PETROLOGY

Petrography and petrology of Cerro Quemado lavas

Cerro Quemado is composed of porphyritic, biotite-bearing hornblende andesite and dacite. The petrography of plug domes and flows is identical, except that domes generally contain more mafic inclusions than do flows. Cerro Quemado rocks possess a disequilibrium phenocryst assemblage that includes plagiocalse, hornblende, titanomagnetite, augite, hypersthene, olivine, and quartz in a pilotaxitic to hyalopilitic groundmass comprised of glass (to 40 volume percent), cryptocrystalline material, plagioclase microlites, titanomagnetite, and sparse granular hornblende augite.

Plagioclase phenocrysts, from 0.3 to 4 mm long, are the most abundant phase in Cerro Quemado lavas, making up to 40 volume percent. Plagioclase commonly exhibits complex zoning, including normals, reverse, patchy and oscillatory zoning, and cellular or sieve-like zones. Two phenocryst populations are observed, An35-45 and An-70-85. Plagioclase microlites (An40-70) are the most common groundmass phase. Rare, subhedral quartz phenocrysts, fractured, embayed and charged with fine needles and clinopyroxene (to 0.1 mm in length) also occur in Cerro Quemado lavas.

Mafic minerals

Amphibole phenocrysts (0.5 mm-2mm in length) make up to 5 volume percent of Cerro Quemado lavas and typically occur as deeply corroded oxyhornblende. Corroded phenocrysts of biotite (0.5 mm to 1 mm) make up to 1-2 volume percent. Augite occurs as subhedral phenocrysts (0.3 mm to 1.0 mm), granular microphenocrysts (0.3 mm <) , and acicular crystals mantling hornblende and quartz. Hypersthene is less common than augite and occurs as phenocrysts ranging from 1 to 2 mm in diameter. Accessory forsteritic olivine (to 2 mm), rarely with skeletal habit, occurs as corroded and embayed phenocrysts with kelyphitic rims. Phenocrysts (<1 mm in diameter) and microphenocrysts of titanomagnetite are ubiquitous throughout Cerro Quemado lavas.

Mafic inclusions

Spheroidal mafic inclusions are abundant in Cerro Quemado lavas and range from 1 mm to more than 30 cm in diameter. They are composed of plagioclase, hornblende, titanomagnetite, and trace amounts of orthpyroxene, clinopyroxene, olivine, and apatite in a matrix of intersertal glass. Acicular plagioclase (averaging about 0.4 mm in the longest dimension) in the compositional range AN60-75 and fresh, green hornblende make up to 25-40 volume percent and 15-30 volume percent, respectively. Equant crystals of titanomagnetite make up to 5% by volume of inclusions. Intersertal glass makes up to 30 volume percent, is commonly clean and unaltered, but dark glass charged with opaque inclusions also occurs. Vesicles are common and range to 15 volume percent.

The petrographic evidence, summarized in the following table, strongly suggests an origin by magma mixing for the Cerro Quemado lavas.

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Petrographic evidence of magma mixing in Cerro Quemado lavas.

Petrology of Cerro Quemado lavas

The rocks of Cerro Quemado are silici andesite adn dacite, with SiO2 in the range of 60-66%. The mafic inclusions observed in Cerro Quemado are mafic andesite with about 56 weight percent SiO2. Cerro Quemado rocks have a calc-alkaline affinity but differ from the nearby suite of Santa María volcano by having higher K2O and lower Zr. Sr isotope values from Cerro Quemado are obut 0.7400, similar to those reported for nearby Santa María and Santiaguito volcanoes.