TRACKING COMPOSITIONAL AND MINERALOGICAL CHANGES ASSOCIATED WITH THE BLUESCHIST-TO-GREENSCHIST TRANSITION IN METABASALTS FROM MIHALIÇÇIK, TURKEY

In many exhumed subduction complexes, high-pressure/low-temperature rocks, such as blueschist, coexist with lower pressure rocks, such as greenschist. This relationship has been interpreted as reflecting: 1) tectonic juxtaposition of rocks equilibrated at different P-T conditions, 2) variations in bulk rock composition, or 3) local variations in fluid availability. In Mihalıççık, Turkey, blueschist and greenschist coexist at the hand sample scale, providing an opportunity to evaluate the mechanism responsible for the transformation of blueschist to greenschist, as well as the mineralogical and compositional changes associated with this transition. The metabasalts range from foliated to non-foliated and contain blueschist interiors consisting of glaucophane + epidote (ep) + albite (alb) ± titanite ± pyrite. Bulk rock compositions of the blueschist domains indicate basalts with MORB and calc-alkaline affinities as the protoliths. The margins of the blueschists have been overprinted by a greenschist facies mineral assemblage consisting of chlorite (chl) + ep + alb ± quartz (qtz). The boundary between the blueschist interior and the greenschist exterior is gradational, and characterized by an increase in the modal abundance of ep + chl and the appearance of mm-to-cm scale vein networks. Vein mineralogy varies between the samples: some veins are predominantly qtz + calcite (cal) + alb, whereas others are almost entirely ep. The highly localized nature of the overprinting, coupled with its spatial association to veins, points to the importance of metamorphic fluid flow in driving the blueschist to greenschist transformation at the sample margins. In the sample with ep-veins, amphibole in the greenschist domains and adjacent to the ep-vein is Ca-rich, whereas amphibole in the blueschist domains have Na-rich cores and Ca-rich rims. Locally, white mica is replaced by ep. In the sample with qtz + cal + alb veins, all matrix amphibole is Na-rich, and white mica is replaced by alb. Bulk rock geochemical data show that the greenschist domains have lower Fe₂O₃, and higher Al₂O₃, CaO and K₂O contents relative to blueschist domains.These compositional trends are consistent with results indicating that Fe³⁺-rich and Al-poor compositions preferentially stabilize epidote blueschist facies mineral assemblages.