THE FLUVIAL GEOCHEMISTRY AND DENUDATION RATE OF THE GUAYANA SHIELD INVENEZUELA, COLOMBIA, AND BRAZIL

The Guayana Shield is composed of Early to Mid-Precambrian igneous and metamorphic basement rocks with a quartzitic platform cover. The comp lete absence of limestones and evaporites allows a clear chemical expr ession in the stream data of the primary weathering of the basement in a humid tropical environment. Total erosion rates are extremely slow, similar to 10 m/m.y., with equal contributions from the dissolved and suspended loads. However, the former is largely silica with ratios of Si to total cation equivalents [Si:TZ(+)] ranging to in excess of thr ee. Weathering is ''complete'' to kaolinite and gibbsite, i.e., the en vironment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate. In basins of relatively homogeneous lithology, Rb/Sr isochrons c onstructed from the river data agree with the whole-rock ages from the drainages; thus, all the common, refractory, Rb-containing minerals( K-feldspar, mica) are completely dissolved. The thick, lateritic regol ith that is accumulating as a result of this intense weathering is a c ommon relict feature on other Southern Hemisphere Shields. In the abse nce of active tectonics or greatly accelerated mechanical erosion, the weathering rates of these basement rocks must be quite insensitive to environmental change.