Geology and Mineral Resources of Paraguay A Reconnaissance

36 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYdespite the variation in names, the rocks are essentiallysimilar. The diabase porphyrite (proterobase) fromTagaruassu in Mato Grosso, Brazil, described by Goldschlag(1913a, p. 25) is also similar. The Acahay basalt(specimen P-58a), however, is chemically more relatedto the group of alkalic rocks discussed below. Further,both the shonkinite from Mbocayaty, and the phonolitefrom Centurion, are chemically similar to the fivebasaltic rocks. The spectrographic analysis (table 4)appears to confirm this relationship.All these rocks (and the alkalic types describedbelow) differ both in major and minor chemical relationshipsfrom the granitic-porphyritic Precambrianrocks. Whether any valid conclusions as to relation ofchemical similarities to age of the rocks are possiblecannot be said from the data at hand.ALKALIC ROCKS OF UNKNOWN AGEAlkalic rocks have been reported from many localitiesin Paraguay and nearby Brazil. One body of theserocks at Mbocayaty, was studied by Eckel and anumber of specimens from it have been analyzed(specimens P 54). A second locality of possibly alkalicrock that was visited is a mica-porphyry dike 10 mileseast of Villarrica (specimen P-50).The Acahay volcanic mass and a group of porphyriesand agglomerates from near Ypacarai are also describedhere, though from the scanty evidence at hand thesebodies appear to be complex and not all of the rocksassociated with them are alkalic.From the available data (Pohlmann, 1886; Carnier,1911, 1913; Goldschlag, 1913), it appears that atCenturion in the northern Rio Apa region and atMbocayaty, in the central region, there are verysimilar occurrences of alkalic rocks. Likewise, similarrocks occur at Cerro Ybytymi, part of the CerroApitagua (Hibsch, 1891), near Sapucai (Milch, 1895),and the Pao de Agucar region in Brazil, and adjacentParaguay (Lisboa, 1909; Carnier, 1913; Gerth, 1935;Oliveira and Leonardos, 1943).Data are not at hand for any satisfactory discussionof the alkalic rocks of Paraguay. The few analysesavailable leave uncertain what relationship may existbetween the undoubted alkalic rock types and thebasaltic rocks of Paraguay.The age of the alkalic rocks is not known. Someseem to be closely related geographically and petrographicallyto the Serra Geral lavas of Triassic orJurassic age and some of particularly youthful appearance,as parts of the Acahay body and the one atPao de Agucar, Brazil, are no older than Late Tertiary.All the alkalic rocks are shown on plate 1 as "ageunknown, possibly Triassic through Tertiary."NORTHERN AREAPHONOLITE AT CENTURIONGoldschlag (1913a) describes phonolite from theRio Apa region, 2-3 kilometers west-southwest ofCenturion. This probably is the "felsite porphyry"mentioned by Carnier (19lie) as occurring in thisgeneral vicinity. According to Carnier, dikes ofsyenite and felsite cut muscovite schist, metamorphicsandstone, and quartzite. This is the only reportedoccurrence of alkalic rocks in the Rio Apa region exceptfor the nepheline basalt and olivine kersantite describedby Pohlmann (1886).Goldschlag says that the phonolite near Centurionis bluish green, characterized in hand specimen byphenocrysts of pyroxene (as much as 1 centimeter long)and secondary zeolites. Under the microscope phenocrystsof aegirine augite, orthoclase and andesinelabradoritein poorly defined crystals, olivine partlyaltered to serpentine, and biotite are seen. Very smallcrystals of noselite, magnetite, and apatite are dispersedthrough a groundmass composed largely of feldspar.The groundmass is zeolitized; there is also secondarycalcite and part of it appears as well-formed crystals.Goldschlag's analysis of the rock is given in table 5(analysis C). It is evident from comparison withMbocayaty shonkinite (P-54g, table 5, analysis B)that the rock analyzed by Goldschlag is similar andthat the phonolite described by Milch from Sapucai(table 5, analysis D) is also a similar rock. Olivinekersantitefrom the limestone of Colonia Santa Mariadel Apa, was described by Pohlmann (1886); it isassociated with nepheline-basalt. Pohlmann does notdescribe the field relations, and no analysis is available.SYENITIC KOCK OF PAO de AgUCAR, BRAZILThe prominent Pao de Agucar is on the Brazilianside of the Rio Paraguay, just north of P6rto Murtinho,Brazil, and about 35 kilometers north of Puerto PalmaChica, Paraguay. It is included here because smallbodies of rock that appear to be related to it appearin Paraguay. The Pao de Agucar rises nearly 400meters abruptly from the river's edge; it is about5 kilometers in diameter. With steep sides and ajagged top, from the air it appears to be a relativelyyoung complex volcanic cone, not unlike the calderaat Acahay (p. 38), but considerably modified byerosion.Several much smaller and lower hills form theParaguayan bank of the river just southwest of thePao de Agucar. From the air they appear to belong tothe same rock mass, separated from it only by the riveralluvium. This mass of rocks is dated as post-Permianby Lisboa (1909, p. 51-53), as of probable Tertiary