Phase relations and dehydration behaviour of psammopelite and mid-ocean ridge basalt at very-low-grade to low-grade metamorphic conditions

Abstract

P–T pseudosections were calculated in the system Na–Ca–K–Fe–Mg–Al–Si–Ti–H–O with the PERPLE_X software package for the pressure–temperature range 1–25 kbar and 150–450 °C to gain a better understanding of the phase relations and the dehydration behaviour of psammopelite and mid-ocean ridge basalt (MORB) during prograde metamorphism at very-low-grade and low-grade. For this purpose, the thermodynamic data set of Holland & Powell was enlarged by end-member data for Fe2+- and Fe3+-pumpellyite, Fe2+- and Mg-stilpnomelane, actinolite, and magnesioriebeckite. In addition, a three-component solid-solution model for pumpellyite, a two-component model for stilpnomelane, and four-component models for amphibole and sodic pyroxene were created. Studied metamorphosed MORB and psammopelite contain around 6 wt.% structural H2O bound in minerals at 150 °C and pressures up to 5 kbar. Prograde metamorphism causes dehydration patterns, which are, for example, important for an understanding of the formation of accretionary-wedge systems: along a relatively high geotherm of 15 °C/km, significant dehydration (1.5 wt.% H2O release) of metapsammopelite can be noted in the temperature range 220–240 °C. We believe that this 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, which then would dehydrate at higher T , to form frontal accretionary prisms. Basal accretionary prisms are generated at lower geotherms ( e.g ., 12 °C/km) by dehydration of metapsammopelite between 260 and 300 °C. Again, metabasic material would dehydrate at significantly higher T and is, therefore, only subordinately involved in accretionary wedges. Along geotherms lower than 7 °C/km, almost no water is released up to temperatures of 400 °C and more. Thus, the corresponding material is subducted to mantle depths as in the subduction/exhumation channels of collision zones. We also hypothesize that accretionary-wedge complexes of the hot subduction zones in Precambrian times should mainly have formed by frontal accretion.