Petrogenesis of voluminous mid-Tertiary ignimbrites of the Sierra Madre Occidental,Chihuahua, Mexico   总被引：1，自引：0，他引：1  

Maryellen Cameron  William C. Bagby  Kenneth L. Cameron 《Contributions to Mineralogy and Petrology》1980,74(3):271-284

The mid-Tertiary ignimbrites of the Sierra Madre Occidental of western Mexico constitute the largest continuous rhyolitic province in the world. The rhyolites appear to represent part of a continental magmatic arc that was emplaced when an eastward-dipping subduction zone was located beneath western Mexico.In the Batopilas region of the northern Sierra Madre Occidental the mid-Tertiary Upper Volcanic sequence is composed predominantly of rhyolitic ignimbrites, but volumetrically minor lava flows as mafic as basaltic andesite are also present. The basaltic andesite to rhyolite series is calc-alkalic and contains 1% K₂O at 60% SiO₂. Trace element abundances of a typical ignimbrite with 73% SiO₂ are Sr 225 ppm, Rb 130 ppm, Y 32 ppm, Th 12 ppm, Zr 200 ppm, and Nb 15 ppm. The entire series plots as coherent and continuous trends on variation diagrams involving major and trace elements, and the trends are distinct from those of geographicallyassociated rocks of other suites. We interpret these and other geochemical variations to indicate that the rocks are comagmatic. Mineral chemistry, Sr isotopic data, and REE modelling support this interpretation.Least squares calculations show that the major element variations are consistent with formation of the basaltic andesite to rhyolite series by crystal fractionation of observed phenocryst phases in approximate modal proportions. In addition, calculations modelling the behavior of Sr with the incompatible trace element Th favor a fractional crystallization origin over a crustal anatexis origin for the rock series. The fractionating minerals included plagioclase (> 50%), and lesser amounts of Fe-Ti oxides, pyroxenes, and/or hornblende. The voluminous ignimbrites represent no more than 20% of the original mass of a mantle-derived mafic parental magma.  相似文献   

Further evidence of tidal influence on earthquake incidence     

John P. Bagby 《Earth, Moon, and Planets》1973,6(3-4):398-404

The rate of release of earthquake energy over the past 65 yr is compared to the relationship of the longitude of perigee of the Moon to the Earth's longitude of perihelion. Certain perculiar phase and local hour angle relationships, of the Moon to earthquake epicenters, are also presented. Finally, the clustering of epicenters near the longitudes of Neptune and Uranus is examined.  相似文献