دانشگاه: Tulane University School of Science and Engineering
: 7.95 MB
ژئوشیمی سنگهای آتشفشانی کواترنر عمومی از کمربند آتشفشانی مکزیک
Abstract: Geochemical analyses of Quaternary monogenetic cinder cones and associated lava flows of basic composition (SiO2 <56%) are used to investigate the magma sources under the Mexican Volcanic Belt (MVB). Magmas from these cinder cones could potentially represent primary magma compositions as there are less likely to be contaminated by the continental crust. The Mexican Volcanic Belt is associated with the subduction of the Rivera (Western sector) and Cocos Plates (Central and eastern sectors) under the North American Plate along the Middle America Trench. Samples were collected from the Central and Eastern sectors of the MVB and analyzed for major and trace elements, in addition to Sr, Nd, Pb and Oxygen isotopes. Most samples were classified as basaltic andesites and basalts. Two adakites from the Central sector were identified. A database of samples of the entire MVB was utilized for comparison. Primitive compositions that could be in equilibrium with peridotitic mantle were identified. These compositions, however, appear to be fractionated. Like in subduction related volcanism samples analyzed in this study, are affected by slab derived components (subduction additions, i.e. hydrous fluids or melts derived from the subduction slab or sediments) indicated by high LILE/HFSE and LILE/LREE. The effects of slab derived fluids are more prominent in the west and diminish towards the east. Unfortunately slab derived components, also carry continental crust signatures and consequently the effects of subduction and crustal components cannot be differentiated. Simple isotopic mixing and assimilation crystal fractionation (ACF) models indicate that the most basic MVB compositions can be explained by mixing with different crustal materials (up to 30%). This study indicates that even in the most primitive (less evolved) MVB rock compositions crustal contamination cannot be excluded.
Keywords: Earth sciences, Basic rocks, Major and trace elements, Mexican volcanic belt, Oxygen isotopes, Radiogenic isotopes, Subduction magmatism