ABSTRACT. This contribution reports the first set of samples and the creation of an internal reference material at the recently opened Radiocarbon Laboratory at the National Autonomous University of Mexico (UNAM). Samples for the initial measurements were selected from archaeological and dating projects on Teotihuacán, one of the largest and best-studied Mesoamerican urban and ceremonial centers. The 14 C dates were compared to results obtained by 2 other laboratories in order to assess the results obtained at UNAM and validate the adopted methodology. As part of the quality assurance protocol, an internal reference material was created that consists of charred wood from the Teotihuacán site with a 14 C activity in the value range expected for samples from Mesoamerican archaeological sites. Results from 7 analyses have a mean of 1750 ± 16 BP (80.43 ± 0.16 pMC).

Cuatro de las catorce muestras de lavas máficas colectadas a lo largo del lineamiento volcánico central de Jalisco (CJVL) fueron datados por el método de K-Ar para evidenciar un decremento sistemático en la edad del vulcanismo a lo largo del lineamiento como lo indican estudios geológicos y geofísicos previos. Los resultados concuerdan con la progresión en edad propuesta. Específicamente, estos resultados indican que el vulcanismo a lo largo del segmento ubicado más al sureste del CJVL (localizado entre los campos volcánicos de Ayutla y Atenguillo) ocurrió hace cerca de 3.0 Ma, el volcanismo a lo largo del segmento al NW (entre los grábenes de Atenguillo y de Mascota) de 1.8 a 1.9 Ma y entre 0.5 y 0.6 Ma en el Graben de Mascota. La ausencia de vulcanismo a lo largo del segmento ubicado más al noroeste del lineamiento (entre el graben de Mascota y San Sebastián) concuerda con la progresión de la edad propuesta. Se realizó un análisis geoquímico en las 14 muestras y se determinaron tierras raras en las cuatro muestras datadas. Estos resultados, junto con estudios previos, indican que el vulcanismo ocurrió a lo largo del CJVL y adentro del campo Auytla mostrando una variabilidad composicional similar a la observada en otros campos volcánicos localizados dentro y en el interior del bloque Jalisco. Además, el afloramineto del cual fue tomada la muestra de Mascota (datada como 0.66 ± 0.038 Ma) es cortado por una falla transcurrente orientada al noroeste (tentativamente identificada según sus estrías rugosas como lateral derecha) indicando que recientemente ocurrió una deformación transcurrente en el graben de Mascota.

No abstract

We present and evaluate lanthanide contents measured by inductively coupled plasma mass spectrometry (ICP-MS) in fluorite samples from the fluorite deposits in Zacualpan and Taxco mining districts in the south of Mexico. The information is used to distinguish different generations of fluorite, to establish a correlation between mineralization episodes and the wall rock nature, and to identify postdepositional processes.The total lanthanide content of the fluorites are variable, and early- stage fluorite samples are usually enriched in LREE. The concentration of REE in fluorite is low in comparison with the volcanic and metamorphic rocks (∑REE > 100 ppm) and is generally high respect the carbonates (∑REE < 30 ppm). There is host rock influence. The higher REE concentra- tions are in fluorites hosted by volcanic rocks. The fluorite that replaced carbonate is characterized by low REE to very low concentrations. Fluorite samples associated with sulfurs are typically enriched in HREE. Nearly all fluorites show a negative Eu anomaly similar to the REE anomaly observed in the volcanic rock. Only some early stage dark, uranium rich fluorites, from la Azul deposit, have a strong positive Eu anomaly. Direct correlation between color and REE patterns is observed in some samples.In the Zacualpan mining district, only an episode of mineralization has been discriminated, where fluorite presents flat to HREE- enriched chondrite-normalized REE patterns.In the Taxco mining district and particularly in the “Mina la Azul”, multiple hydrothermal events of mineralization have been determined. The first generation of fluorite is formed by replacement of carbonates and is characterized by very low contents of lanthanides, chondrite- normalized REE patterns similar to the limestone, high strontium content and primary textures (e.g. massive fluorite and rhythmites). The second generation of fluorite is related to the entry of new fluid to the system and has higher REE concentrations, chondrite-normalized REE patterns similar to volcanic rocks, low strontium content and secondary textures (i.e. breccias, nodules). Most of the samples show a genetic relationship between fluorite and fluids of magmatic origin.

No abstract