Composition of modem stream sand derived from a mixture of sedimentary andmetamorphic source rocks (Henares River, Central Spain)

The Henares River, central Spain, flows westward from the Iberian Range (Me sozoic sedimentary rocks) under semiarid climatic conditions. In the middle and lower reaches, the Henares River receives sediment from three tributar ies (Canamares, Bornova and Sorbe rivers) that drain the Central System (Pa leozoic crystalline rocks, low-grade metamorphic rocks, and minor amounts o f Mesozoic sedimentary rocks). Modern sands from the Henares River and its tributaries offer an excellent opportunity to evaluate the importance of li thology and physiography in determining detrital modes from mixed metamorph ic and sedimentary source terrain. Sand modes from the Henares River and it s tributaries are quartzolithic. They plot in a restricted area on a QmFLt diagram, with low contents of feldspar and variable amounts of quartz, and carbonate, metamorphic and minor siliciclastic lithic fragments. Higher fel dspar abundances in the Canamares and Bornova stream sands are related to t he erosion of gneissic rocks. By contrast, sands from Sorbe River are low i n feldspar, reflecting the absence of coarse metamorphic sources. Thus, pro portions of bedrock lithologies in the drainage sub-basins are the main con trol on detrital modes of the tributaries. In addition, slope acts on the s and productivity of source rocks. Sedimentary source rocks in the upper rea ches of the three tributaries have a poor productivity in the carbonate lit hic fragments (Lsc), compared with the Lsc productivity of sedimentary sour ces located downstream, with higher relief. The percentage of bedrock types in the source area versus petrographic indices (Lm/L, Lss/L and Lsc/L) pro vide a useful contrast between source and sand composition in the tributari es. Thus, the high content of metamorphic lithic grains in the lithic grain population over-represents this lithology (slate plus schist) at the sourc e terrain. Lithic grains from sedimentary elastic sources generate few reco gnizable grains (Lss) in the lithic grain population. Local supplies from c arbonate sources may produce important increases of Lsc in short reaches of the channels. Sand compositions in the Henares River seem to be very homog eneous in a QmFLt diagram, with only minor differences caused by the suppli es from the tributaries. These differences can be modeled in terms of end-m ember mixing processes between Henares detritus and detritus from the relev ant tributary. These mixing processes are more evident when lithe grain con tents (LmLvLs and LmLssLse diagrams) are compared. Changes produced by inpu ts from the Canamares, Bornova and Sorbe rivers are not permanent along the Henares course. An important homogenization of Henares sand composition ta kes place by the mixing of tributary deposits with previously deposited Hen ares River terraces. Abrasion during transport does not appear to cause sig nificant changes in the sand composition along the Henares River. (C) 2000 Elsevier Science B.V. All rights reserved.