U-Pb geochronology of rutile: deciphering the cooling history of the Oaxacan Complex granulites, southern Mexico

Abstract

Rutile (TiO₂) is a heavy mineral, commonly found as accessory in many lithologies, such as basic igneous rocks, high-grade metamorphic units, as well as a detritus in sedimentary clastic rocks. Its chemical composition is sensitive to the crystallization environment, allowing a characterization of either metabasic or metasedimentary protoliths in metamorphic rocks. Thanks to the capability to accept U in its crystalline network, at least in metasedimentary, high-grade protoliths, rutile can be dated by U-Pb geochronology. Furthermore, its closure temperature of ca. 600 °C for the U-Pb system makes rutile a suitable chronometer, complementary to zircon, to unravel provenance and exhumation paths in both sedimentary siliciclastic cover and basement units. Besides, the Zr-in thermometer allows for a very precise calculation of the rutile crystallization temperature.

In the example case presented here, focused on granulite facies units of the Grenvillian Oaxacan Complex (OC), rutile crystallisation took place in the range 808–873 °C. Data for different localities indicate that cooling and exhumation after the Zapotecan granulite facies event (ca. 990 Ma) was heterogeneous among the different tectonic slices that constitute the OC. Cooling occurred in the central sector (Nochixtlán-Oaxaca) right after the granulite peak, with fast cooling rates of ca. 40 °C/Ma. To the north and south, the cooling to ca. 600 °C was much slower, with calculated cooling rates of ca. 3 °C/Ma for the northern OC outcrops in Coatepec (Puebla) to ca. 6 °C/Ma south of Ejutla (Oaxaca). This can be related to a combination of factors, such as an early collapse of some sectors of the orogen, a change of conditions in the subducing plate, or more in general, to a sudden change in the geodynamic conditions during the Zapotecan orogeny and Amazonia-Baltica amalgamation.

This application example to some metasedimentary lithologies belonging to the OC demonstrates how the exhumation after the Zapotecan granulite facies event (ca. 990 Ma) was heterogeneous among the different tectonic slices that compose the OC, having occurred in the central sector (Nochixtlán-Oaxaca) right after the granulite peak, with fast cooling rates of ca. 40 ºC/M.y., whereas to the North and South the cooling to ca. 600 ºC was much slower, with calculated cooling rates of ca. 3 ºC/M.y. (north, OC outcrops in Coatepec, Puebla) to ca. 5.5 ºC/M.y. south of Ejutla (Oaxaca). This can be related to a combination of factors, such as an early collapse of some sectors of the orogen, change of conditions in the subjecting plate, or more in general, to a sudden change in the geodynamic conditions during the early stages of the Rodinia amalgamation.

This example sharply illustrates the advantage of employing microanalytical techniques, able to resolve restricted crystal-domain chemical variations, to obtain accurate and precise temperature and age values. Furthermore, it is paramount to combine several mineral species with different closure temperatures, and collected in well-defined, recognized tectonic slices, to understand their behavior and construct meaningful cooling curves through geologic time, capable to better characterize and interpret their tectonic evolution.