EVIDENCE FOR SUBMARINE WEATHERING FROM METAMORPHOSED WEATHERING PROFILES ON BASALTIC ROCKS, TANANAO METAMORPHIC COMPLEX, TAIWAN

The metamorphosed paleo-weathering horizon(s) developed on basaltic ro cks in the Tananao Metamorphic Complex forms distinct Al-Fe-rich rocks and occurs as two parallel belts, demonstrating its potential as a st ratigraphic correlation unit in this structurally complicated metamorp hic terrain. These Al-Fe-rich rocks have been metamorphosed under amph ibolite-/greenschist-facies conditions, leading to the formation of ga rnet-chloritoid rocks/chloritoid-rich rocks. The chemical changes alon g three profiles, from metabasite to garnet-chloritoid rock, show prom inent K, Rb, Cs and Ba enrichment, yet, also, a large depletion of oth er elements if it is assumed that Ti, Zr, Nb, Ta and Hf remained relat ively immobile. The REE's, however, behaved non-coherently. These chem ical characteristics are attributed to pre-metamorphic processes and a re shown to be similar to those observed with submarine weathering of basaltic rocks. In contrast, the chloritoid-rich rocks reveal distinct K, Rb, Cs and Ba depletion. It is suggested that this resulted from s ubaerial weathering processes. K enrichment is not an uncommon feature in Paleozoic/Precambrian paleosols. The present case study demonstrat es that submarine weathering should be considered as a probable cause.