RAMAN-SPECTRUM OF CARBONACEOUS MATERIAL - A POSSIBLE METAMORPHIC GRADE INDICATOR FOR LOW-GRADE METAMORPHIC ROCKS

Raman spectral analyses of carbonaceous material (CM) extracted from p elitic samples along two sections traversing the metamorphic belt of T aiwan were carried out in the present study. The results show similar spectral variations of CM with metamorphic grade as those documented i n the literature. However, continuous sampling from zeolite facies thr ough prehnite-pumpellyite facies to greenschist facies metamorphic roc ks in the present study does reveal some interesting features on the R aman spectra of CM that were not noted before. Both the Raman D (disor dered-)/O (ordered-) peak area (i.e. integrated intensity) ratio and t he D/O peak width (i.e. full width at half maximum, FWHM) ratio of the CM decrease with progressive metamorphism, but the most prominent cha nge in the D/O peak area ratio occurs in samples of lower greenschist facies metamorphic grade, while the most significant decrease in the D /O peak width ratio occurs in samples near the boundary of prehnite-pu mpellyite facies and greenschist facies. This phenomenon is interprete d as a result of the decoupling of the changing rates of in-plane crys tallite size and degree of defects of CM with progressive metamorphism . It is postulated that the Raman spectrum of CM can serve as a metamo rphic grade indicator to distinguish samples of prehnite-pumpellyite f acies metamorphic grade from those of greenschist facies metamorphic g rade.