Margarita Reyes-Salas

Late Ordovician–Early Silurian continental collisional orogeny in southern Mexico and its bearing on Gondwana-Laurentia connections

New zircon and monazite U-Pb data, tectonic mapping, and petrologic studies in key units of the Acatlan Complex show a previously undocumented phase of continental collision orogeny of Late Ordovician–Early Silurian age in southern Mexico. The event involved the partial eclogitization of oceanic lithosphere and continental crust, which traveled westward more than 200 km over siliciclastic metasedimentary rocks of the trench-forearc of an opposing continental margin. The overriding eastern margin was the Oaxaquia microplate attached to Gondwana, and the western overridden margin is considered to have been the eastern margin of Laurentia. This event, which we name the Acatecan orogeny, was roughly synchronous with the possible closure of Iapetus along the Appalachian margin, which involved, according to current models, either the docking of peri-Gondwanan terranes such as Avalonia and Carolina or the direct collision between Gondwana and Laurentia. The permanence of Oaxaquia in northwestern Gondwana until the end of the Silurian, as suggested by Tremadocian to Silurian marine faunas in the cover of Oaxaquia, is more consistent with the direct collision of Gondwana and Laurentia at the end of the Ordovician, forming the Acatlan Complex between.

Polyphase, High-Temperature Eclogite-Facies Metamorphism in the Chuacús Complex, Central Guatemala: Petrology, Geochronology, and Tectonic Implications

This paper describes the first discovery of eclogite-facies rocks in the Paleozoic Chuacús basement complex of north-central Guatemala. In this area, the complex comprises a thick, polydeformed sequence of high-Al metapelite, amphibolite, and quartzofeldspathic banded gneisses and schists characterized by garnet, phengite, and kyanite. Detailed petrographic, electronprobe microanalyses, and a late Carboniferous U-Pb zircon apparent age indicate that this deeply rooted orogenic terrane may be related to the Alleghenian suturing between Gondwana and Laurentia. Eclogite-facies metamorphism is established by assemblages with omphacite-garnet-rutile ± phengite ± zoisite in mafic rocks, which are consistent with garnet-kyanite-zoisite-rutile-quartz-phengite ± staurolite ± chloritoid assemblages in pelitic rocks, and amphibole-calcite/dolomite/aragonite?- rutile-quartz-zoisite ± clinochlore ± diopside in marbles. Moreover, various textural and mineralogical features (such as radial cracks in garnet and kyanite around quartz inclusions; palisade-like coronas of a silica mineral around quartz in some carbonates; lamellar inclusions of a titaniferous phase in garnet, zoisite, and phengite; and plagioclase or white mica in some omphacite; as well as the relatively high Na2O content of garnet [up to 0.12 wt%]), suggest relict ultrahigh-pressure metamorphism (UHPM). These conditions predated high-temperature-high-pressure hydration and decompression melting that occurred between 18 and 23 kbar and 700-770°C. This decompressional melting event of eclogitic rocks is dated as late Carboniferous by U-Pb on discordant zircons from a leucocratic neosome, and may be associated with the initial closure of Pangea. K-Ar ages of ~70-75 Ma on micas and amphibole, stable at 14 kbar and 597°C, are interpreted to record the Cretaceous obduction of Caribbean ophiolites and arc assemblages onto the Chuacúús complex and the southern edge of the Maya block, along the paleo-Motagua fault zone.

Petrology of high-grade crustal xenoliths in the Chalcatzingo Miocene subvolcanic field, southern Mexico: buried basement of the Guerrero-Morelos platform and tectonostratigraphic implications

The Miocene Chalcatzingo trondhjemitic volcanic field, sited along the southern margin of the Trans-Mexican Volcanic Belt, is a newly discovered locality with deep-seated crustal xenoliths that provide fundamental petrologic information on the nature of the unexposed metamorphic basement. The volcanic field lies along the eastern edge of the Cretaceous Guerrero-Morelos platform, which juxtaposes the Guerrero and Mixteco terranes of southern Mexico. Xenoliths consist of high temperature to ultra-high temperature metapelites as well as mafic and quartzofeldspathic gneisses, all of which show evidence of multiple granulite to amphibolite facies metamorphism and ductile deformation. A detailed petrologic study of representative xenoliths indicates a metamorphic evolution that apparently followed a clockwise pressure–temperature path leading from biotite-sillimanite1/kyanite(?)-quartz assemblages (M1) to the assemblage plagioclase-garnet-sillimanite2-rutile/ilmenite (M2) with a peak at ∼9–11 kbar and >870°C. These conditions were followed by rapid uplift to <6 kbar and >800°C, which produced the decompression assemblage spinel-cordierite-sillimanite3-corundum ± orthopyroxene ± quartz (M3) before shallow emplacement of the xenolith-bearing trondhjemitic magma. Three possible sources for the xenoliths are considered: (1) early Mesozoic metasediments buried in the middle crust; (2) Precambrian lower crust; and (3) subducted Cenozoic sediments trapped in the mantle wedge. Based on the deep-seated, polymetamorphic nature of the xenoliths, the Nd depleted mantle model age of an orthogneissic xenolith, and on regional tectonostratigraphic considerations, we suggest that the xenolith source was Proterozoic continental crust. Although old zircon inheritance in the host trondhjemite is minimal, it may be explained by a lack of interaction of the magma with the traversed lithosphere. Studies of Palaeogene shallow intrusions exposed 140 km west of Chalcatzingo in the Guerrero terrane (Pepechuca plug) and 80 km southeast of that place in the Mixteco terrane (Puente Negro dikes) reveal the presence of similar very high-grade aluminous xenoliths. However, these magmas were probably generated by partial melting of Triassic–Jurassic metasediments of the Guerrero terrane underplated by basaltic magmas in Jurassic–earliest Cretaceous times or from Precambrian crust assimilated by underplated mafic magmas of Oligocene age, respectively.

