STALLED PROGRADE ULTRAHIGH-PRESSURE METAMORPHISM: WHY SOME ROCKS TRANSFORM DURING CONTINENTAL SUBDUCTION AND OTHERS DON'T

A long-standing conundrum in tectonics and metamorphic petrology is how felsic continental crust subducted to the depths of ultrahigh-pressure (UHP) metamorphism shows little evidence of transformation to an assemblage stable at these conditions. If equilibrium is assumed for the prograde path, explaining this paucity of UHP mineralogy has led to hypotheses requiring pervasive retrogression during exhumation, catalyzed by deformation and fluid influx. How this reconciles with typically dry orthogneissic crust, and recent studies showing relatively low levels of strain in exhumed UHP terranes, is unclear.

Insight into this problem can be gained from studying felsic rocks from the ultrahigh-pressure Western Gneiss Region (WGR) of Norway. Using thermodynamic modeling (Perpl_X) and split-stream LA-ICPMS zircon geochronology, we evaluated the metamorphic history in four compositions common to the WGR and many other orogenic terranes: paragneiss, pelitic schist, granodioritic and granitic rocktypes. We seek to understand whether the extant mineral assemblages are prograde or retrograde with respect to the (U)HP metamorphism.

The chronology of the WGR is relatively well understood: subduction lasted from 430-400 Ma, and exhumation took place until approximately 375 Ma. Our results indicate that much of the mineral development in these rocks took place on the prograde path: a) zircon growth begins in all samples at 440-430 Ma; b) flatter zircon HREE patterns from garnet-bearing pelite and paragneiss indicate competition with this phase after 425 Ma; and c) suppressed Eu anomalies in some samples suggest plagioclase breakdown after 413-409 Ma. Although not directly datable by zircon techniques, zoisite–epidote, white mica, albite and rutile also grew in various rocktypes, replacing plagioclase and biotite. Overall, granite gneiss, pelite and paragneiss contain a similar zircon record, but granodioritic rocktypes contain very thin rims that indicate little zirconium mobility, even at temperatures >600-700ºC. Therefore, equilibrium on the prograde path cannot be assumed and is quite dependant upon lithology. The availability of water is probably the most significant kinetic factor in growing a high-P mineralogy, and deformation appears to be less important in either growth or preservation.