| In a recent paper (Eur. J. Mineral. 20), Massonne and Willner (2008) presented P-T
pseudosections for common rocks involved in accretionary wedge systems and argued that
the dehydration of psammopelitic rocks could be an essential process for the formation of
these systems. These authors assumed that this dehydration process leads to softening of the
sedimentary cover of oceanic crust during early subduction so that this material can be
scraped off the basic crust. Since many accretionary wedge systems contain metamorphosed
calcareous sediments it was tested which influence carbonates, ignored by Massonne
and Willner (2008), could have on the dehydration behaviour of these sediments.
For this purpose, P-T pseudosections were calculated for a calcareous greywacke
and a marly limestone in the system Na-Ca-K-Fe-Mn-Mg-Al-Si-Ti-C-O-H with
the PERPLE_X software package (Connolly, 2005) for the pressure-temperature
range 1-25 kbar and 150-450°C. In addition to the thermodynamic data and solid
solution models already used by Massonne and Willner (2008), a newly created
quaternary (Ca-Mn-Mg-Fe2+) solid solution model was applied to carbonate with
calcite structure together with an existing dolomite-ankerite model. Aragonite was
considered as a pure phase. The Mn end-member was added to the previously used
stilpnomelane model in order to calculate the P-T conditions of garnet formation at high
pressure.
Along a low geotherm of 10-12°C/km, the dehydration behaviour of a calcareous
greywacke resembles that of the previously studied psammopelite. However, the relevant
dehydration event (release of about 1 wt% H2O) occurs in the temperature interval
270-330°C and, thus, at temperatures about 30°C higher than in an ordinary psammopelite.
The calculated compositions of fluids generated at low geotherms ( |