Geology and Mineral Resources of Paraguay A Reconnaissance

IGNEOUS AND METAMORPHIC ROCKS 43agglomerate contains inclusions of sedimentary rocksand shows considerable evidence of replacement of sedimentsby igneous material; one would therefore expectgreat variability in chemical composition from onesample to another.The brown phase of the porphyry (specimen P-35)is a dark-brown dense rock, with red (not whitishgray)feldspathic and glassy quartz areas. It is fragmental,as seen microscopically, though this is not obviousin hand specimens. The photomicrograph (fig.32) shows an elongated fragment of very fine-grainedrock. The irregular dark areas are chloritized hornblende.Several embayed quartz phenocrysts may beseen, as well as flow structure in the rhyolite. Nearthe center are several strongly sericitized plagioclasephenocrysts (gray).The brown, relatively fine grained rock, such asshown in figure 32, grades into much coarser volcanicbreccia, with strongly contrasting fragments of finegrained,light- to dark-gray sandstone or siltstone,cemented by quartz rhyolite porphyry (specimen P-36,fig. 33).The dark phase (specimen P-34) of the porphyry isfine grained dark gray with whitish-gray feldspar andglassy quartz. Microscopically, it is seen to consistof andesine and quartz, with some chloritized biotitein a felsic groundmass. The feldspars are only slightlysericitized. The analysis of this rock is given in table 1.It is illustrated in figure 34.BASALT FLOW BETWEEN PARAGTJARI AND CAEAPEGuAThe broad grassy flat plains drained by ArroyoCaanabe, which heads not far from Ybytymi and flowswestward to the swampy land north of Lago Ypoa, isshown on figure 2 as underlain by basaltic flow rocks.This designation is based on observation of only oneexposure, supported to some extent by an interpretationof physiographic forms as seen in the field and inaerial photographs.Near a bridge on the main road between Paraguariand Carapegua, and about 4 kilometers northeast ofthe latter town, there is an extensive shallow quarryin basalt. In that vicinity the loamy cover of the plainis underlain at depths of 60 to 130 centimeters by alayer of basalt that is at least 5 meters thick. Locallythe top of this flow rock is dull red and scoriaceous, butmost of the rock is dense, dark gray to black, andcryptocrystalline.FIGUEE 33. Porphyry (specimen P-36) from highway quarry at Ypacaral. Photomicrograph shows rounded resorbed shate fragments, sh, and quartz, q,in rhyolite porphyry. X 20.