Abstract
High pressure metamorphic rocks in two contrasting geological settings
have been studied and interpreted.
Occurrences in exposed high grade gneiss complexes (especially in west
Norway) have been documented. Petrogenetic interpretations of these rocks have
involved the integration of field, mineralogical and microstructural observations with whole rock and mineral composition data, isotopic age data and
calculated equilibration temperature and pressure values. Deduced pressure - temperature -time paths for both the prograde (subduction related) and retrograde (uplift related) metamorphic stages have been used to establish tectonothermal models for the formation and survival of such high pressure rocks in
orogenic belts involving collision between continental lithospheric plates.
Particular emphasis has been placed on interpretation of the chemical, mineralogical and tectonic evolution of alpine -type Mg -Cr rich peridotites of deduced
sub -continental mantle origin. However, it has been shown that high pressure
mineral asemblages have also developed in original, low pressure, crustal
protoliths due to the imposition of high lithostatic pressures during transient
A -type subduction.
Complementary studies of high pressure assemblages in xenoliths, brought
up in volatile charged magmas of deep mantle origin, have provided important
data on the chemical and mineralogical composition of the lower crust and
uppermost mantle beneath continental cratonic areas. Particular attention has
been paid to evaluation of the reliability of the application of mineral
exchange reaction thermometers and barometers to assessment of the pressure - temperature conditions for formation, and depths of origin, of the various
xenolith types. It has been demonstrated that, for the garnet lherzolite
xenolith suite in the kimberlites of northern Lesotho, earlier pressure - temperature estimates purported to indicate a marked thermal perturbation in
the upper mantle palaeogeotherm are invalid. Instead revised pressure - temperature estimates are interpreted to be indicative of only a slightly
elevated craton margin geotherm, compatible with derivation of the higher
temperature deformed xenoliths in a thermally convecting asthenosphere beneath
a ca. 150 kms. thick, thermally conductive, lithosphere.
A revised, three fold, temperature based classification scheme for the
formation of high pressure, eclogite facies, mineral assemblages in proposed.
Low temperature (<550 °C) eclogites have formed in subordinate, fluid deficient,
rocks associated with blueschists in B -type subduction zones. Medium temperature (550- 900 °C) eclogites have been stabilised in tectonically thickened
continental crust sequences in A -type subduction zones. High temperature
( >900 °C) eclogites and associated garnet lherzolites, as witnessed as xenoliths
in kimberlites, have equilibrated in the upper mantle.
