SSC samples have high Zr, P2O5, and Nb, indicating a different magma source.The northern and southern spatial boundaries of the field (the surface area with monogenetic volcanoes) became smaller with time: 78km for PMVG, ~40km for the Older CMVG and ~25km for the Younger CMVG, concentrating the volcanoes in the central part of the area. The alignment of the cones changed progressively from NNE-SSW to NE-SW to E-W through the time, associated with the changes in the stress field which appears also to have caused the gaps.Results suggest that the Sierra Chichinautzin is actually four different volcanic fields, some partially overlapping, instead of one as previously considered. The differences in age, emplacement orientation and geochemistry support this conclusion.