"Rock of the Month # 235, posted for January 2021" ---

Pico de Orizaba

The Pico de Orizaba (elevation 5,700 m) is the highest volcano in Mexico, and the third-highest point in North America (after Mount Logan in the Yukon and Denali [Mount McKinley] in Alaska). The Nahuatl (Aztec) name for Orizaba is Citlaltepetl, meaning "Star Mountain". Orizaba lies on the border of Puebla and Veracruz states. It is situated east of Mexico City and the state capital, Puebla, and southwest of the beautiful city of Jalapa. Orizaba is the highest point of the Trans-Mexican Volcanic Belt: heading west, one comes to the volcanoes of La Malinche (4,461 m, northeast of Puebla), Popocatepetl (5,452 m) and Ixtaccihuatl (5,286 m), and - on the south side of Mexico City, the Chichinautzin volcanic field. North of Orizaba rises the Cofre de Perote (4,282 m). On a clear day one can see from the summit of Orizaba, across Veracruz state, to the Caribbean Sea, the Gulf of Mexico.

The high peak and its volcanic crater have attracted a lengthy history of exploration and research. This work was historically dominated by German scientists, as well as French, Belgian, Swiss, American and other visitors, since 1804, when Humboldt explored the area north of the summit (Crausaz, 1986). Sulphur was mined on Orizaba, 1875- 1910 (Crausaz, 1987). The crust in the Orizaba region is quite thick, capped by tall stratovolcanoes (Urrutia-Fucugauchi et al., 1995). A range of dating studies have been conducted on the regional volcanic suites (e.g., Cantagrel and Robin, 1979; Hoskuldsson and Robin, 1993). The regional volcanism (including peaks west of Mexico City, such as Nevado de Toluca and Nevado de Colima) began some 10 Ma (million years ago), with the formation of Ixtaccihuatl and the first cone of Popocatepetl. The present cones of the three highest peaks were formed in a second cycle beginning circa 2.5 Ma (Secor, 1981). Thousands of years into the past, major eruptions have occurred and muddy lahar deposits were emplaced along channels (Carrasco-Núñez et al., 1992; Carrasco-Núñez, 1994). In the late Pleistocene, mountain-top glaciers and ice caps were more extensive than today, and water from eruption-melted glaciers would promote hydrothermal alteration, weakening the summit of the volcano. Alteration and consequent debris flows have been mapped by remote sensing (Hubbard et al., 1998). Romero (1991) has reviewed the structure and stratigraphy of the region, including the nearby geothermal field of Los Humeros. While modern volcanoes are often wreathed in vapours, with at least a trace of sulphur in the air, the principal natural hazards in their vicinity involve edifice collapse, eruptions and their slow to fast-moving products, such as lava flows, pyroclastic flows (ignimbrites), landslides and lahars. The last historic eruption of Orizaba was in 1687 (Medina, 1980; see also Zimbelman et al., 2004).

Three samples are illustrated here (Figs. 1-2). The evolution of the mountain, in its present form, is geologically very young, starting some 1.5 million years ago and developing in 3 stages. The third stage began around 13,000 BP, and samples of this phase have been described from above 4,500 m elevation (Jackson et al., 1984).

Andesite

General:

Andesite is one of the most important and widely-distributed varieties of volcanic rock. It can be defined as follows (Le Maitre et al., 1989, p.46): "An intermediate volcanic rock, usually porphyritic, consisting of plagioclase (frequently zoned from labradorite to oligoclase), pyroxene, hornblende, and/or biotite". The glossary entry extends the definition by aspects both petrographic (defining modal proportions of the essential minerals) and geochemical (a field on a plot of total alkalies versus silica). Andesites are named for the Andes mountain chain of western South America, and typify volcanic arcs around subduction zones, as around the Pacific "Ring of Fire". Far older examples are found in volcanic sequences back into the Archean, which may be interpreted as evidence of a relatively early start to plate tectonic mechanisms in our internally hot and still-active planet.

See also two other examples of andesite, from Poas volcano in Costa Rica (with a beautiful example of basaltic hornblende with oxidized rim) and veined and mineralized lava from Guanajuato in central Mexico .

