Journal of the Virtual Explorer

Detrital grains, including quartz, plagioclase and felsic volcanic fragments indicate felsic volcanic and plutonic rocks in provenance of the Shimozaisho meta-clastic rocks. In addition to these felsic igneous fragments, highly weathered rocks and mafic rocks should be in their provenance due to the following reasons.

The Si-free Al-mineral in the protolith of the Shimozaisho meta-clastic rocks should be detritus of weathering products which were mostly derived from feldspar through the weathering hydration process. Gibbsite is generated at low activity of aqueous SiO₂ and low ^aK⁺/^aH⁺ activity ratios (Aagaard et al., 1983). In other words, the source of the protolith was located in a humid and well-drained surface (Barshad, 1966). In tropical climates, hydration removes most of the Si from soils because of high humidity (Retallack, 2001). Thus, the presence of detrital gibbsite, as main soil derived detritus in the Carboniferous Shimozaisho Group, suggests that the provenance was located in a tropical climate zone (Fig. 8). In addition, high TiO₂ content also suggests a provenance from highly weathered soil because high TiO₂ contents are frequent in intensely weathered modern tropical soils (e.g., Brazilian soil contains 2.3 wt.% TiO₂; Cornu et al., 1999), whereas in average soils the TiO₂ content is 0.35 wt.% (Pendias and Pendias, 1984).

Source rocks of the Shimozaisho metapelites are shown in the A-CN-K diagram (Fig. 7).

Figure 7. A-CN-K diagram.

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Arrow indicate ideal weathering trend. Cross symbols shows average igneous rocks (Nesbitt and Young, 1984). Abbreviations: Ad = Adamellite, G = Granite, Ga = Gabbro, Gd = Granodiorite, Cld = Chloritoid, Pl = Plagioclase, Ksp = K-feldspar, Sm = Smectite, Mu = Muscovite, Ka = Kaolinite, Gb = Gibbsite, Chl = Chlorite, T = Tonalite.

On the A-CN-K diagram, ideal weathering trend (Nesbitt and Young, 1984) suggests that chemical compositions of the Shimozaisho meta-clastic rocks are comparable with ideally weathered rocks originated from mafic to felsic rocks. Abundant sericite in the matrix of type 1 rocks suggests their potassium-rich rock origin and relevant felsic rocks in their provenance. Chlorite as major component of type 2 rock matrix suggests their iron- and magnesium-rich rock origin and relevant mafic rocks in their provenance.