Authors: SURENDRA P. VERMA
Abstract: If both the Mexican Volcanic Belt (MVB) and the Central American Volcanic Arc (CAVA) have been related to the subduction of the Cocos plate beneath the North American and Caribbean plates, respectively, their magmas, and especially the less evolved basic varieties, should show considerable similarities. The conventional multielement normalized diagrams indicate more complex petrogenetic processes for the MVB than the CAVA. Forty-five statistically coherent tectonomagmatic discrimination diagrams were used to infer the tectonic setting of the controversial geological subprovince of the eastern part of the MVB (E-MVB). Basic rocks from the E-MVB indicated a continental rift setting, whereas the intermediate rocks were more consistent with a transitional setting of rift to collision. The acid rocks, presumably having a larger crustal component than the intermediate and basic rocks, showed inconclusive results. The volcanic rock data from the CAVA were used to successfully test these diagrams. The expected arc setting was consistently indicated for the CAVA from basic, intermediate, and acid rocks, confirming the satisfactory functioning of the diagrams. The data for all three types of rocks from the E-MVB and CAVA were then objectively compared for their similarities and differences. Specially designed computer programs were used to efficiently apply discordancy and significance tests at the strict 99% confidence level. Most (43 out of 50) chemical elements and (25 out of 28) log-ratio parameters in basic rocks from the E-MVB and CAVA showed statistically significant differences. For intermediate rocks and, to a lesser extent, for acid rocks, a large number of parameters also showed differences between the E-MVB and CAVA. The differences in the inferred tectonic settings for basic and evolved rocks from the E-MVB are likely related to the different magmatic sources.
Keywords: Tectonomagmatic discrimination diagrams, discordancy tests, significance tests, continental rift, continental arc, collision tectonic setting
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