Alfredo Aguillón-Robles

Late Miocene adakites and Nb-enriched basalts from Vizcaino Peninsula, Mexico: Indicators of East Pacific Rise subduction below southern Baja California?

A typical slab melt association was emplaced from 11 to 8 Ma in the Santa Clara volcanic field, Vizcaino Peninsula, Baja California Sur. It includes adakitic domes and associated pyroclastic flow deposits, together with lava flows of niobium-enriched basalts. The trace element and isotopic (Sr-Nd-Pb) signatures of adakites are consistent with melting of altered mid-ocean ridge basalts, and the sources of the Nb-enriched basalts contain an enriched mantle wedge component. Such associations commonly form at depths of 70–80 km during low-dip subduction of very young oceanic crust. However, the Santa Clara field is relatively close (100 km) to the paleotrench, which suggests that the genesis of its adakites and Nb- enriched basalts occurred in a very high thermal regime linked to the subduction of the then-active Guadalupe spreading center of the East Pacific Rise. Our data suggest that the asthenospheric window documented below northern Baja California also developed beneath the south of the peninsula during the Neogene. This hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region.

Spatial and temporal evolution of basalts and magnesian andesites (“bajaites”) from Baja California, Mexico: the role of slab melts

Late Miocene to Quaternary basalts and associated magnesian basaltic andesites and andesites, locally referred to as ‘‘bajaites’’, occur in the central part of the Baja California (BC) Peninsula. They form five volcanic fields (Jaraguay, San Borja, San Ignacio, Santa Rosalia, La Purisima) delineating a 600-km-long array parallel to the Gulf of California. They range in age from Late Miocene to Pleistocene, and display very specific geochemical characteristics: SiO2=50% to 58%, high MgO contents, very low FeO*/MgO ratios usually less than 1.5, highly fractionated rare earth element patterns with low Yand heavy rare earth element, very high Sr (commonly between 2000 and 3000 ppm) and Ba (up to 2300 ppm) contents. The geochemical study and K–Ar dating of ca. 50 samples of these rocks allow us to show that most of their incompatible element ratios, which vary significantly in space and time, reflect source heterogeneities rather than partial melting, fractional crystallisation or crustal contamination effects. Their slab melt imprint increases from northwest to southeast and with time. It is best expressed in the geochemical signatures of Quaternary lavas from La Purisima volcanic field. These features reflect the origin of the ‘‘bajaites’’ by melting of mantle peridotites previously metasomatised by slab melts, in connection with the opening of an asthenospheric window below the Baja California Peninsula during Early and Middle Miocene in northern Baja California, and during Late Miocene in southern Baja California. Melting was initiated by the high thermal regime accompanying ridge subduction or slab tearing/breakoff, and later by Plio-Pleistocene thermal pulses linked to the opening of the Gulf of California. We show that the incongruent melting of metasomatic pargasitic amphibole, leaving a garnet-rich residue, accounts for most of the specific geochemical features of the magnesian andesite suite. This breakdown started at ca. 1000 jC at depths of 70–110 km, and amphibole was probably not entirely consumed during the melting process. D 2002 Elsevier Science B.V. All rights reserved.

Geochemical Diversity of Late Miocene Volcanism in Southern Baja California, Mexico: Implication of Mantle and Crustal Sources during the Opening of an Asthenospheric Window

Five main petrologic and geochemical groups can be identified among the Middle to Late Miocene lavas from the western part of southern Baja California: (1) calc‐alkaline and K‐rich andesites emplaced between 15.5 and 11.7 Ma; (2) adakites and (3) associated niobium‐rich basalts erupted between 11.7 and 8.5 Ma in the Santa Clara volcanic field, Vizcaino Peninsula; (4) 10.6–9.2 Ma tholeiitic basalts and basaltic andesites that form large tabular plateaus near San Ignacio; and (5) magnesian and basaltic andesites of adakitic affinity whose emplacement started at 11.7 Ma south of San Ignacio and between 9.7 and 8.8 Ma near La Purisima. These lavas, although spatially and temporally related, display very different geochemical signatures. Their trace elements and isotopic characteristics suggest that three different magma sources were involved in their genesis. Partial melts of subducting altered oceanic crust produced the adakites when erupted directly at the surface. These magmas were eventually trapped in the mantle wedge where they reacted with ultramafic lithologies. Such slab‐melt‐metasomatized mantle could then melt to produce niobium‐rich basalts or magnesian andesites, depending on the pressure that controlled the stability of garnet into the mantle wedge. The melting of fluid‐metasomatized mantle wedge led to the emplacement of andesites. In southern Baja California, the opening of a slab window following active ridge subduction resulted in the additional contribution of partial melts from the suboceanic mantle uprising through the tear in the slab. This process might be responsible for the occurrence of tholeiitic basalts and basaltic andesites near San Ignacio. The studied association can be considered as a modern analog of high‐thermal‐regime Archean subductions.

