| Geology, tectonic setting, paleomagnetic data, and copper mineralization of the mid-to-Late Jurassic continental La Quinta Fm. in the Sierra de Perija of NW Venezuela are described. It consists of red clastic sedimentary rocks, interbedded basaltic-andesite and hornblende-andesite flows, and felsic volcaniclastics that accumulated in a series of grabens. Granitic plutons of Jurassic age crop out in the area. Alkalic basalt dikes cut the entire sequence.;Native copper occurs in amygdular mafic flow tops, copper sulfide in arkosic sedimentary rocks with and without associated hydrocarbons, and bornite replaces pyrite in a quartz latite with associated Ag-Te veins. Mineral assemblages, fluid inclusions, zircon fission-track ages, and S isotopes constrain temperature of mineralization to between 100 and 300(DEGREES)C. Hydrothermal solutions "buffered" by redbeds leached copper from sedimentary and volcanic rocks, and deposited it in reducing and/or S-rich environments provided by mafic rocks, pyrite, or organics.;Paleomagnetic data (150 samples, 32 sites) of dikes, redbeds, and mafic flows yield separate vectors for dikes relative to redbeds and mafic flows. Dike vectors are northerly (comparable to South American craton data), whereas the other vectors are northwesterly, suggesting counterclockwise rotation of the Perija block during the Late Jurassic. A reconstruction based on paleomagnetic data and published reconstructions of Noth America-South America-Africa places the Perija block along the southern margin of Mexico, rather than in the Gulf of Mexico. This implies that the Perija block was affected by subduction, rifting, and transcurrent motion during separation of South America from North America and emplacement of the block in its present position.;Whole-rock geochemistry includes major elements and Cr, Rb, Sr, Zr, Cu, Zn, Ni, Pb, Y, Nb, Ba. Variation diagrams reveal a silica-enrichment trend (calcalkaline volcanics) and a silica-depletion trend (alkalic dikes), suggesting that dikes and volcanic rocks are unrelated chemically. Electron microprobe analyses (EDA) of phenocrysts indicate that amphibole, feldspar, cpx, and magnetite controlled the calcalkaline trend, whereas cpx fractionation controlled the dike trend. Whole-rock Sr-iso-tope ratios, zircon U-Pb data, garnets, and chemistry indicate an origin by anatexis and/or contamination of an upper mantle melt. |
