Sedimentary mixing forcing Al-rich metapelites chemistry

In the previous section we suggested that source rocks for Shimozaisho pelite included highly weathered soils (suggested by presumed detrital gibbsite), felsic igneous rock (indicated by detrital volcanic fragments, quartz and feldspar) and mafic rocks (suggested by chemical composition) (Fig. 8).

Figure 8. Schematic block diagram

Schematic block diagram

Schematic block diagram illustrating the suggested provenance of the detrital materials in the Carboniferous Shimozaisho Group with source in a tropical climate zone. Not to scale.

In addition to this, we emphasize two implications concerning the Shimozaisho meta-clastic rocks: mixing of weathered and non-weathered materials during sedimentary process; and wide variation in source rock types including felsic and mafic rocks with highly weathered soil. A key to the understanding of the mechanism of complex detritus supply for the Shimozaisho basin is intercalating volcaniclastic layers.

Carboniferous shelf deposits in the Hida Mountains commonly include volcaniclastic layers. These indicate that the sedimentary basins were not far from volcanic activities.

The suggested provenance of the Al-rich horizons from highly weathered materials implies a stable surface - whereas the volcanic horizons suggest an active surface. This necessitates a change in tectonic setting from presumably passive margin to active margin. In support of this assumption is the occurrence of the Al-rich horizons, characteristically towards the base of the pelitic sequence overlying thick limestone beds in the Unazuki Metamorphic Rocks (Hiroi, 1978). In this case, the mixing of highly weathered and non-weathered materials and the supply of both detritus to Carboniferous basins was caused by volcanic activity and/or associated tectonic uplifting. We here mention that the wide distribution of Japanese Carboniferous Al-rich metaclastic rocks might be result of regional-scale detrital supply of intense weathered materials, which was caused by the change in tectonic setting. Interpretation of the petrology and geochemistry of metapelites in the Carboniferous Shimozaisho Group of the Hida Marginal Belt, Central Japan as the result of a mixed tropical pedogenesis of the protolith sheds light on simultaneous tectonic activity and paleo-latitude of this Gondwana derived terrane.