Evidence for submarine weathering from metamorphosed weathering profiles on basaltic rocks, Tananao Metamorphic Complex, Taiwan

The metamorphosed paleo-weathering horizon(s) developed on basaltic rocks in the Tananao Metamorphic Complex forms distinct Al-Fe-rich rocks and occurs as two parallel belts, demonstrating its potential as a stratigraphic correlation unit in this structurally complicated metamorphic terrain. These Al-Fe-rich rocks have been metamorphosed under amphibolite-/greenschist-facies conditions, leading to the formation of garnet-chloritoid rocks/chloritoid-rich rocks. The chemical changes along three profiles, from metabasite to garnet-chloritoid rock, show prominent K, Rb, Cs and Ba enrichment, yet, also, a large depletion of other elements if it is assumed that Ti, Zr, Nb, Ta and Hf remained relatively immobile. The REE's, however, behaved non-coherently. These chemical characteristics are attributed to pre-metamorphic processes and are 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 subaerial weathering processes. K enrichment is not an uncommon feature in Paleozoic/Precambrian paleosols. The present case study demonstrates that submarine weathering should be considered as a probable cause.