Pedro Corona-Chávez

Uplift and subduction erosion in southwestern Mexico since the Oligocene: pluton geobarometry constraints

Details of the late Oligocene to Middle Miocene uplift and tectonic erosion episodes of the southwestern continental margin of Mexico can be inferred using Altot geobarometry of igneous hornblendes, geochronology, and field relations. On the basis of such analyses carried out between Acapulco and Huatulco we find the following: (1) Calc-alkaline batholiths exposed along the coast from Acapulco to Huatulco, mostly in the 35-25 Ma age range, were emplaced at depths between 13 and 20 km. (2) The contact relationships between these plutons and their host rocks, and the exposure of volcanic counterparts, 70 km from the coastline, indicate a landward decrease in the amount of uplift. (3) A comparison of the time differences between intrusion and cooling ages of batholiths along the coast suggest that cooling rates were, in general, higher between Acapulco and Huatulco than those along the margin between Puerto Vallarta and Manzanillo, 700 km northwest of Acapulco. (4) The uplift of this coastal belt occurred during the late stages of magmatism and after its cessation, triggering intensive subaerial erosion of supracrustal rocks and the exposure of midcrustal rocks such as granitic batholiths and amphibolite facies metamorphic assemblages of the Xolapa Complex. These findings, in conjunction with the geometry of the present continental margin, as well as the offshore tectonic and stratigraphic features, support previous interpretations of very active late Oligocene to Middle Miocene subduction erosion after the onset of strike-slip tectonics related to the detachment and subsequent eastward displacement of the Chortis block. Subduction erosion involved both trench sediments and crystalline (continental framework) rocks. Different rates of continental framework erosion are assessed on the basis of the bathymetric fluctuations of the upper slope trench sediments and the age of the accretionary prism. Subsidence of the offshore continental basement suggests intense episodes of basal erosion of lower continental crust, whereas the construction of the present day accretionary prism and the uplift of the upper slope indicate a decline in the frontal and basal erosion of the continental framework. Comparing the calculated depths of pluton crystallization with the present depth of the continental crust-subducted slab boundary, interpreted using previously published seismic refraction and gravity models, we conclude that onshore basal erosion played a subordinate role during Miocene episodes of subduction erosion. Major removal of lower crustal sections was probably restricted to offshore regions. Plate reconstructions of the Cocos plate and its predecessors with respect to North America indicate that the uplift and probably the offshore subduction erosion in this region coincided with the initial stages of the subhorizontal trajectory of the Guadalupe plate beneath southwestern Mexico.

Further evidence for magnetic susceptibility as a proxy for the evaluation of heavy metals in mining wastes: case study of Tlalpujahua and El Oro Mining Districts

Magnetic susceptibility is nowadays used in most areas of environmental research as a proxy for heavy metal pollution in industrial and urban areas. Although the relationship between magnetic susceptibility and concentration of toxic elements in different environments has been pointed out in several studies, mining wastes (tailings) have hardly been investigated by magnetic methods. We report the relationships between magnetic susceptibility and potentially toxic elements monitored at 12 vertical ground profiles of the Tlalpujahua and El Oro mining districts, western Mexico. Specific bulk magnetic susceptibility (k) measurements, percentage frequency-dependent susceptibility (%XFD) determinations as well as the identification of the magnetic carriers within the samples were accomplished using standard rock-magnetic techniques on geochemically well characterized sister samples. Magnetite and/or Ti-poor titanomagnetite seem to be the main magnetic carriers in the samples. Tight correspondence between k and Fe concentrations, as well as Pb and As with the iron content were found. This association seems to hold also for pH variations.

Miocene andesitic lavas of Sierra de Angangueo: a petrological, geochemical, and geochronological approach to arc magmatism in Central Mexico

ABSTRACT The early and middle Miocene andesitic lavas of the Sierra de Angangueo (MALSA) represent one of the most prominent landforms of intermediate magmatism in Central Mexico. In this paper, we present new petrological, geochemical, thermobarometric, and geochronological data in order to discuss the conditions of MALSA’s emplacement in the lithosphere. MALSA comprises a voluminous eroded early Miocene andesitic structure, emplaced on a Mesozoic basement. MALSA shows a wide variety of textures, from glassy and aphanitic to porphyritic, mainly composed of plagioclase (An10-55), clinopyroxene (Wo60-65; En35-40), amphibole (Mg-hornblende/edenite), and rarely olivine (Fo75–86) or orthopyroxene (En72-80). Major and trace elements plots follow a typical calc-alkaline trend with relatively increasing fractionation. The 87Sr/86Sr isotopic signatures range between 0.703343 and 0.704459 and εNd values from +1.37 to +4.84; apparently without a significant contribution of an old, thick, or highly radiogenic continental crust. Lead isotope values 206Pb/204Pb vary between 18.68 and 18.83, 207Pb/204Pb from 15.57 to 15.65, and 208Pb/204Pb from 38.39 to 38.67, suggesting contamination of magmas by juvenile crust. Thermodynamic calculations indicate equilibrium conditions at ≤900°C and ~2 kbar and an average log ƒO2 ≈ −10. Ar–Ar and K–Ar dating carried out on whole rock, matrix, amphibole, and plagioclase phenocrysts yielded ages between 13.0 ± 0.5 and 23.9 ± 0.3 Ma. These dates indicate a series of pulses with maximum magmatic activity between 24 and 18 Ma. The geochemical and petrologic data suggest partial melting processes in the lower or middle crust associated with possible magma mixing during its ascent; such a mechanism could explain a hybrid mantle-lower crust origin of this large volume of andesites. The MALSA, as well as the early to middle Miocene magmatism from Western Mexico, could represent two coeval and independent magmatic arcs prior to the present Trans-Mexican Volcanic Belt (TMVB) in Central Mexico.

Combined rock-magnetic and geochemical characterization of Angangueo mining district, central Mexico

Results of a combined rock-magnetic and geochemical investigation carried out at Angangueo mining district are presented. The different thickness and color of layers observed in field are also sketched from the magnetic susceptibility vertical ground profile accomplished. Additionally, an area with enhanced magnetic susceptibility values is clearly outlined from a radial survey on the top of one of the biggest tailing. The internal differentiation of the tailing was also supported by a full set of laboratory rock-magnetic and physical–chemical parameters. Geochemical results of matched samples reveal a complex relationship along the profile between some heavy metal concentrations (i.e., Cu and Pb) and magnetic susceptibility, never reported in mining wastes nor in environmental studies. Contrary to environmental investigations of topsoils, magnetic susceptibility enhancements zones in a tailing do not correspond necessarily to more heavy metal-contaminated zones. These outcomes evidence the intricate nature of mining wastes and the need for a better and intensive investigation of these barely studied objects.