Geology and Mineral Resources of Paraguay A Reconnaissance

IGNEOUS AND METAMORPHIC ROCKS 13and shown on plate 1 as "older Precambrian metamorpliicrocks" whereas the granitic and porphyritic rocksare grouped as "younger Precambrian igneous rocks."SOUTHERN AREAOne of the two large bodies of Precambrian rocks isin the southern part of the country between the latitudesof Carapegua and San Juan Bautista. The mappingof this area (pi. 1) is based in large part on EckePs fieldobservations, supplemented by Harrington's map (1950)and by interpretation of aerial photographs.OLDER METAMORPHIC ROCKSThe eastward-trending belt of older Precambrianrocks between San Juan Bautista (pi. 1) arid VillaFlorida is very poorly exposed in most places, and itscharacter is known only in a general way. The northerncontact of this belt with the younger granitic rocks iscovered in all places examined, but is thought to besomewhere near the Eio Tebicuary at Villa Florida, asshown on the map. Along the ridge west of San JuanBautista the Triassic sedimentary rocks south of thebelt rest uri conformably on the metamorphic rocks,with a thick basal conglomerate. This same relationshippossibly exists along the entire contact, but theapparent absence of basal conglomerate just north ofSan Juan Bautista and strong shearing within themetamorphic rocks near San Miguel indicate that partof the contact between Triassic and Precambrian rocksmay be a fault.The best exposures are along the highway from SanMiguel to a point about 6 kilometers north of that town.At and near San Miguel most of the rocks consist ofwhite to gray glassy quartzite with a little feldspar,most of which has been intensely crushed and containsmuch bright-green muscovite along fracture surfaces.The lineation of this crushed material strikes N. 80° E.and dips 45° N. There are also two sets of prominentjoints that strike due north and N. 45° E. and dip 30°E. and 60° NW., respectively. The quartzite is conspicuousbecause of its bright-green mica. Small flakesof graphite are fairly abundant in the specimens athand, and numerous grains of rutile are seen under themicroscope.From San Miguel northward toward Villa Floridathe rocks are fairly well exoosed in roadside ditches(fig. 5). They consist of a wide variety of rocks inparallel bands that trend N. 40° E. and dip northwestat angles of 30° to 60°. Nearly all the rocks arecrushed and their constituent minerals are strung outalong the direction of the bands so that they resembleflow banding in glassy volcanic rocks. The rocksrange from granite, aplite, and pegmatitic quartzFIGURE 5. Precambrian rocks exposed in a gully along the highway just north of San Miguel. The weathering profile is distinct. The rocksare mostly granite and aplite, but also contain some gneiss, schist, and talc, the latter probably derived from basic rocks.