PHYSICAL AND CHEMICAL WEATHERING IN A COLD-SEMIARID, TECTONICALLY ACTIVE PROGLACIAL SYSTEM IN NORTHERN PERU

Sediments derived from granodiorites within the Cordillera Blanca Batholith, northern Peru were studied to evaluate the effects of climate, transport, and tectonics on sediment texture, mineralogy, and geochemistry in cold-arid, proglacial systems. We collected fluvial sediment every 500 m along two proglacial streams (Rio Parón and Llanganuco), which cross a major detachment fault, and sampled proglacial lake sediment. Sediment samples were sieved to separate the gravel (>2 mm), sand (63µm – 2 mm), and mud (<63 µm) fractions. Petrographic analyses of bedrock and sand thin sections yielded mineralogy and textural data. The mud fraction was further analyzed to determine Brunauer-Emmett-Teller (BET) surface area measurement, X-ray diffraction (XRD) mineralogy, and bulk chemistry.

The fluvial and lake sediments are predominantly mud (Rio Parón 3.6-97 %) and sand (Llanganuco 2.96-76.5 %). The bedrock contains plagioclase, quartz, K-feldspar, biotite, hornblende, and accessory muscovite, sphene and apatite. The Rio Parón sand fraction shows evidence of plagioclase alteration to sericite, supported by higher concentrations of muscovite observed in the mud fraction compared to bedrock analyses. Chlorite was also observed in muds from Llanganuco, which is absent in the bedrock, suggesting biotite alteration to chlorite. XRD analysis of sediments showed similar concentrations of quartz, plagioclase and K-feldspar in the bedrock and sediments from the two transects. Chemical weathering is further recorded in the REE chemistry; the mud fraction displays a negative Eu anomaly, which is absent in the bedrock, suggesting preferential weathering of plagioclase. While Chemical Index of Alteration (CIA) values in the two transects indicate relatively weak weathering (Bedrock = 60, Rio Parón transect = 61 to 63, Llanganuco = 62 to 68), the mud fraction displays a clear weathering trend in the A-CN-K diagram departing from bedrock composition. Both CIA and BET surface area values decrease within and beyond the detachment fault zone at the distal end of the transects, indicating the introduction of fresh material associated with tectonic fracturing. Despite the generally cold and arid climatic conditions, glacial grinding produces voluminous fines that promote chemical weathering in this system.