Paper No. 9
Presentation Time: 3:30 PM
Geochemistry of Volcanic Rocks of the Chivillas Formation: Evidence of Jurassic-Cretaceous Alkaline OIB Volcanism in Eastern Mexico
The Chivillas Formation is exposed in a key region for understanding the tectonic evolution of southern Mexico, because it is located at the convergence of the Zapoteco and Cuicateco Terranes, to the northeast of Tehuacán, Puebla. It is the most eastern exposure of Mesozoic marine volcanism in Mexico. This formation was interpreted as part of an aulacogen by Carfantan (1981) and even as part of arc by Alzaga and Pano (1987). Thus their geochemical characterization was lacking. The Chivillas Formation consists of thick packages of pillow lavas interbedded with siliciclastic turbidites. The last containing clasts derived from metamorphic, sedimentary and volcanic sources. It contains microfossils of Late Jurassic - Early Cretaceous age. Peperites are also found. Basement of the Chivillas Formation is unknown but it is inferred to be Proterozoic-Paleozoic metamorphic rocks, since grenville-age metamorphic rocks are exposed 30 km to the southwest and Paleozoic granitoids have been cut by drilling eastward from the Chivillas Formation. Fourteen samples were analysed of spilite and basalt from five stratigraphic levels.
Lavas are mafic to intermediate in composition, with SiO2 (40.9-52.8%); trace element composition indicate that the lavas are alkaline basalts, basalts and andesites. Chondrite-normalized REE patterns show enrichment in REE, and a slight europium anomaly. Diagrams show a slight enrichment in LREE. MORB-normalized trace element profiles show enrichment in incompatible elements Rb, Sr and a slight negative Nb anomaly . The eNd values range from -0.16-2.98.
Our results indicate that the lavas of the Chivillas Formation were not formed in an arc environment. All samples except one fall in the field of E-MORB in the Th-Hf-Ta, Cr/Y, Y/V, TiO2-Y/Nb and Th/Yb-Ta/Yb tectonomagmatic discrimination diagrams. These results suggest that the alkaline lavas of the Chivillas Formation were originated in a seamount developed within a rift basin , in a continental intraplate alkaline setting.