PETROLOGIC EVIDENCE FOR EPISODIC SUBDUCTION

Three approaches have been used to constrain the prograde subduction P-T paths in blueschists and eclogites from the Cyclades blueschist belt, Greece: trace element thermobarometry, garnet zoning, and pseudosection analysis. Ti in quartz ranges from 0.2-0.8 ppm in mica schists to 4-20 ppm in some eclogites and Zr-in-rutile ranges from 10-50 ppm in mica schists to 40-150 ppm in some eclogites, which combine to yield conditions of equilibration for the mica schist of ca 540 C, 22 kbar and for the eclogite of 600 C, 18 kbar. Chemical zoning of some rutile grains show lower Zr concentrations towards the rims, which requires an increase in pressure/temperature that is steeper than the slope of the Zr-in-rutile isopleth; that is, nearly isothermal compression. Quartz inclusions in garnet with higher Ti concentrations than some matrix grains also hint at prograde P-T paths that are steeper than the isopleths. Analysis of chemical zoning in garnet yields prograde P-T paths that have saw-tooth shapes and periods of nearly isothermal loading, similar to thrust nappe terranes, suggesting rapid tectonic burial. In addition, the sequence of phase assemblages development as revealed by inclusion suites in garnet and matrix assemblages require at least two episodes of major glaucophane growth. Pseudosection analysis reveals that there is only one major glaucophane producing reaction in rocks of these bulk compositions, requiring that the reaction is crossed twice in the same direction. This could readily be accommodated by a saw-tooth P-T path involving loading, heating (± decompression), loading, and heating. These observations all suggest that the tectonic history of subducted rocks is similar to that observed in continental collision zones, and is not the same as the calculated steady state subduction geotherm. This implies that subduction of individual slabs is episodic, involving rapid burial, presumably followed by heating with or without decompression, and that there may be several cycles of burial and heating before the peak conditions are reached. Such episodic underthrusting might give rise to episodic earthquake activity, fluid release, and arc-related volcanic activity.