Geology and Mineral Resources of Paraguay A Reconnaissance

IGNEOUS AND METAMORPHIC ROCKS 39came pile itself. The shape may be due to erosion ofa single volcanic cone, but it seems more likely to havebeen formed as a series of more or less coalesced cones,since dimly seen flow structures along the rim appearto be related to individual cones. No ash or cinderswere seen, but in at least two places along the Acahayroad there are exposures of red to black scoria.The plains east and northeast of the volcano aredotted with smaller cones and a number of dikes canbe seen on aerial photographs.The age of the Acahay volcanism is not known, but itmust be relatively young. The Acahay volcanic pileitself, as well as the smaller nearby cones, are so typicalin shape that they could not have been greatly eroded.Moreover, except for a little soil that supports smallfarms in the crater of the volcano and along its outerslopes, all the volcanic rocks seen in this area are veryfresh, even at the surface.Another bit of evidence that strongly indicates theyouthfulness of the volcanism is that the flows andcones are related to the present topography in valleysor on broad plains that could hardly have survivedsince Triassic time.Harrington (1950, p. 43) refers briefly to this occurrencein his description of the "necks and small plutonsin the Carapegua-Ybycui-Quiindy district," and notesthe presence of "granular phanerocrystalline faciesgabbros, syenites, or basic diorites" besides the dominantdark olivinc basalt.Three specimens collected from the outer fringes ofthe volcanic cone, along the road between Carapeguaand Acahay, represent three different rock types. All,however, are such as have been observed in manyplaces in connection with basaltic eruptions: basalticrock proper (P-58a), rhyolitic agglomerate (P-58b) andscoriaceous material. The first two may be flows, thethird ejected material.The basalt is illustrated in figure 29. It is a finegrainedbrown rock, with few if any phenocrysts. Microscopically,it is an even textured aggregate of minutepale-green pyroxene and opaque iron oxide in a groundmassof sodic plagioclase, with a few porphyritic iddingsitizedbrown olivine crystals. Some of the groundmassmay be glass. It resembles the Serra Geral basalt fromthe Foz de Iguacii, Brazil, referred to Triassic or Jurassicage.A rhyolitic agglomerate (specimen P-58b) that occursin the vicinity of the Acahay caldera, along the roadFIOUBE 29. !(specimen P-58) from flow just east of the Acahay caldera. Photomicrograph shows uniform fine-grained texture, with sparse alteredolivine (?) hi upper left corner. X 20.