Detrital Zircon Ages from the El Chilar Accretionary Complex and Volcaniclastic Rocks of the San Juan De La Rosa Formation, Toliman, Queretaro, Mexico

The Toliman region is formed mostly by calcareous rocks of the Cretaceous Mexican seaway of eastern Mexico. Underneath the limestone, there are two siliciclastic units: El Chilar Complex and San Juan de la Rosa Formation. The El Chilar Complex is constituted by highly disrupted turbidites (quartz-rich sandstone, shale and conglomerates). This succession shows a block in matrix tectonic structure. Most of the blocks are made of the quartz-rich sandstone, few small blocks of black chert and a large (500 m in diameter) block of chert and microgabbro. The matrix contains scarce fossil bivalves of unknown age and ichnofossils.

Major and trace element were determined from thirteen microgabbro samples. They have low silica (38 to 44%) and are enriched in TiO2 (4-2%) and MgO (9-6%). REE patterns are enriched in light rare earth elements (LREE) (LaN 115 to 219) and some samples have Eu anomaly. Their patterns are similar to those from OIB. Other elements, such as Hf, Ta, Th, Y, and Nb plot in the Within Plate Basalt fields of different petrotectonic diagrams. These suggest a within plate oceanic setting for the exotic block found within the El Chilar Complex. One sample from the quartz-rich sandstone matrix was analyzed for detrital zircon provenance, yielded ages that cluster around 300 Ma, 600 Ma, and 1 Ga. Such clusters are similar to those found in Triassic rocks of central Mexico. El Chilar Complex is recording subduction at the western margin of Oaxaquia, the Grenvillian basement of eastern Mexico.

The El Chilar complex is unconformably overlain by the San Juan de la Rosa Formation, formed by marine felsic volcanic and clastic rocks. These rocks were originally considered as part of the Middle Jurassic continental arc of central Mexico. However, new U-Pb detrital zircon ages form an Early Cretaceous cluster around 139 Ma.