Long-term evolution of Volcán de Santa María, Guatemala from 40Ar/39Ar geochronology, paleomagnetic stratigraphy, and geochemsitry

Paleomagnetic directions and whole-rock geochemical compositions of 22 basaltic andesite lava flows, coupled with precise 40Ar/39Ar ages determined for 13 of these flows exposed in the crater wall formed during the 1902 eruption reveal a strongly episodic growth history for the upper half of Volcán de Santa María. Results from forty-four 40Ar/39Ar incremental heating experiments indicate that the main cone grew in two rapid pulses at 72.1 ± 4.6 and 35.6 ± 2.0 ka. During the intervening period, a much smaller volume of flows erupted between 60.2 ± 8.1 and 46.0 ± 5.4 ka. Consistent with these three distinct time periods, paleomagnetic directions obtained from the same 22 flows also are clustered, indicating rapid emplacement of flows during the two main pulses. New major and trace element XRF data from the basaltic andesites also fall into three distinctive groups that are consistent with the radioisotopic and paleomagnetic data. These lavas evolved in step-wise fashion from 53% to 55% SiO2 and 0.7 to 1.2% K2O. Modeling of the volume preserved indicates that the uppermost 4.0 km3 of the 8.5 km3 Santa María edifice erupted predominantly in pulses of 2.0 km3 at both 72.1 ± 4.6 and 35.6 ± 2.0 ka. An order of magnitude less magma, 0.2 km3, leaked out during the intervening period. Following a period of greatly subdued activity or repose of ca. 35 ka the volcano re-awakened violently in 1902 with Plinian eruption of about 10 km3 of crystal-rich plag-hbl-px-mt phyric dacite deposited as pumice and ashfall over 106 km2. The 1902 dacite contains 65.3 to 68.1 % SiO2, 1.6 to 2.0 % K2O, but also comprises a small percentage of basaltic andesite similar to that which formed the preceding cone. In 1922 the Santiaguito dome complex began to form in the 1902 collapse crater with intermittent eruptions continuing to the present. The dome complex comprises 1 km3 of plag-hbl-px-mt-phyric dacite with 62.5 to 65.0 % SiO2 and 1.5 to 1.7% K2O. Since 72.1 ka the overall extrusion rate is 0.2 km3/kyr, but the 40Ar/39Ar age determinations indicate that extrusions during at least two brief intervals of not more than a few thousand years each occurred at more than twice the average value. The change from basaltic andesite to dacite at 1902 marks an abrupt shift in the behavior of the volcanic system. Episodic, but decellerating, growth of the basaltic andesitic stratovolcano was finally punctuated by growth of a much more silicic magma body at shallow depth. During the last 34 ka an explosive subvolcanic reservoir accumulated and then erupted. Despite the exceptionally large volume of the 1902 dacitic eruption, the overall and peak eruptive flux rates for the Santa María stratocone over the past 75 kyr fall at the lower end of those determined for several other arc volcanoes in the southern Andes, Cascades, and Aleutians that have not experienced such large eruptions. Considering the state of eruptive activity and flux rates during the historical period (~500 yrs) of other volcanoes in the Guatemalan volcanic front, recognition of the different stages of evolution of Volcán de Santa María, suggests possible differences in the evolutionary state of these volcanic systems and may have important implications for hazards.