Geology and Mineral Resources of Paraguay A Reconnaissance

44FJGTJEE 34. Porphyry (specimen P-34), dark phase, from the highway quarry at Ypacarai. Photomicrograph shows partly resorbed quartz crystals, g,and slightly sericitized andesinic plagioclase, /, in rhyolitic groundmass. X 25.No other exposures of this rock are known in theextensive plain along the Arroyo Caanabe, but theuniformity of its virtually flat surface and of the vegetationgrowing on the plain suggests that the entirearea is, as mapped, underlain by basalt that is coveredby a thin mask of silty loam. This conclusion isstrengthened by the fact that several volcanic conesrise from the plain not far east of the Paraguari-Carapegua road; these and the nearby volcanic pilesof Acahay and Cerro Apitagua would seem to be likelysources for lava flows that moved westward down thebroad valley of Arroyo Caanabe to or beyond thelongitude of Yaguaron. No specimens were collectedso that it is impossible to state whether this basalt issimilar to the alkalic basalt (specimen P-58b) (p. 39),that came from closer to the Acahay caldera.NEPHELINE SYENITE AND SHONKINITE AT MBOCAYATYAn interesting assemblage of alkalic shonkinites, verysimilar to those in the Highwood Mountains of Montana,are exposed in a quarry at the northwest edge ofMbocayaty. The quarry is on a small rounded hill,about 25 meters high and 75 meters in diameter, thatstands above the nearly level plain cut on tillites andother sediments of the Tubarao series of Pennsylvania!!age. A fairly large quantity of rock has been quarriedfor road surfacing and rough building stone; most ofthe quarried rock apparently consisted of loose blocksthat had been separated from the main mass by weathering,but some shallow drilling and blasting was alsodone. Figure 35 shows one of the larger quarryopenings.The hill is made up of rocks that differ widely intexture, though all of them are shown by microscopicexamination to be alkalic rocks, such as nephelinesyenite, shonkinite, and closely related types. Themost abundant type is a dark-gray coarse-grained rock,containing clear green crystals of olivine that resemblesgabbro in the hand specimen. There is also muchmaterial that appears to be an intrusive breccia andcontains angular fragments of most of the textural typesfound on the hill. Smaller quantities of a spheruliticphase, and of an exceedingly tough rock made up ofcoarse interlocking crystals of sanidine in a dense blackgroundmass, are also to be seen. The relations of the