PETROLOGY

Geochemistry

Sixteen samples from Tacaná volcano were chosen for chemical analysis. They were analyzed for major and trace elements by XRF (X-ray fluorescence) following the method of Rose et al. (1986), with a precision of ±1% for major elements and ±5% for trace elements. Accuracy is comparable to other standard techniques.

The volcanic rocks of Tacaná are calc-alkalic, typical for Central America. They are medium-K, according to Gill classification.

AFM graph of Tacaná's extrusive products (lithics, lavas, and ash).

AN-CI graph of the extrusive products of Tacaná. These products are broadly andesitic.

Normative An-Ab'-Or graph of Tacaná's extrusive products.

Graph of the classification of Tacaná andesites. The volcanic rocks of Tacaná are medium-K enriched.

All rock samples are of intermediate silica concentration, with SiO2 ranging from 58 to 64 weight percent. All samples contain normative quartz. Harker variation plots of SiO2 vs. major oxides show that Tacaná's rocks exhibit coherent trends and lack wide variations. K2O and Na2O depict a positive correlation with SiO2, while all other oxides show negative trends.

Mercado and Rose (1991) compared Tacaná's andesites with other Guatemalan and Mexican volcanoes, using data from Tajumulco, Tolimán, Santa María-Santiaguito, Nicolas Ruiz, and El Chichón. Rocks from all these volcanoes are broadly andesitic and differ mainly in the degree of enrichment of K and other incompatible elements. Tacaná andesites show an average K enrichment similar to Tolimán, higher than Santa Marí:a and Nicolas Ruiz, but lower than Tajumulco or El Chicón.

The K/Zr ratios of all these volcanoes range from 75 to 125. K/Rb ratios range from 250 to 500. Santa María is greatly depleted in Rb compared to Tacaná. Nicolas Ruiz and El Chichón are slightly enriched in Rb relative to Tacaná. Zr/Rb plots separate the Mexican volcanoes (Nicholas Ruiz and El Chichón) from Tacaná and other Guatemalan volcanoes.

Chemical composition of Tacaná ashes is shown in the following table.

Chemical analysis of volcanic ashes from Tacaná.

Airfall ash from La Haciendita is nearly compositionally identical to the Tacaná lavas. This indicates that andesitic plinian activity has occurred at Tacaná. Samples from thinner distal sites thought to be correlative are altered and show a pattern of losses and enrichment consistent with glass alteration to montmorillonite.

Mercado and Rose (1991) conclude that Tacaná's recent eruptives (lava and ashes) are dominantly silicic calc-alkalic andesites with normative quartz. Chemistry of the rocks shows a moderate K enrichment, similar to other Guatemalan volcanoes found along the northern side of the volcanic front. The chemistry and mineralogy of Tacaná's lavas resemble those of the Guatemalan volcanic front more than the Chiapas Volcanic belt.

Click here for Mineralogy of Tacaná's eruptive products

(Mercado and Rose, 1992)