SPACE-TIME VARIATION OF GEOCHEMICAL CHARACTERS AND VOLCANIC OUTPUT IN THE WESTERN HALF OF THE TRANSMEXICAN VOLCANIC BELT

We analyze the evolution in space and time of the volcanism in the western half of the Trans-Mexican Volcanic Belt (TMVB) (up to Long. 101°W) which is underlain by the young Rivera (western TMVB) and Cocos (central TMVB) plates subducting at different angle. We used all the geologic, geochemical and geochronologic data available (about 550 ages and 1180 geochemical analyses) compiled into a Geographic Information System. After filtering for quality geochemical data were assigned to one of the following age interval: 0 – 3 Ma; 3 – 5 Ma; 5 - 7.5 Ma; 7.5 – 11 Ma; 11 – 15 Ma. These time slices correspond to specific volcanic episodes in the TMVB (Ferrari, this volume). Volcanic output was estimated by computing the area of each geologic unit using the GIS and estimating the thickness of each unit from field observation, borehole data and topographic maps. The analysis of the data reveals several interesting features. 1) Silica contents and Mg# in rocks younger than 7.5 Ma correlate with distance from the trench in the western TMVB but are more homogeneously distributed in the central TMVB. Late Miocene rocks (7.5 – 11 Ma time slice) show a constant mafic character. 2) On average rocks younger than 7.5 Ma display a negative correlation between Ba/Nb and distance from the trench. Rocks far from the trench span a larger Ba/Nb range. In 7.5 – 11 Ma rocks Ba/Nb does not correlate with distance from the trench, suggesting a genesis unrelated from flux melting of the mantle. 3) Adakitic rocks are found among the late Pliocene to Quaternary volcanoes of the northwestern part of the TMVB, which agree with their position above the northwestern edge of the young Rivera plate. 4) Rocks showing an OIB affinity are found preferentially far from the trench. 5) Volcanic output in the western TMVB correlate with relative convergence rate between Rivera and North America plates. Volcanic output along the arc may vary as much as 5 times even in the same time slice. The higher volcanic output in all times corresponds to a broad zone located above the subducted boundary between the Rivera and Cocos plate. Volcanic output shows a rough increase with distance from the trench in the western TMVB whereas in the central TMVB the correlation is reverted.