Carbon, hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos, Greece

At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C.

The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ¹⁸O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ¹⁸O_H2O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite.

The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δD_H2O values for the metamorphic fluid decrease from −5‰ in the glaucophane zone to an average of about −70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend.

Theδ¹⁸O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism.

Detailed data on 20 marble units show systematic variations of δ¹⁸O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ¹⁸O gradients at the margins and large δ¹⁸O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ¹⁸O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates.

Almost all the δ¹³C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ¹³C values of most marble units show a general correlation with δ¹⁸O values.

The CO2H2O">CO₂H₂O mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ¹³C values of the CO₂ range from −8.0 to 3.6‰ and indicate that at some localities CO₂ in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles.