THE DEVELOPMENT OF UNUSUAL ROCK WEATHERING FEATURES IN GRANODIORITE BOULDERS ON LATE QUATERNARY MORAINES IN THE CORDILLERA BLANCA OF CENTRAL PERU

Differential weathering rates of granodiorite erratics on late Pleistocene and Holocene moraines in the Cordillera Blanca, Peru (8.5-10°S, 77-78° W) have generated unusual rock weathering “posts”. These posts range in height from a few centimeters on early Holocene moraines to between 20 and 25 cm on moraines that date to between 13 and 15 ka, and to between 30 and 35 cm on moraines deposited around 30 ka. Weathering posts that exceed 1 meter in height have been measured on moraines that are more than 200,000 years old. The weathering posts are not significantly different than the host boulders with respect to major element chemistry, but display several petrographic differences. Petrographic observations reveal that post samples contain an interlocking mosaic of quartz (28%), plagioclase (44%) and alkali feldspar (17%), along with secondary biotite, chlorite and sphene. The quartz in the weathering posts shows little weathering and contains a wide range of grain sizes (2-4mm). Plagioclase is characterized by albite twinning and concentric oscillatory zoning, and abundant melt inclusions in the core. The K-feldspar grains (6mm) feature perthite exsolution and few inclusions. Post samples contain the largest biotite grains (1.5-4mm), and these show chloritization on rims extending towards the grain center, however chloritization of biotite decreases away from the posts. Samples from the weathered rock away from posts possess an abundance of quartz (39%) with lesser plagioclase (21%) and K-feldspar (13%). Off-post granite boulder samples also have veins of micro-quartz surrounded by biotite/muscovite (0.2-2mm). These veins appear to flow around larger K-feldspar and plagioclase and are sheared with a preferred orientation. Extending further away from the posts, the mineral grains become increasingly fractured and contain grain boundary vacancies (0.1-0.3mm). Given the differences in texture and modal abundances, the weathering posts suggest the mingling of multiple batches of granitic magma, in which diapirs of less dense magma rise through the host granitic mush.