Weathering assessment in the Achala Batholith of the Sierra de Comechingones, Córdoba, central Argentina. I: Granite-regolith fractionation

CAMPODONICO, V.A., MARTÍNEZ, J.O., VERDECHIA, S.O., PASQUINI, A.I., DEPETRIS, P.J.

CATENA

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2014 vol. 123 p. 121 - 134

0341-8162

Weathering assessment in a weathering-limited environment is a difficult task because a significant proportion of the regolith (i.e., the original saprock, saprolite and overlying soil) has been removed by tectonically-induced denudation. In such an environment, the remnants of the altered mineral debris must be studied with care in order to attain a meaningful picture of the rate and intensity of ongoing weathering. A monolithologic (i.e., granite), small (~2 km2), and mountainous (~1500 m elevation) drainage basin in the Sierras de Comechingones (31°54´07"S 64°45´28"W ? 31°53´11"S 64°44´16"W, Córdoba, Argentina) was selected as a pilot area to survey the nature of weathering in a weathering-limited erosional setting, and a semiarid climatic regime. Relatively thin coarse-grained regolith and scattered sediment (i.e., fine-grained regolith) that had accumulated in valleys and topographic depressions were analyzed. The most abundant identified clay mineral in the regolith is illite followed by kaolinite >smectite. Smectiteis the only clay mineral clearly associated with weathering.The petrographic observations, CIW, ternary diagrams, multielement plots and statistical analysis (i.e., mass balance calculations) define weathering as incipient, in contrast with the denuded landscape. This apparent controversy can be explained by peneplanation during Gondwana times. Petrographic observations indicate that plagioclase and biotite are the main mineral phases affected by alteration, which is more intense in the fine-grained regolith. Coarse- and fine-grained regoliths are chemically similar, among them and with the country rock, with statistically significant losses of MgO, MnO and P2O5 in the fine fraction. Depletions of trace elements and REE, which are best explained by sorting than by actual rock alteration, are no statistically significant. Ternary diagrams reveal that the masses of Al2O3, CaO, Na2O and K2O were not significantly altered during weathering and transportation into or within the basin, and that regolith samples correspond to a coarse residue relatively enriched in feldspars (and quartz), where the mud-fraction with high clay mineralcontents has been removed by high-energy processes from the drainage basin.