The high pressures necessary for the stabilisation of eclogites in
metabasic rocks andgarnetperidotitesinultrabasic rocks havebeen long
recognised and experimentally established. Xenoliths of such rocks
brought up in volatile charged alkaline magmas, such as kimberlites, are
widely accepted to be mostly ofupper mantle derivation (Chapter 13).
Eclogites are predicted to be thermodynamically stable also in the lower
crust beneath cratonic regions. However, xenolith suite studies indicate
that kinetic and/or compositional factors limit their distribution in
the lower continental crust relative to granulite fades assemblages
(Chapter 12). Occurrences ofeclogitesand gamet peridotites in exposed
crustal metamor- phic terrains have been interpreted in the past as
exotic tectonic blocks of deeper (largely mantle) origin, because of
their apparent difference in metamorphic grade compared with the
encompassing rocks. Only in recent years have metamorphic petrologists
begun to recognise that such crustal terrains sometimes preserve
co-facial (eclogite fades), high pressure mineral parageneses in other
spatially associated lithologies such as metapelites and metagranitoids.
Placed in a modern, global geotectonic context, it is now apparent not
only that eclogites can be expected to be stabilised in oceanic crust
subducted at continental plate margins (Chapter 9), but also that
eclogite fades mineral parageneses may be stabilised in a wider range
ofcontinental crust lithologies, where substantial tectonic thickening
has occurred in continental plate collision zones (Chapters 8-10).
Recent exciting evidence from the Western Alps(Chapter 10)suggeststhat
continental crust may be subducted to depths approaching 100km and iyet
exhumed during subsequent orogenic uplift.
