Isotopic Composition of Carbonaceous Material in Metamorphic Rocks from the Mountain Belt of Taiwan

Abstract

Carbonaceous (organic) material was extracted from pelitic rocks and marbles collected along three highways transecting the mountain belt of Taiwan. δ13C values of carbonaceous matter show a general increasing trend with elevated metamorphic grade. The first prominent, systematic change in δ13C values occurs in samples of the middle greenschist facies, with δ13C values higher than -24%‰. Carbon isotope fractionation between coexisting calcite and carbonaceous material suggests that this marked change might have taken place at around 300-350°C. From systematic temperature variations, two thermal maxima, 430°C and 460-490°C, were delineated in the central part of the Hsuehshan Range and the Backbone Range, respectively, and are attributed to Cenozoic mountain building. High temperatures (>500°C) were detected in some marble samples adjacent to Mesozoic granite intrusions, indicating that pre-existing thermal records are preserved in these marbles. The carbon isotope fractionation preserved in rocks across the mountain belt therefore may not have been synchronous.