Petrology and geochemistry of the Valle de Santiago lower-crust xenoliths: Young tectonothermal processes beneath the central Trans-Mexican volcanic belt

We present a comprehensive petrologic study of lower-crust mafic and felsic xenoliths hosted by Quaternary alkaline basalts of the Valle de Santiago monogenetic volcanic field. This is the only locality along the entire Trans-Mexican volcanic belt where the abundance and size of xenoliths allow the understanding in great detail of processes associated with interactions of young subduction-related magmatism and the deep continental crust. Mafic xenoliths (two pyroxene ± spinel granulites and metanorthosites), olivine-rich gabbroic xenoliths, and transitional xenoliths compose the bulk of the population, although a few belong to the charnockitic suite (enderbite and faersundite). Thermobarometric calculations (two-pyroxene, ilmenite-magnetite, Ti-in amphibole, amphibole-plagioclase, and phase equilibria in the system NCMAS (Na-Ca-Mg-Si)) result in pressures around 9 kbar and temperatures of 1000–1100 °C for the granulite-facies metamorphism, which would give a very hot lower crust, ∼33 km thick, beneath Valle de Santiago and a mean geothermal gradient of ∼30 °C/km. Igneous zircons (Th/U = 0.03–0.87) extracted from one of the felsic granulites yielded a major peak of latest Cretaceous age (67.1 Ma), interpreted as the crystallization age of the granitic protolith, without inheritance from Precambrian or Paleozoic crust. Minor peaks at 45.1 and 25.5 Ma are interpreted as partial Pb losses from some of the Cretaceous zircons. Trace-element geochemistry, as well as Sr, Nd, and Pb isotopic studies performed on two granulites, is consistent with the juvenile and coeval nature of both the mafic metagabbroic xenoliths and the alkaline basaltic magmas that lifted the xenoliths from the lower crust. Two intermediate stages in the thermal evolution of the sampled xenoliths include the emplacement at different depths of volatile and K-Fe-Ti–rich oxidized melts represented by igneous assemblages with kaersutite, biotite, titanomagnetite, spinel, plagioclase, Fe-rich epidote, clinopyroxene, fayalitic olivine, and glasses that pervasively invaded most granulite xenoliths before being taken to the surface. A preferred plumbing system model is presented depicting a protracted Miocene to Quaternary basaltic intraplate magmatic system that sampled former basaltic batches stationed in the lower crust, together with the Late Cretaceous deep-seated granitoids beneath Valle de Santiago in the backarc of the central Trans-Mexican volcanic belt. Both components were later subjected to granulite-facies conditions in the lower crust, most probably related to the continued heating of the crust by basaltic magmas underplated in the central Trans-Mexican volcanic belt backarc region.

Direct Electrochemical Determination of Hydroxymethylfurfural (HMF) and its Application to Honey Samples

Abstract In this work an electrochemical method is evaluated for direct hydroxymethylfurfural (HMF) content determination in untreated honey samples. The HMF presented a single, well‐defined reduction signal at −1100 mV vs. Ag/AgCl. Borate was confirmed as being the most suitable supporting electrolyte for determining HMF content in honey because it allowed better definition and selectivity of the HMF reduction signal. The detection limit for the differential pulse polarography method was 48 ppb and it was successfully applied to three samples of Mexican honey, using the standard addition method.

Ca‐poor Pyroxene Raman Characteristics in H Ordinary Chondrites

In this work we present the zoning of the Ca‐poor pyroxenes polymorphs and its Raman characteristics of crystal within chondrules in H3.3, H4, H5 and H6 ordinary chondrite suite. Applying the Principal Component Analysis (PCA) method to the Raman data and plotting the first and second principal components, we were able to differentiate their petrologic types in four separate fields.