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Traveling south from Chillahuita, near the Tatio geothermal fields, we come across Copacoya, yet another large silicic dome of the APVC. Like its brethren, it takes on the typical “torta” shape with steep sides and a flat top.
Also like its brethren, Copacoya is dacitic in composition and crystal rich (40%+). Mineralogically it contains phenocrysts of plagioclase, quartz, hornblende and some devitrified glass in a glassy matrix. The matrix is much denser than seen at previous domes, due to the lack of microvesiculation within the lava. Mafic enclaves are also seen frequently – more andesitic, finer grained.
There are lines of evidence pointing to this dome being around the same age and close to the source of the Sifon Ignimbrite as the Sifon is greater than 300m think in this area.