A "FOSSIL" OF 2.5 GA WEATHERING FROM PRONTO PALEOSOL

Weathering processes in the late Archean to early Proterozoic remain unknown because the ancient weathering profiles were altered chemically and mineralogically by later diagenesis and metamorphism. We examined chlorite in the weathering profile and parent granite in Pronto, Canada by field-emission scanning electron microscopy and high-resolution transmission electron microscopy (HRTEM). In parent granite, biotite was completely altered to chlorite that is not interstratified with biotite as observed by HRTEM. In slightly weathered granite, where chlorite and partly sericitized plagioclase seem texturally the same as those in the parent granite, we found biotite layers are inserted in some chlorite grains. The chlorite does not contain K as revealed by energy dispersive X-ray analysis (EDX) because of few occurrence of such biotite layers in chlorite. We, then, observed intermediately weathered granite where quartz and K-feldspar are almost intact but chlorite and plagioclase are replaced by sericite. Chlorite grains of a few tens of micrometers in size, cut by sericite, are found in the sericite matrix. HRTEM and EDX revealed that the chlorite contains K and are interstratified with biotite. The above microstructural changes of chlorite cannot be explained by chloritization after weathering. Our results strongly suggest that chloritization of biotite of parent granite occurred prior to weathering, the newly formed chlorite was partly dissolved and interstratified with secondary vermiculite (Ito et al., 2000) during weathering, and the vermiculite layers in chlorite were altered to biotite by supplied with K during K-metasomatism (hydrothermal alteration) that formed sericite.