Characterization and tectonic environment of the mafic rocks of the El Chilar Complex: evidence for a pre-Late Jurassic accretion prism in central Mexico

The El Chilar Complex, located in Queretaro State, is a strongly deformed sedimentary unit that forms a tectonic melange. This complex contains autochthonous blocks of sandstone or sandstone interbedded with shale, as well as exotic blocks of radiolarian chert. It is pre-Jurassic in age, and contains sandstone or sandstone with interbedded shale autocthonous blocks as well as chert exotic blocks. The largest of the exotic blocks is 1.5 km. long, and crops out in the town of El Terrero. This chert block is intruded by microgabbro dikes, from which 13 samples were analyzed. Petrography, XRF, ICPMS, XRD and microprobe analyses were done in order to characterize the tectonic setting of the mafic rocks. Whole rock major and trace element analysis, including rare earth element analysis, allowed us to characterize two groups of samples. Group I comprises microgabbro with abundant clinopyroxene and albite and scarce mica. Clinopyroxene shows rims of amphibole, and albite is partially replaced by hyalophane. Group I is enriched in Ba (9000 ppm) which is concentrated in micas and hyalophane. Group II is formed by Na feldspar-rich microgabbro with few relicts of clinopyroxene. All samples show geochemical signatures of typical enriched mantle source (oceanic island basalts), similar to those reported from recent oceanic island basalts on the Pacific Ocean floor. Microgabbro dikes described in this paper have geochemical signatures similar to those of mafic rocks that form exotic blocks within an accretionary prism in northern Zacatecas State. This and other similarities suggest that both localities belong to the same accretionary prism of regional extent. Our results show that the exotic block of El Terrero contains dikes of geochemical affinity, We consider that shallow subduction fluids altered the microgabbro dikes during accretion, originating large ion lithophile elements enrichment (mainly Ba and K).