ABSTRACT

To investigate factors affecting the evolution of nitrogen system in rocks during progressive metamorphism, unmetamorphosed to greenschist-facies pelitic samples were collected from both pelitic and (pelitic members of) psammitic rock sequences along two transects of the Taiwan mountain belt. Among all samples, greenschist-facies metapelites from the Tananao metamorphic basement of this mountain belt exhibit the most variable and depleted nitrogen content and δ¹⁵N values. The observations are difficult to interpret and can only be accounted for by complicated tectonic/metamorphic history of the basement complex. On the other hand, irrespective of the different metamorphic grades, samples from the metamorphosed cover strata, which have been subjected to only one stage of metamorphism, show comparable nitrogen and N-isotope composition. The only exception is that greenschist-facies metapelites from the pelitic members of psammitic rock sequence (i.e., the G(Ps) group samples) of the cover strata are enriched in the ¹⁵N isotope by at least 1‰. Taking Al content as a normalizing factor, averaged N/Al and LOI/Al show systematic decrease through progressive metamorphism. Given the non-homogeneous nature of the sedimentary protoliths, the comparable N-isotope composition among the unmetamorphosed to zeolite-facies (i.e., S-Z) group, prehnite-pumpellyite-facies (i.e., PP) group and greenschist-facies (i.e., G) group samples from pelitic rock sequences can be explained by rock devolatilization with limited nitrogen depletion (<10%). High δ¹⁵N composition of the G(Ps) group samples might be mainly due to more extensive nitrogen depletion (∼25%). It is suggested that during metamorphism, the pelitic rock sequence may approximate a closed system, while thick psammitic rock sequence may behave more like an open system, facilitating nitrogen depletion and N-isotope fractionation even for thin pelitic members within a thick psammitic sequence. Lithology, in addition to other factors, therefore plays a role in controlling the evolution of the nitrogen system during low-grade metamorphism. The present study implies that element transport by dehydration fluid in subduction zones may largely take place along fluid channel ways.