Abstract Oligo-Miocene volcaniclastic sedimentation in northern New Mexico occurred before and during initiation of the Rio Grande rift. Reconstruction of dispersal patterns and paleogeography of extensive volcaniclastic aprons, using geochemical, paleocurrent, textural and petrofacies analyses, places important constraints on models for the tectonomagmatic initiation and evolution of the rift. K-Ar ages and chemical analyses of volcanic clasts from conglomerate provide details concerning three primary volcanic centers: 1) San Juan Mountains (29–27 Ma, high-K andesite and rhyodacite); 2) Latir volcanic field (28–25 Ma, high-K andesite and rhyolite); and 3) the previously unrecognized Servilleta Plaza center (23–22 Ma, Iatite, high-K andesite and rhyodacite), which may have been a southern extension of the Latir field. These three volcanic centers provided detritus to the following units, respectively: I) Esquibel Member of Los Pinos Formation, upper Abiquiu Formation, middle Picuris Formation, and Bishops Lodge Member of Tesuque Formation; 2) Cordito Member of Los Pinos Formation and uppermost Abiquiu Formation; and 3) Chama-EI Rito Member of Tesuque Formation and upper Picuris Formation. Correlation of these units is based on both conglomerate and sandstone petrofacies. Detailed point counting of volcanic lithic-grain varieties (vitric, granular, seriate, microlitic and lathwork) is especially useful in discriminating petrofacies. The Esquibel petrofacies has abundant plagioclase, seriate lithics and dense minerals, with low quartz. The Cordito petrofacies has abundant sanidine, quartz and vitric-to-granular lithics, with few dense minerals. The Plaza petrofacies is similar to the Esquibel, but with higher quartz and microlitic lithics, and lower total feldspar and dense minerals. Use of petrofacies simplifies the chaos of stratigraphic nomenclature and allows regional correlations of poorly exposed units. Paleocurrent and textural data from these volcaniclastic petrosomes define volcaniclastic aprons derived from the above volcanic centers. The coordinated use of clast ages and petrology to define source characteristics, sandstone petrofacies to define petrosomes, and paleocurrent and textural analyses to define dispersal patterns provides a powerful method for reconstructing ancient volcaniclastic systems.

Abstract The first part of the trip travels northwest from San Luis Potosi to Zacatecas. An extensive collection of Tertiary volcanic rocks covers the base sediments west of San Luis Potosi. Rocks are mainly rhyolite tuffs, latites, and layered volcanic limolites. Another important volcanic center is the Los Pinos, which is a complex formed by clastic rocks, red beds, trachytes, and rhyolite tuffs. Historically, mining in the area has been active and was first reported in epithermal Au-Ag vein mineralization related to the mid-Tertiary sequence formed by domes, associated lava flows, and ash-flow tuffs. Peñon Blanco (San José) is an emergent mining district that has been explored recently. Rocks reported in the area include the basement, formed by Cuesta del Cura limestone and rocks of the Chilitos Formation. This sequence of rocks is intruded by the Peñon Blanco batholith, which has created a halo of marble rock in the contact with the sedimentary and volcanosedimentary rocks. Mineralization occurs in quartz veins and skarn, and main ores are silver, lead, zinc, and little copper. The city of Zacatecas is situated within a horst formed by NE-trending normal faults. The Mesozoic Chilitos Formation, siltstones, phyllites, and rhyolites are the main host rocks for mineralization in the region. Two of the major producer mines are located nearby: The Francisco I. Madero and the Cozamin deposits. The Madero deposit is hosted in thin-layered siliceouscarbonaceous units, with chlorite, calcite, and pyrite alteration that can be seen only underground. The Cozamin deposit occurs in veins hosted by the Chilitos Formation, andesites, phyllites, and siltstones. General vein trending is N 40° to 65° W dipping 65° to 75° NE, and the entire group of veins is known as the Mala Noche vein system. Good morning! Take the side road, San Luis Potosi-Mexico, to the east. 0.4 Turn left on Colorines street and take the San Luis-Mexico Highway to the west. 0.9 Jump into the Distribuidor Vial Benito Juarez and take the road San Luis Potosi-Matehuala to the north. 2.1 Turn left and follow the Boulevard Rio Santiago to the west. 2.6 Turn right on Fray Diego de la Magdalena Street. 1.1 To the right you'll see the Tangamanga local park, a Formula 3 racing course. Continue straight to Zacatecas by way of the Federal Highway no. 49. 4.1 Pass over the bridge of the Anillo Periferico Norte. 14.6 Route passes by cultivated areas and a series of small