Obsidian balls (marekanite) from Cerro Tijerina, central Nicaragua: petrographic investigations

Abstract

The paper describes a find of volcanic glass in central Nicaragua. We investigated obsidian samples in perlite from a large profile of volcanic rocks (Cerro Tijerina) in the vicinity of the town of Matagalpa. The obsidian occurs in the form of up to 3-cm dark-coloured balls (spheres) randomly distributed in brittle grey perlite, which also has spherulitic texture. These balls, previously called marekanite, are characterised by an abundance of microphenocrysts (feldspar, biotite, amphibole, apatite and magnetite). Microprobe analysis of the homogenous glass of obsidian balls and perlite has confirmed their subalkaline, and particularly high-K calc-alkaline character (SiO 2 ≥ 70 %, Na 2 O + K 2 O < Al 2 O 3 ), which is typical of rhyolitic melts. A sharp contact was observed between the core and the surface layer (approx. 0.1-0.2 mm) of the obsidian balls, reflecting changes in the H 2 O content.

Pantasma: Evidence for a Pleistocene circa 14 km diameter impact crater in Nicaragua

“…We note that only two other occurrences of obsidian have been described in Nicaragua, despite intensive investigation by archeologists (see review in Aggen ). These occurrences (toward the south of the site described in Mrazova and Gadas ) correspond to a few loose gravels recovered. Overall, obsidian appears to be rare in Matagalpa Group volcanic rocks.…”

“…Na 2 O+K 2 O versus SiO 2 plot for Pantasma dry glass samples P1–P3 (circles), rocks, P4 glass, and soil (black, blue, and brown diamonds), as well as Mrazova and Gadas () obsidian (red squares; maximum and minimum values). For the glasses, ICPMS data are shown as a big circle, whereas SEM EDS data are shown as a small circle.…”

“…1a) (Ehrenborg 1996;Arengi and Hodgson 2000;Elming et al 2001) show that the structure formed in Oligocene volcanic flows of the Matagalpa Group, which is estimated to range from 0.3 to >1.5 km thick and is dated to between 25 and 32 Ma (Elming et al 2001). Bedrock compositions range from andesite to rhyolite, and lithologies include lava flows and ignimbrites (welded tuffs, volcanoclastic layers) interbedded with fluvial sediments, including rare occurrence of obsidian (Mrazova and Gadas 2011). The overlying Miocene to early Pleistocene age (based on map relations) Coyol Group contains more basaltic flows and is exposed south of the Pantasma structure (Fig.…”

“…). Bedrock compositions range from andesite to rhyolite, and lithologies include lava flows and ignimbrites (welded tuffs, volcanoclastic layers) interbedded with fluvial sediments, including rare occurrence of obsidian (Mrazova and Gadas ). The overlying Miocene to early Pleistocene age (based on map relations) Coyol Group contains more basaltic flows and is exposed south of the Pantasma structure (Fig.…”

“…While P1-P3 are more or less layered compact glass, with masses from 3.6 to 64 g, P4 is a larger angular fractured cobble (circa 200 g) with a clear perlitic structure. Obsidian gravels were described by Mrazova and Gadas (2011) in a river 40 km south of Pantasma, derived from local outcrops of Matagalpa Group ignimbrites. We note that only two other occurrences of obsidian have been described in Nicaragua, despite intensive investigation by archeologists (see review in Aggen 2007).…”