Major element, REE, and other trace element behavior in amphibolite weathering under semiarid conditions in southern India

A body of komatiitic amphibolite, an enclave within the Archean high-grade orthogneisses in southern India, shows mild chemical weathering under semia rid conditions. Along fractures, chemical weathering has advanced (Chemical Index of Alteration [CIA] = 53; CIA of fresh rock similar to 26) to the ex tent that secondary Mg-Fe-Al clay minerals have formed and the rock has tur ned brownish red, soft, and fine grained. The weathering process has result ed in the mobilization and redistribution of the so-called immobile element s Fe, Al, Ti, and REE effected by the nature of secondary mineral formation (talc vs, aluminous clay minerals) and also possibly by soil microbes. In the initial stages of secondary mineral formation, there is a small loss of Fe, Al, and REE (noticeably Eu). However, in the fracture zone as well as in the incipiently altered zone, there is significant REE enrichment, proba bly effected by different precipitation mechanisms. Mobilized REE may have come from a minor alteration of clinopyroxene.