The evolution of Santa María volcano, Guatemala
Samples of 26 successive lavas from the high composite volcano of Santa María in southwestern Guatemala, demonstrate variations in magnetic declination and inclination which can be roughly-correlated with similar data from cores in the Gulf of Mexico and lake sediments near Tlapacoya, Mexico, and suggest that Santa María's constructive history goes back to approximately 30,000 years B.P. The basaltic andesites and associated pyroclastics, which make up this volcanic succession and comprise the latter 40% of the volcano's eruptive output, show systematic increases in , and Zr, and decreases in MgO and CaO, up the succession. These evolutionary changes anticipate the dacites which first appeared after a period of repose in Santa María, first as pyroclastic debris in the catastrophic flank eruption of 1902, and later as the endogenous dacite dome of Santiaguito which appeared in 1922 and has continued to grow to the present day. The increasing effects of crystal fractionation in the latter part of Santa María's history, as reflected in the compositional changes in the basaltic andesites and its culmination in the Santiaguito dacites, is thought to be due to the inhibiting effect which increasing cone height has had on the volcano's eruptive frequency through time, so progressively increasing the lengths of the repose periods when fractionation could proceed undisturbed in the top of the feeding magma column. Data from other high composite cones in Guatemala suggests that 3,500-4,200 meters represents a threshold cone height above which basaltic andesite is not erupted in this region, and it is believed that this limitation led to the historically documented dormancy of Santa María during which the Santiaguito dacites evolved. A peridotitic mantle source, rather than eclogite, for the basaltic andesites is consistent with their chemistry, provided that the primary liquids have been depleted in Ni and Cr by approximately 10% olivine and clinopyroxene fractionation.
The Journal of Geology
Rose, William I.; Grant, N. K.; Hahn, G. A.; Lange, I. A.; Powell, J. W.; Easter, J.; and Degraff, J. M., "The evolution of Santa María volcano, Guatemala" (1977). Department of Geological and Mining Engineering and Sciences Publications. 164.