Metamorphic Geology of the San Lucas del Maíz Area, State of Mexico

Abstract

This study is based on detailed geological mapping of a 32 km area of the Tierra Caliente Complex, exposing the deepest structural levels of the region. The area is located 120 km SSW of Mexico City and 6 km north of Tejupilco, State of Mexico.
The studied metamorphic sequence is approximately 2,000 meters thick and consists as follows: the lower part consists of sericite schist with abundant graphite, with intercalations of actinolite/tremolite schist and cordierite schist; the middle part is composed of green schist and chlorite-biotite schist, quartz-sericite schists, and sericite intercalated schists; the upper part is represented by metarhyolite, sericite schist with scarce graphite, metacalcareous schist, and calcite sericite schist.
The sequence is intruded by Tertiary igneous bodies, including an adamelite stock, a diorite stock, and aplite dikes, and is discordantly overlain by Pliocene and Quaternary andesitic-basaltic lava flows. The apparent basement of the sequence is exposed in the Tizapa area, 3 km NNW of the studied area, and consists of a mylonitic augengneiss originally adamelitic.
Structurally, three phases of deformation were recognized in the sequence: the first, likely the most intense, is characterized by isoclinal reclined folds with penetrating axial foliation (Si) and mylonitization in the more competent intercalations; the second displays angular folds with non-penetrating axial foliation (S2); and the third has rounded folds with no associated foliation, seemingly related to a doubly plunging anticline about 8 km long and 3 km wide, located between Tejupilco and Zacazonapan.
Four phases of metamorphism affected the sequence. The first two were contemporaneous with Si and S2 and are restricted to the green schist facies; the third is post-S2, developed under static conditions, and is characterized by typical associations of the lower amphibolite facies, such as cordierite-phlogopite-clinoclore in pelitic schist and clinopyroxene-tremolite in basic schist. The fourth phase was retrogressive.
A paleogeothermal gradient of approximately 70-90°C/km was estimated based on the metamorphic zoning established and structural relief measurements. This gradient is characteristic of low-pressure, high-temperature metamorphic terrains and suggests that the Tierra Caliente Complex evolved under the tectonothermal conditions of an island arc.
The chronology of these events is not well understood due to the scarcity of paleontological and isotopic data, though a tectonothermal evolution, likely beginning in the Late Paleozoic and continuing into the Cretaceous, has been proposed. The discovery of the augengneiss as the apparent basement of the Tierra Caliente Complex and the recognition of its complex tectonothermal evolution are crucial for making correlations with other metamorphic sequences in southwestern Mexico.