Hydrogen and oxygen isotope evidence for fluid–rock interactions in the Menderes massif, western Turkey

Abstract.

The Menderes Massif comprises an inner crystalline core with gneissic rocks and an outer surrounding schist belt with predominantly metasedimentary rocks. Both units have a complex metamorphic history including a late Alpine overprint. Temperatures inferred from oxygen isotope compositions of coexisting minerals increase from 420 to 600°C from the rim to the center. More positive δ¹⁸O values in all minerals from the schist belt may reflect a higher abundance of sedimentary precursor material, whereas biotites and muscovites in core and rim are indistinguishable in hydrogen isotope composition. δD values of muscovites range from –35 to –60‰, whereas δD values of biotites range from –65 to –125‰, indicating normal values for muscovite but anomalously negative values for some biotites. For muscovite the trend can be interpreted in terms of increasing loss of water with rising metamorphic temperature. For biotite the δD values decrease with increasing H₂O content and decreasing Na₂O+K₂O content, which provides evidence for alteration processes or exchange of K and Na with water from interlayers of biotite forming hydro-biotite. The data suggest isotopic resetting of pre-Alpine characteristics during Alpine metamorphism. The hydrogen isotope composition of biotite was later disturbed, probably during extensional neotectonic movements in this region, as this allowed infiltration of and exchange with D-depleted meteoric water; however, the muscovites retained its Alpine characteristics.