Systematic Variations in the Trace Element and Isotopic Compositions of High-K2O, Medium-K2O, and High-TiO2 Volcanic Rocks From the Zit cuaro-Valle de Bravo Region of Central Mexico

Within the Zit cuaro-Valle de Bravo region of the central Mexican Volcanic Belt, three series of lava have erupted during the Quaternary period. These lava series are defined based on their major and trace element compositions to be: 1.) Shoshonitic (high-K₂O), 2.) Medium potassium (med-K₂O) basaltic andesites through dacites, and 3.) High-TiO₂ basalts and basaltic andesites. The distribution of these distinct compositions is systematic; with the high-K₂O volcanism at the arc front, the med-K₂O volcanism concentrated along the central axis of the arc, and the high-TiO₂ volcanism at the middle to back portion of the arc. The high-K₂O compositions are extremely enriched in large ion lithophile elements (LILE's) and light rare earth elements (LREE's), with a narrow range of distinctly enriched ⁸⁷Sr/⁸⁶Sr values from 0.704245 to 0.704507 and ɛ _Nd values ranging from 4.27 to 5.63. The med-K₂O compositions are only moderately enriched in the LILE's and LREE's and have the largest range of ⁸⁷Sr/⁸⁶Sr values from 0.703227 to 0.704287 and ɛ _Nd values ranging from 3.39 to 6.03. In contrast, the high-TiO₂ compositions have no enrichments in LILE's or LREE's and instead are enriched in the high field strength elements (HFSE's). The high-TiO₂ lavas have ⁸⁷Sr/⁸⁶Sr values ranging from 0.703549 to 0.704095 and a narrow range of ɛ _Nd values from 2.98 to 3.47. The isotopic compositions of the Zit cuaro-Valle de Bravo lavas correlate with their major and trace element characteristics, as well as the spatial distribution of the lavas within the arc. The systematic spatial distribution of these distinct lava types is similar to that of the Quaternary lavas in the Michoacan-Guanajuato Volcanic Field, directly to the west. However, in distinct contrast, the Quaternary lavas in the Sierra Chichinautzin Volcanic Field, to the east, are not spatially distributed according to composition. Instead, their distribution appears to be predominantly controlled by several conspicuous faults.