Eclogite Facies Rocks

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.

Inhalt
1 Eclogites and the eclogite facies: definitions and classifications.- 2 Eclogite facies mineral parageneses.- 3 Experimental studies on the stability of eclogite facies mineral parageneses.- 4 Mineral barometry and thermometry.- 5 Role of kinetics in the formation and preservation of eclogites.- 6 Isotopic systemsgeochronology of eclogites.- 7 Occurrence and stability conditions of low-temperature eclogites.- 8 Formation and exhumation of medium-temperature eclogites in the Scandinavian Caledonides.- 9 Eclogite formation and distribution in the European Variscides.- 10 Formation and distribution of eclogite facies rocks in the Alps.- 11 Petrogenesis of Mg-Cr garnet peridotites in European metamorphic belts.- 12 Eclogite stability near the crust-mantle boundary.- 13 High-temperature and ultra-high pressure eclogites.- References.