Origin of the adakite–high-Nb basalt association and its implications for postsubduction magmatism in Baja California, Mexico: Discussion

Constraining the origin of the adakite–high-Nb basalt (HNB) association in Baja California, Mexico, is critical to a better understanding of global arc magmatism. Currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted Farallon-Cocos plate, thus providing support for the slab-melting origin of adakites elsewhere. Moreover, a tectono-magmatic model involving the production of both adakite and HNB from slab melts offers a comprehensive explanation for the origin of the atypical, arc-related, postsubduction magmatism in Baja California. This paper proposes alternative models for the origin of HNB and postsubduction magmatism in Baja California, wherein the unusual geologic setting of western Mexico and westward movement of North America permitted the influx of Pacific asthenosphere beneath the adjacent Gulf of California after the cessation of subduction. Unlike the previous tectono-magmatic model, the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in Baja California as tholeiites and HNB, respectively. Fractional crystallization of some of the HNB magmas plus assimilation of tholeiitic materials produced Nb-enriched basalts (NEB). The influx of Pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower Baja California crust, producing adakite rocks, and the preexist-ing metasomatized mantle wedge, producing bajaites and calc-alkaline magmas.

Tectono-volcanic control of fissure type vents for the 28 Ma Panalillo Ignimbrite in the Villa de Reyes graben, San Luis Potosí, México.

The volcano-tectonic events at the Villa de Reyes Graben (VRG), in the southern Sierra Madre Occidental, Mexico, include 1) a regional NNE fault system developed before 32 Ma, 2) this pre-32 Ma faulting controlled the emplacement of 31.5 Ma dacitic domes, 3) NE faulting at 28 Ma that displaced the 31.5 Ma dacitic domes and formed the VRG, as well as the oblique grabens of Bledos and Enramadas oriented NW, 4) emplacement of Panalillo ignimbrite at 28 Ma filling the VRG and erupting from fissures related to the oblique grabens, and eruption of Placa basalt apparently also from fault-controlled vents.

Assessment of the acid drainage neutralization capacity and the toxic metals lixiviation of tailing from Guanajuato mining district, Mexico

In Mexico, many environmental problems are generated by large mining activities taking place in several mining districts. These mining activities produce great quantities of residues; large majorities of these have high sulfur content, which could generate acid drainage due to their interaction with the oxygen in the environment. The study area was located in the Mining District of Guanajuato, Mexico with abandoned tailings generated mainly by the gold and silver production. Two areas, called as Monte de San Nicolás (SN) and Peregrina (P) were selected for this study. The results study shows that there was no risk of production of acid drainage, since these tailings contained high amount of carbonates, which neutralized the generation of acidity and consequently decreased the possibility of leaching of some elements. However, not all elements leach in acid pH, as arsenic bound to oxyhydroxides, which is in a basic environment and its increased release by increasing the pH.

Geologic setting of the Peña de Bernal Natural Monument, Querétaro, México: An endogenous volcanic dome

Pena de Bernal is a natural monument located near the town of Bernal, in Queretaro State, central Mexico. It is one of the tallest monoliths of the world, with a maximum height of 433 m. Pena de Bernal was recently declared Intangible Cultural Heritage of Humanity Patrimony by United Nations Educational, Scientific, and Cultural Organization (UNESCO). In spite of being both a natural and cultural monument, little is known about its origin, physical characteristics, and chemical composition. It is a leucocratic-igneous rock intruding marine Mesozoic sedimentary rocks and has been misinterpreted as a pluton of Eocene or older age. However, this study shows that Pena de Bernal is a dacitic dome with SiO 2 = 67 wt% and an age of 8.7 ± 0.2 Ma. The complete Pena de Bernal body includes three plugs that crop out in an ∼3.5 × 1.5 km area elongated N40°E. Texture of the rock is porphyritic, nearly holocrystalline (80 vol% crystals and 20 vol% glass), with a mineral assemblage of pyroxene, hornblende, biotite, plagioclase, and quartz, plus accessory apatite and zircon. Pena de Bernal dacite is a spine-type endogenous dome that was forcefully intruded through the Mesozoic sequence practically as a solid plug.