Relations volcanology-Magmatology-Geodynamic: The transition between andesitic and alkaline volcanisms in southern Mexico

Abstract

This thesis is subdivided into six. parts : 1 - The definition and the geological survey of the volcanic provinces which outcrop in south-eastern Mexico: The N-S directed Alkaline Province, widespreading along the eastern Mexican coastal plain, and the eastern part oí the Mexican Volcanic Belt. On the basis of their age and their spatial relations, these provinces remain difficult to reconcile: with the classical concept of a Circum-Pacific volcanic: zone. 2 - The volcanological study of major andesite composite volcanoes in this zone. lt was necessary to define the magmatic evolution during the last 2 Ma. The study leads to propose a volcanological model valid for this type of continental andesitic volcano-complexes. 3 - 4 :In parts 3 and 4 the petrological characteristics and the evolution (both in time and space) of each province are presented. 500 new analyses (major and minor elements) and mineralogical data support this study. In the major volcanoes such as Nevado de Toluca, Pico de Orizaba and Popocatepetl, the comipositional characteristics of the lavas are correlated with the volcanological stages of development. 5 - According to trace-elements geochemistry (REE, Sr isotopes, LIL Elements) hypotheses on the genesis of primary magmas are discussed. 6 - The compilation of these data, in the light of geophysical results on the Pacific plate subduction in South Mexico leads to a new interpretation of the andesitic province and allows to propose a model which explains the transition between the two types of magimatism during the last 4 Ma.