L. Hecht

Petrology and geochemistry of metaigneous rocks from a Grenvillian basement fragment in the Maya block: the Guichicovi complex, Oaxaca, southern Mexico

Abstract The Guichicovi complex is the southeasternmost exposure of Proterozoic granulites in Mexico and forms the basement at the western edge of the Maya block. Its Grenville-affinity is similar to other basement rocks in Mexico that comprise the Oaxaquia microcontinent. Several authors have interpreted Oaxaquia as a volcanic arc terrane formed ∼1.2xa0Ga ago outboard the Grenville Province. This volcanic arc was intruded by igneous rocks of the anorthosite-mangerite-charnockite-granite suite (AMCG-suites) prior to collision during the Grenville orogeny and subsequent granulite facies metamorphism ∼1.0xa0Ga ago. The Guichicovi complex consists of metasedimentary sequences and basic to acid metaigneous rocks. We analyzed major, trace, and rare earth elements of metaigneous rocks from the Guichicovi complex that comprise two major lithologic units: (1) the Zacatal unit composed of felsic granulites (charnockites) and orthogneisses, and (2) the Northern Guichicovi unit composed of anorthositic-tonalitic gneisses, mafic granulites (ferrodiorites), and amphibolites. These two major lithologic units can clearly be distinguished by their chemical compositions. Most of the analyzed rocks indicate a volcanic-arc tectonic environment, but there are exceptions with within-plate characteristics. The Zacatal unit may have formed by crustal anatexis within the Oaxaquia arc-terrane, partly in a within-plate environment. One of those granitic gneisses with within-plate characteristics has a 1.23xa0Ga protolith age, which indicates that either the Oaxaquia arc was formed earlier, or that protoliths of this granitic unit have an origin distinct to the Oaxaquia arc terrane. We consider different models to interpret the Northern Guichicovi unit. It cannot conclusively be explained by an anorthosite suite, although the ferrodiorites can. Partial melting of the lower crust at a volcanic-arc basement may have formed the anorthositic-tonalitic gneisses and ferrodioritic melts.

Reactivity of sandstone and siltstone samples from the Ketzin pilot CO2 storage site-Laboratory experiments and reactive geochemical modeling

To evaluate mineralogical-geochemical changes within the reservoir of the Ketzin pilot CO2 storage site in Brandenburg, Germany, two sets of laboratory experiments on sandstone and siltstone samples from the Stuttgart Formation have been performed. Samples were exposed to synthetic brine and pure CO2 at experimental conditions and run durations of 5.5xa0MPa/40xa0°C/40xa0months for sandstone and 7.5xa0MPa/40xa0°C/6xa0months for siltstone samples, respectively. Mineralogical changes in both sets of experiments are generally minor making it difficult to differentiate natural variability of the whole rock samples from CO2-induced alterations. Results of sandstone experiments suggest dissolution of the anorthite component of plagioclase, anhydrite, K-feldspar, analcime, hematite and chloritexa0+xa0biotite. Dissolution of the anorthite component of plagioclase, anhydrite and K-feldspars is also observed in siltstone experiments. In an inverse modeling approach, an extensive set of equilibrium simulations was set up in order to reproduce the experimental observations of the sandstone experiments. Simulations generally show fairly good matches with the experimental observations. Best matches with measured brine data are obtained from mineral combinations of albite, analcime, anhydrite, dolomite, hematite, illite, and kaolinite. The major discrepancies during equilibrium modeling, however, are reactions involving Fe2+ and Al3+. The best matching subsets of the equilibrium models were finally run including kinetic rate laws. These simulations reveal that experimentally determined brine data was well matched, but reactions involving K+ and Fe2+ are not fully covered. The modeling results identified key primary minerals as well as key chemical processes, but also showed that the models are not capable of covering all possible contingencies.

Provenance, age constraints and metamorphism of Ediacaran metasedimentary rocks from the El Triunfo Complex (SE Chiapas, México): evidence for Rodinia breakup and Iapetus active margin

ABSTRACT Metasedimentary rocks from the El Triunfo Complex (Jocote Unit) in the southern Chiapas Massif (SE México) are constituted mainly by sillimanite-rich micaschist, locally intercalated with marble and calc-silicate rocks. Mafic rocks (now amphibolite) intruded the sequence prior to deformation and folding. Peak metamorphic conditions are estimated by geothermobaromerty at ~6.0 kbar and ~650ºC. The timing of the metamorphic event is dated by LA-MC-ICPMS analysis on zircon rims at 438+23/–12 Ma. Furthermore, detrital zircon grains yield mainly Stenian–Tonian and minor early Mesoproterozoic ages, indicating provenance from Grenville-type orogens (such as Oaxaquia) and some older cratonic sources. The 87Sr/86Sr values of 0.70775–0.70777 and the δ13C values from +1.9‰ to +2.7‰ in associated calcite marble define the time of deposition between 600 and 580 Ma. Geochemical markers from metapelite samples (such as La/Th > 3.94, La/Sc > 3.72, Th/U > 8.19, Th/Co > 0.42 and CIA = 74 to 83), as well as Sm–Nd isotope data (εNdi = −8.1 to −4.0, TDM(Nd) = 1.65–1.32 Ga) suggest weathering of Mesoproterozoic felsic rocks during temperate to warm climate. Furthermore, Zr/Sc values (9.1–21.0), chondrite-normalized REE patterns [La/Yb]N = 10.3–23.3, Eu/Eu* < 0.64), and ΔHf values (1.98–10.02) are indicative of pelagic and zircon-depleted sediments of a passive margin. The results suggest that the Jocote Unit was deposited during the opening of the Eastern Iapetus Ocean in the Ediacaran Period. This is the first evidence for Rodinia breakup in southern México. Besides that, the Ordovician tectonothermal event is probably related to compression during subduction and accretion along the western margin of Gondwana.