Geology and Mineral Resources of Paraguay A Reconnaissance

IGNEOUS AND METAMORPHIC ROCKS 45FIGURE 35. Quarry in alkaline rocks (mostly shonkinite) on the outskirts of Mbocayaty.different textures could not be deciphered on hastyexamination.Different parts of the rock body differ widely in theirdegree of weathering, but there is no apparent relationshipbetween weathering and the different texturaltypes. Some parts are almost completely disintegratedwhereas other parts are extremely fresh. Weatheringtends to proceed along the strong vertical and nearlyhorizontal joints that characterize the body; there issome tendency toward spheroidal weathering in places.There are no indications of the shape of the Mbocayatybody, but it appears most likely to be a plug orsmall stock, intruded into the surrounding beds of theTubarao series, that has suffered multiple injectionsof alkalic magma. No similar rocks were found elsewhereby Eckel, but several of the other small massesof igneous rock shown on plate 1 in the general vicinityof Mbocayaty are reported by highway engineers andothers to be similar in appearance to this material.Seven specimens, respectively P-54, a-g, were studiedand are described in order below. All except specimenP-54b are dark rocks; this one is light colored and resemblesa coarse porphyritic syenite. The others havea gabbroic aspect.P-54a, shonkinite. Specimen P-54a (fig. 36) is a coarsely crystallizedgabbroic-textured rock, with dark mafic patches strewnin feldspathic groundmass. The abundant mafic minerals, moreor less subhedral, are strewn in a groundmass consisting largelyof sanidinic orthoclase. It has replaced nepheline, which is convertedto the hydronephelite aggregates shown in figures 36 to42, inclusive. The replacement appears to be progressive, thefinal stage being the graphic structures shown in figures 36 and38. Apatite and ore grains are in the feldspar. The maficminerals include colorless (in section) olivine, brown biotite,pale-green titanaugite, and titaniferous amphibole related tokataforite. The latter is described in detail (p. 46), in connectionwith specimen P 54g, where it also was noted."Hydronephelite" is a rather ill-defined replacement of nephe­line. It is listed as a species by Dana (6th edition p. 609) andmore recently by Troger (1952). Dana lists it with ranite andspreusteiri. In Maine it replaces sodalite, in Norway, nepheline(Dana). Apparently it consists of natrolite, epiriatrolite, hydrargillite,and diaspore (Thugutt, 1932).P-54b, nepheline syenite. Specimen P-54b (fig. 37) is a rathercoarsely crystallized feldspathic rock, with dark sodic(?) augiteas the chief visible mafic. The feldspar laths are several millimeterslong, well shown on the weathered surface of the rock.The dark constituents are interstitial to the feldspar.Microscopically the rock has the features shown in figure 37 asfollows: coarsely crystallized orthoclase (sanidine?) comprisesthe bulk of the rock. Near the lower right corner a large blackgrain of iron oxide has hydronephelite (on the left of it) withrectangular outlines; it is uniaxial positive, with indices lowerthan feldspar and birefringence slightly higher; hexagonal crosssections show irregular triangular fields under crossed nicols.With it is green aegirine(?) augite (the smaller dark grains).Another large augite is seen at the middle right edge. Small