Orizaba:

The most common rock type appears to be andesite. Some samples (e.g., Orizaba sample 303, Fig. 1a) are volcanic breccias, preserving magmas of contrasting compositions. A thin section of Orizaba sample 302 (Fig. 2) reveals an estimated mode (volume percent) of 50% glass plus tiny crystallites, 44% plagioclase, 2% clinopyroxene, 2% orthopyroxene, 1% opaque grains (all or mainly magnetite?) and 1% amphibole. The latter is basaltic hornblende (lamprobolite) with dark oxidized rims. The plagioclase displays at least four twin laws, is often strongly zoned, and the composition is estimated optically at An₄₄ (andesine). Orizaba sample 304 (Figs. 1b,c) is a hornblende-phyric andesite, with lustrous black hornblende prisms up to 10 mm long. Estimated mode is 50% plagioclase, 44% glass, 6% basaltic hornblende, with an abundant trace of clinopyroxene, and scattered opaque (Fe oxide) grains <0.1 mm in size (more detailed studies of the volcano have also noted Cr spinel). Andesites and other volcanic rocks such as dacite and rhyodacite compose the volcanoes of the Belt (e.g., Jackson et al., 1984; Kudo et al., 1985). Volcanic glasses such as obsidian are also known in the region (Crausaz, 1987; Barca et al., 2019), as well as basalt, hawaiite, basaltic andesite, rhyolite, shoshonite and pyroclastic rocks (Negendank et al., 1985; Romero, 1991). Some of the lavas contain relatively “primitive”, high-temperature phases such as olivine and Cr spinel. Magnesium-rich material may be xenocrysts, remnants of lower crustal rocks assimilated by the rising magmas (Singer and Kudo, 1986). Upper crustal contamination is also indicated, by fused calc-silicate and diorite xenoliths, common in some andesite flows (Singer et al., 1987).

The magnetic susceptibility of the suite (measured using a ZH Instruments SM-30 model with a 50-mm coil) is about 6-8 (303), 7-11 (302) and 11-15 (304) x10^-3 SI units, uncorrected for sample size and geometry. This is weakly but distinctly magnetic, consistent with the likely identity of most of the opaque accessory grains as magnetite and related iron oxides.

Fig. 2a,b,c,d: Sample 302 is from the wind-scoured summit of the peak at 5,700 m (18,700 feet) elevation, and is presumed to come from the third and latest phase of volcanism, beginning around 13,000 BP. Gleaned from outcrop, loose ("effectively in situ"). Grey, feldsparphyric andesite, with pale felsic cognate xenoliths (fine-grained diorite? - one example is seen at the upper left - rock surface wetted to improve contrast). These blebs of first-formed lava, entrained by and preserved within the andesite, are holocrystalline, and up to 1 cm in diameter. The sample is fresh, with abundant 1-4 mm feldspar phenocrysts, sparse black pyroxene, and minor ironstaining on the exterior surface.

A thin section of this sample (lower left) displays the texture and essential mineralogy. Twinned laths of plagioclase feldspar and scattered pyroxene crystals appear like distorted stars in a groundmass of isotropic glass. The feldspar phenocrysts exhibit subparallel alignment, probably a flow texture, acquired as the lava carried the elongate crystals inside it, prior to final cooling. Some of the feldspars have collected into multigranular aggregates, known as glomerocrysts ("glomeroporphyritic texture"). Photo in crossed-polarized, transmitted light, 20X magnification, long-axis field of view 5.7 mm. A second view (lower right) shows an example of brown hornblende with oxidized rims, in an oxide-speckled glassy matrix with the outlines of clear, tabular feldspar crystals. Photo in plane-polarized, transmitted light, 80X magnification, long-axis field of view 1.4 mm.

Some basic petrography was conducted on the samples collected in 1983 (Wilson, 1987a,b), and a suite was subsequently selected for radiocarbon measurements by accelerator mass spectrometry, in samples from California, Colorado and three Mexican volcanoes, the results consistent with high-altitude ¹⁴C production by cosmic rays (Jull et al., 1989).