Petrology and geochemistry of shield volcanoes in the central Mexican Volcanic Belt.

摘要

More than 100 small shield volcanoes and 1,200 cinder cones occur interspersed throughout the central Mexican Volcanic Belt (MVB). The shields are predominantly medium-K calc-alkaline basaltic andesites. Compositional variation within single shields is minor with silica variation restricted to less than 3%. The most evolved Shield lavas (dacite) occur in small volume late-stage eruptive products. Shield phenocryst mineralogy is generally plagioclase ;Shield isotopic ratios of Pb (;Roughly one third of the shield lavas have phosphorous and titanium contents which are high relative to other arc volcanics. High phosphorous and high titanium shield lavas are spatially and compositionally related to transitional shield lavas. Least squares analysis indicates that these high phosphorous magmas are derived through the modification of a transitional parent by progressive fractionation and assimilation.;Comparison of shield compositions with published information on the associated cinder cones reveals a close similarity. The primary differences are that shield compositions are on average more evolved and more restricted in range. Similar fO;The geographic distribution of both shields and cinder cones in the central MVB is very similar in regional extent and character. Both vent types occur in geographic clusters identified as local regions (