Origin of frictional melt in the Jurassic accretionary complex of the Mino-Tamba Belt, central Japan on the basis of trace element and isotope analyses
Abstract
Pseudotachylytes in exhumed accretionary complexes have great significance for understanding the dynamics of earthquake faulting in subduction zones. Recently, the pseudotachylyte-like dark vein was found in Jurassic accretionary complex of the Mino-Tamba Belt, central Japan. The dark vein is 2 mm-thick and shows the appearance of fault and injection-like veins, which are typically observed in pseudotachylytes. The microstructures of the dark vein are characterized by intensely cracked gray chert fragments in the matrix and locally embayed wall rocks, suggesting thermal cracking and thermal erosion, respectively. Thus, the dark vein appears to be originated from pseudotachylyte, but extensive hydrothermal alteration of the matrix obscures the evidence for frictional melting. Here, we analyzed major and trace elements and strontium (Sr) isotopes of the milligram-level samples taken by micro-milling technique along the structures of the dark vein, cataclasite and gray chert, as well as the black chert and black carbonaceous mudstone outcrop near the dark vein. The dark vein exhibits remarkably high trace element concentrations compared to gray chert. The trace element composition of the dark vein requires the involvement of black carbonaceous mudstone, which is currently absent in the dark vein. This suggests melting of the mixture of gray chert and black carbonaceous mudstone, with the latter having been vanished due to low-melting-point characteristics of illite-rich mudstone. Sr isotope composition of the dark vein is also consistent with the mixture of gray chert and black carbonaceous mudstone, and its distinctly high rubidium(Rb)/Sr ratio relative to the chert-cataclasite-mudstone compositional trend can be explained by melting-induced Rb-Sr fractionation. These features suggest that the dark vein was originated from frictional melt. Geochemical analyses are thus useful for the identification of past frictional melting on faults even after hydrothermal alteration, which shed light on the issue whether pseudotachylytes are rarely generated or rarely preserved.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR31B0075O
- Keywords:
-
- 3625 Petrography;
- microstructures;
- and textures;
- MINERALOGY AND PETROLOGYDE: 8004 Dynamics and mechanics of faulting;
- STRUCTURAL GEOLOGYDE: 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICSDE: 8160 Rheology: general;
- TECTONOPHYSICS