Petrography of metaclastic rocks
General features of the Shimozaisho metaclastic rocks
Protolith texture and detrital grains are preserved in metaclastic rocks of the Shimozaisho Group (Figs. 4A; 4B). The protoliths are fine-grained to very coarse-grained lithic wackes or arenites containing detrital felsic volcanic fragments, quartz and plagioclase. Detrital grains are sized 0.2-1.5 mm. Most quartz grains show undulatory extinction. Plagioclase grains are mostly converted to aggregates of sericite and deficient calcite. Felsic volcanic fragments exhibit felsitic texture. Felsic volcanic fragments have suffered mostly sericitization or merely chloritization.
Shimozaisho Al- or Fe-rich metaclastic rocks
Metamorphic chloritoid, sericite and anatase are observed in the Shimozaisho Al-rich metaclastic rocks that also contain detrital grains as pseudomorphs. Metamorphic minerals overprint the protolith texture. Felsic volcanic fragments have reaction rims composed of sericite (Fig. 4D). Plagioclase grains also show reaction texture (Fig. 4E). Chloritoid porphyroblasts overgrow the sedimentary texture and the detrital quartz and ilmenite (Fig. 4F). Prismatic anatase grains occur parallel to the pressure solution cleavage (Fig. 4G). Chloritoids are prismatic crystal with size changing from 0.05 x 0.30 mm to 0.75 x 2.00 mm. The grain size of chloritoid is almost constant in hand specimen.
Two types of chloritoid-bearing metapelites are classified on the basis of megascopic observations. Type 1 rocks are white to dark-gray with indistinguishable detrital grains. Type 2 rocks are dark-gray with distinguishable detrital grains. Type 1 rocks have matrix composed of abundant sericite with no chlorite. Quartz in the matrix decreases in inverse proportion to the amount of sericite present, to the extent where quartz is absent. These rocks contain fewer detrital quartz grains than the chloritoid-free rocks. Detrital grains are poorly preserved and converted into sericite. Outlines of detrital grains are blurred. Type 2 rocks have matrix composed of abundant chlorite and lesser sericite. Detrital grains are mostly preserved and show partial replacement by chlorites.
Figure 3. Outcrop-map of the Shimozaisho Limestone and Slate Alteration Formation, and locations of analyzed samples.
Figure 4. Photograph (A) and photomicrographs (B-G) of the Shimozaisho Al-rich metaclastic rocks.
(A): Cross lamination. (B): Preserved sandstone texture (wacke) and overprinting metamorphic minerals. Pressure solution cleavages are well developed. Open polarized light. (C): Cross polarized light of photomicrograph (B). (D): Reaction texture showing sericite replacing felsic volcanic fragment. Dotted line indicates original shape of detrital felsic volcanic fragment. (E): Reaction texture showing sericite replacing plagioclase. Dotted line indicates original shape of the detrital plagioclase. (F): Metamorphic chloritoid overprinting ilmenite. (G): Metamorphic anatase occurring parallel to the cleavage.
Abbreviations: Fvo = felsic volcanic fragments, Pl = plagioclase, Ilm = ilmenite, Cld = chloritoid, Ser = sericite, Ant = anatase.