A comparison among organic and inorganic indicators of diagenesis and low-temperature metamorphism, Tertiary Shimanto Belt, Shikoku, Japan

Tertiary rocks of the Shimanto Belt represent the youngest subaerial part of the accretionary margin of southwest Japan. Measurements of mean vitrinite reflectance (%R (sub m) ) from shales and cleaved metapelites show that the Eocene through early Miocene strata on the Muroto Peninsula of Shikoku Island were exposed to temperatures of approximately 140 degrees C to 315 degrees C. Analyses of inorganic phases corroborate these findings. Values of illite crystallinity index (CI) range from 0.87 Delta degrees 2{theta} to 0.21 Delta degrees 2{theta}. Most of the CI data are consistent with conditions of advanced diagenesis and anchimetamorphism (transition into greenschist facies), and a few CI values fall within the zone of epimetamorphism (lowermost greenschist facies). A significant statistical correlation exists between %R (sub m) and CI, with the best-fit curve corresponding to the following equation and correlation coefficient: %R (sub m) = 0.57 - 5.99 log (CI); r = 0.84. This curve conforms reasonably well with the boundaries of the anchizone, as established by independent compilations. Calibration of CI values with paleotemperature estimates (T, in degrees C), as derived from R% (sub m) data, results in the following correlation: CI = 1.197 - 0.0029(T). However, because of error propagation, uncertainties in the validity of extrapolation, and potential differences in the boundary conditions of low-grade metamorphism, this relation between CI and paleotemperature should be applied with caution to studies of other orogenic belts. Measured values of illite b (sub o) lattice spacings range from 9.001Aa to 9.031Aa; these data are consistent with moderate amounts of (Mg+Fe (sub total) ) in the illite unit cell. By analogy with other orogenic sequences, Shimanto metamorphism evidently was governed by intermediate pressure gradients. Maximum burial pressures were probably less than 2.5 kbar, and maximum burial depths were 9 km or less. Thus, at least when viewed within the blueschist-facies paradigm of subduction zones, the Tertiary Shimanto Belt must be regarded as somewhat unusual.