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The Compostela area is located in the western Trans-Mexican Volcanic Belt, which consists of volcanic rocks that produced during the Pliocene to Recent volcanism. In this paper, we present petrography, whole-rock major- and trace-element concentrations, Ar–Ar ages, and Sr–Nd isotopic data of volcanic rocks from the Compostela area in the south of the city of Tepic, Nayarit, Mexico. These volcanic rocks are of intermediate composition and belong to the transitional series: basaltic trachyandesites [52.72–53.94 wt% SiO2; 0.69–2.53 wt% MgO] and subalkaline series: low-Si [55.11–60.94 wt% SiO2; 0.79–2.74 wt% MgO] and high-Si [61.60–62.71 wt% SiO2; 0.36–0.80 wt% MgO] andesites. The 40Ar/39Ar dating of two basaltic trachyandesites yields plateau ages of 1.05 ± 0.15 Ma and 1.07 ± 0.17 Ma and the andesites yielded a plateau age of 2.42 ± 0.36 Ma. These results indicate that the studied rocks were formed during the Pleistocene in two discrete episodes. The basaltic trachyandesites show enriched light rare earth elements patterns relative to high rare earth elements [(La/Yb)N = 5.81–8.07] with negligible Eu anomalies. The andesites display enriched large-ion lithophile elements (Ba, K) with anomalies of Nb (Th/Ce) and Ti in the three groups identified in the subalkaline series. The basaltic trachyandesites appear in a tight cluster of initial 87Sr/86Sr ratios (0.703519–0.703882) as compared to the basaltic andesite 87Sr/86Sr (0.704073) and positive ɛNd(t) values of +5.6 to +3.4, respectively, indicating that the basaltic trachyandesites were derived from intermediate magmas from a shallow mantle source. Geochemical modelling reveals that both basaltic trachyandesite and andesite rocks were derived by a process of crystal fractionation accompanied by assimilation of crustal rocks at the lower or middle level. Geochemical ratios and multidimensional discrimination diagrams, combined with the cross-section of the subduction zone indicates that basaltic trachyandesites were gener, Consejo Nacional de Ciencia y Tecnología (México), The Royal Society, Depto. de Mineralogía y Petrología, Fac. de Ciencias Geológicas, TRUE, pub

Weappliedboththeordinarylinearregression(OLR)andthenewuncertaintyweightedlinearregression(UWLR)modelsfor thecalibrationandcomparisonofaXRFmachinethrough59geochemicalreferencematerials(GRMs)andaprocedureblank sample. The mean concentration and uncertainty data for the GRMs used for the calibrations (Supplementary Materials) (available here) filewere achieved from an up-to-date compilation of chemical data and their processing from well-known discordancy and significance tests. The drift-corrected XRF intensity and its uncertainty were determined from mostly duplicate pressed powder pellets. The comparison of the OLR (linear correlation coefficient r∼0.9523–0.9964 and 0.9771– 0.9999, respectively, for before and after matrix correction) and UWLR models (r∼0.9772–0.9976 and 0.9970–0.9999, respectively) clearly showed that the latter with generally higher values of r is preferable for routine calibrations of analytical procedures. Both calibrations were successfully applied to rock matrices, and the results were generally consistent with those obtained in other laboratories although the UWLR model showed mostly narrower confidence limits of the mean (slope and intercept)orloweruncertaintiesthantheOLR.Similarsensitivity(∼2.69–46.17kc·s1·%1 fortheOLRand∼2.78–59.69kc·s1·%1 for the UWLR) also indicated that the UWLR could advantageously replace the OLR model. Another novel aspect is that the totaluncertaintycanbereportedforindividualchemicaldata.Iftheanalyticalinstrumentswereroutinelycalibratedfromthe UWLR model, this action would make the science of geochemistry more quantitative than at present.

