Provenance of granitic gravestones in graveyard of feudal lords evaluated by multiple non-destructive rock analyses

Abstract

• The Yamauchi clan gravestones were sourced from southwestern Kochi Prefecture, Japan. • Magnetic susceptibility is a handy indicator, but not enough to identify the origin. • Fraction and grain size of colored minerals are useful for determining provenance. Magnificent graveyards were constructed for feudal lords during the Edo period throughout Japan. The large gravestones of feudal lords, which measured up to several meters high, reflected the political and economic status of the people commemorated at these sites. While the locations of the quarries and the methods used to transport the stones that were used for gravestones also provide clues about the culture of the time, few documents describing the origin of the stones remain. There are several granite gravestones of feudal lords of the Yamauchi clan in Kochi Prefecture in southwestern Japan. However, there is some debate as to their origin and whether they are made of granite from southwestern Kochi Prefecture or Sanyo granite from areas around the Seto Inland Sea. In this study, we conducted multiple non-destructive rock analyses on the granitic gravestones of feudal lords of the Yamauchi clan to clarify their origin. These analyses included magnetic susceptibility measurements, grain-size analysis of colored minerals, and color measurement of K-feldspar. The results were compared with those for granite obtained from southwestern Kochi Prefecture and Sanyo granite. The K-feldspar in the gravestone was white in color. The magnetic susceptibilities of the granite blocks for the gravestones were similar, and followed a Gaussian distribution. The grain size of colored minerals in the granite followed a log-normal distribution, and the areal fraction and grain size of colored minerals were larger than those in other granites. Similarities in color, magnetic susceptibility, and grain size were observed with granite from southwestern Kochi Prefecture. For Sanyo granite, no granite had features that matched all of the measured characteristics. The results indicate that the granite used for the Yamauchi clan gravestones was most likely sourced from southwestern Kochi Prefecture. Stone structures in Kochi Prefecture that were built in the Middle Ages before the Edo period were previously considered to be derived from Sanyo granite. The results of this study provide important insights into whether the routes used to transport stone materials in Kochi Prefecture changed before the Edo period, and if so, what triggered this change. Further, the origin of medieval stone structures in Kochi Prefecture should be reevaluated.