Geomorphological and pedological aspects of epimetamorphic basic intrusive rocks near Lavras , Minas Gerais, Brazil

Abstract

Epimetamorphic basic intrusive rocks of Precambrian age associated with the Lavras Complex occur in the region of Lavras, Minas Gerais, Brazil (CAVALCANTE et aI., 1979). This work deals with the morphologlcal, chemical, physical and mineralogical characteristics of the soil cover that has developed upon these intrusive rocks. According to CAVALCANTE et ai. (1979), the metabasics· of the area are represented by ophitic epimetamorphic gabbros consisting of labradorite in a matrix cf augite. The rock has an ophitic texture, and the labradorite with albite twinning is sometimes saussuritized. Green uralite is seen at the edges of the augite, and the other accessory minerais are opaques, red biotite. apatite, and some quartzo The host rock is a leucocratic gneiss of the Lavras Complex. The soil cover was studied in trenches two to three meters deep (Fig. 1). Pedological analysis has shown that at points 1 and 2 the soils have latosolic characteristics (ROCHA, 1982), that is, they comprise material with high porosity and permeability, with a Munsell color of dark reddish brown (2.5 YR 3/4); soil structure is typically microgranular and friable, with clay content more than 70%. The material is much weathered, predominantly positively charged as a result of the high content of glbbsite In the clay fraction; there is also some kaolinite. In the sand fraction, light minerais (70 to 80%) predominate, followed by heavy minerais (10 to 20%), and finally by magnetic minerais (5 to 10%). At point 3 (Fig. 1), the soil is podzolic (CAMARGO et aI., 1987), red in color (2,5 YR 4/8 - Munsell notation), with well-developed prismatic structure, with Iittle porosity, and clay content less than 50%. The soil here is not as thick as at points 1 and 2 and has a C horizon at a depth of 1.5 m.