We applied both the ordinary linear regression (OLR) and the new uncertainty weighted linear regression (UWLR) models for the calibration and comparison of a XRF machine through 59 geochemical reference materials (GRMs) and a procedure blank sample. )e mean concentration and uncertainty data for the GRMs used for the calibrations (Supplementary Materials) (available here) filewere achieved from an up-to-date compilation of chemical data and their processing from well-known discordancy and significance tests. )e drift-corrected XRF intensity and its uncertainty were determined from mostly duplicate pressed powder pellets. )e comparison of the OLR (linear correlation coefficient r∼0.9523–0.9964 and 0.9771–0.9999, respectively, for before and after matrix correction) and UWLR models (r∼0.9772–0.9976 and 0.9970–0.9999, respectively) clearly showed that the latter with generally higher values of r is preferable for routine calibrations of analytical procedures. Both calibrations were successfully applied to rock matrices, and the results were generally consistent with those obtained in other laboratories although the UWLR model showed mostly narrower confidence limits of the mean (slope and intercept) or lower uncertainties than the OLR. Similar sensitivity (∼2.69–46.17 kc·s1·%1 for the OLR and ∼2.78–59.69 kc·s1·%1 for the UWLR) also indicated that the UWLR could advantageously replace the OLR model. Another novel aspect is that the total uncertainty can be reported for individual chemical data. If the analytical instruments were routinely calibrated from the UWLR model, this action would make the science of geochemistry more quantitative than at present.

The Granjeno Schist is a meta-volcanosedimentary upper Paleozoic complex in northeastern Mexico. We suggest different tectonic settings for metamorphism of its serpentinite and talc-bearing rocks based on petrographic and geochemical compositions. According to the REE ratios (LaN/YbN = 0.51–20.0 and LaN/SmN = 0.72–9.1) and the enrichment in the highly incompatible elements Cs (0.1 ppm), U (2.8 ppm), and Zr (60 ppm) as well as depletion in Ba (1 – 15 ppm), Sr (1–184ppm),Pb(0.1–14ppm),andCe(0.1–1.9ppm)the rocks have mid-ocean ridge and subduction zones characteristics.TheserpentinitecontainsAl-chromite,ferrian chromiteandmagnetite.TheAl-chromiteischaracterized byCr#of0.48to0.55suggestingaMORBorigin,andCr#of 0.93 to 1.00 for the ferrian chromite indicates a prograde metamorphism.Weproposeatleasttwoserpentinization stagesoflithosphericmantlefortheultramaficrockofthe GranjenoSchist,(1)afirstinanocean-floorenvironment atsub-greenschisttogreenschistfaciesconditionsand(2) later a serpentinization phase related to the progressive replacement of spinel by ferrian chromite and magnetite atgreenschisttolowamphibolitefaciesconditionsduring regional metamorphism. The second serpentinization phase took place in an active continental margin during the Pennsylvanian. We propose that the origin of the ultramaficrocksisrelated toanobductionandaccretional eventatthewesternmarginofPangea. Keywords: ultramafic rocks; serpentinite; Granjeno Schist;northeasternMexico,Gondwana,Pangea

La parte central del Cinturón Volcánico Mexicano (CVMC) es una zona de gran importancia para el estudio del origen y la evolución magmática presente en el Cinturón Volcánico Mexicano, ya que incluye una gran diversidad de estructuras volcánicas : (a) estratovolcanes andesíticos-dacíticos (Nevado de Toluca, <2.6 Ma; Tlaloc-Telapón, ~1.8 Ma; Iztaccíhuatl, <2.0 Ma; Popocatépetl, <0.73 Ma); (b) una sierra volcánica del Plioceno-Pleistoceno con una orientación NNW-SSE (Sierra de las Cruces, 3.5-0.5 Ma), la cual está constituida por flujos de lava porfirítica de composición dacítica; y (c) un campo volcánico de edad Cuaternaria con una orientación E-W (Sierra de Chichinautzin, <40 ka), que incluye conos monogenéticos, volcanes escudo y domos. Sin embargo, existen varias localidades en esta región para los que la información petrográfica, geoquímica e isotópica es limitada o inexistente. Este es el caso de la denominada Sierra de Santa Catarina (SSC) y el Campo Milpa Alta (CMA), las cuales son el área de enfoque para este trabajo. Con el fin de contribuir a una mayor compresión sobre el vulcanismo en el CVMC, se plantea como objetivo principal, el de proponer un modelo de origen y evolución magmática para la SSC y el CMA. La SSC está constituida por una serie de conos de escoria que se alienan en una dirección E-W a través de 10 km de longitud, muestran alturas relativas de 60-300 m, con pendientes de 30-40°, conformadas de fragmentos angulosos de color negro a café, que pueden alcanzar hasta 20 cm de diámetro, y que estan intercaladas con capas de ceniza del mismo material. Las características petrográficas que se observan en las rocas de SSC son: (i) texturas porfíriticas, con presencia de vesículas; (ii) fenocristales subhedrales de clinopiroxeno, ortopiroxeno y olivino inmersos en una matriz vítrea, sin orientación preferencial de los cristales; y (iii) sin evidencia de desequilibrio o alteración. El análisis químico de esta zona reveló: (i) que domina una composición