Geology and Mineral Resources of Paraguay A Reconnaissance

IGNEOUS AND METAMORPHIC ROCKS' 37age by Carnier (191 Ic) as part of the Serra Geraleruptive rocks of Triassic age by Gerth (1935, pt. 2,p. 244) and as Triassic (Rhaetic) by Oliveira andLeonardos (1943, p. 99). The apparent youthfulnessof the cone strongly suggests a late Tertiary age.Carnier (1913) describes the Pao de Acucar asaugite syenite. Lisboa (1909, p. 51-53), however,says that it is made up principally of nepheline syenite(foyaite) with lesser amounts of augite syenite, bostonite,and phonolite. He characterized the mass as arelatively young volcano, but dates it only as post-Permian. He says that the nepheline syenite, whichforms the major part of the body, has a granitoidtexture and is made up mainly of orthoclase, nepheline,and acmite, which is nearly opaque and without crystalform. The augite syenite, which Lisboa says formsthe north part of the mass and extends into Paraguay,is coarse-grained, with anorthoclase the only feldspar.Biotite, with some acmite and magnetite, is abundant,and sphene is a very abundant accessory. Blackpyroxenes give the rock a porphyroid appearance. Henotes that this differs materially from the rock describedby Evans (1894) from the same locality. That rockcontained both orthoclase and plagioclase, as well ashornblende, augite, apatite, and sphene. Lisboa'sphonolite is a nearly black rock with fluidal texture.It contains large phenocrysts of orthoclase and rareones of nepheline and acmite in a fine-grained tospherulitic groundmass of the same minerals. Thebostonite is very fine grained, with trachytic texture.It consists of a microcrystalline aggregate of orthoclaseand prisms of pyroxene that are partly altered tomagnetite. Treatment with hydrochloric acid producesthe gelatinization characteristic of nepheline.There is confusion in some bibliographies betweenthis Pao de Agucar and a mountain of the same namein Departamento de Maldonado of southeasternUruguay; the latter is remarkably similar in compositionto the Brazilian Pao de Acucar on the banks ofthe Rio Paraguay (Willmann, 1915).CENTRAL AREAMilch (1895) describes limburgite from Cerro Tacumbu;an abstract of his description with analysis(Milch 1905) is given in the section of this report onthe Serra Geral lavas (p. 33) but the rock may possiblybelong with the alkalic rocks.NEPHELINE BASALT AT YBYTYMfAccording to Hibsch (1891), the top of Cerro Ybytymi,south of the town of that name, is made up of nephelinebasalt. The exact location of this hill is not known,but the rock is doubtless a part of, or closely related to,the elongate body of alkalic volcanic rocks shown onplate 1 as Cerro Apitagua. The mapping of thisrelatively large body, incidentally, is based on interpretationof aerial photographs and on the twodescriptions by Hibsch and Milch. The igneous rocksmay cover less of the area than shown on the map,but the entire area appears to be nearly homogeneouson the photographs. The rock described by Hibsch isdark gray to black, with brown weathered surfaces;it is porphyritic, with phenocrysts of augite and cavitiesfilled with zeolites with radial structure. Microscopically,it is seen to be made up of phenocrysts ofaugite and olivine in a groundmass that containsgrains of magnetite, small crystals of augite andmagnesian mica, nepheline, and sanidine.PHONOLITE FROM SAPTJCAIA sample of phonolite described by Milch (1895) camefrom a railroad cut through a ridge at or near the townof Sapucai. The size or character of the rock bodyfrom which it came is not known, but it is almostcertainly related to the large body of alkalic rocksthat make up Cerro Apitagua and vicinity. Milchdescribes the rock as yellow brown, compact, andmarked by phenocrysts of a dark mineral in thickprisms. Biotite, hornblende, and aegirine-augite alloccur as phenocrysts, but they are much less abundantthan sanidine, a member of the hauyne-nosean family,and nepheline. All the phenocrysts are obscured byalteration weathering, resorption, and zeolitization.The groundmass consists of aegirine-augite, with someaegirine, small prisms of feldspar, small grains of ore;and an aggregate of decomposition products that includezeolites, carbonate, and iron ore. The alkalifeldspar sanidine is the chief constituent of the groundmass.It appears as long prisms which ramify astrichites. This structure, together with the almostcomplete zeolitization of the nepheline, gives a trachytoidappearance to the rock.In 1905 Milch published an analysis of this rock,(see p. 47, table 5, D). Washington (1917, p. 905)lists the rock as altered by weathering, a reasonableinference from the nearly 4 percent H2O, and 1 % percentCO2 . Milch likewise mentions the weathering ofthis rock. Nevertheless, a norm has been computed,which (1-6-2-4) places the rock with other nephelinesyenites and phonolites in Washington's tables.MICA PORPHYRY FROM EAST OF VILLARRICAThe mica porphyry (specimen P- 50) occurs as a dike,4 meters thick, trending N. 25° E. in sandstone nearthe base of the Independencia series; locally it spreadsout as a thin sill along a bedding plane in the sandstone.It does not resemble any of the other specimens fromParaguay, but because of the highly alkalic composition