|
Dirk Spengler, Kyoto University (Japan)
Gill M Pennock, Universiteit Utrecht (Netherlands)
Martyn R Drury, Universiteit Utrecht (Netherlands)
Herman LM Van Roermund, Universiteit Utrecht (Netherlands)
|
|
Mineral exsolution microstructures of garnet and pyroxene preserved in tectonically exhumed mantle fragments (orogenic peridotites) have been proposed to form in two contrasting geodynamic environments, either during long-term isobaric cooling of cratonic lithosphere or during fast near-isothermal compression/decompression in ultra-high pressure metamorphic (UHPM) continental subduction zones. Here we compare exsolved samples from both settings that differ significantly in confining temperature conditions.
First we investigated the topotaxial orientation relationship between pyroxene precipitates in garnet from Norwegian orogenic peridotite. The crystallographic orientation of lamellae type clino- and orthopyroxene enclosed in garnet follow a six-fold symmetry by rotation around [-102]Cpx and [001]Opx parallel <111>Grt. The other two orthogonal major planes in pyroxene, (100)Pyx and (010)Pyx, are parallel {110}Grt and {112}Grt, respectively. Previous measurements of the topotaxial relationship in this microstructure from cratonic mantle xenoliths revealed the same results. Equality in crystallographic orientation relationships and similar microscopic scales of exsolution suggest that the exsolution microstructure preserved in garnet from both types of mantle fragments has the same origin, which has been shown to be related to cooling from high mantle temperatures greater 1300 °C, much higher than in any reported UHPM subduction setting.
The inverse microstructure, garnet lamellae in pyroxene, reported from both orogenic peridotite with a high temperature thermal history and mantle xenoliths, ranges in scale from sub-microscopic to macroscopic and has either the identical orientation relationship or shows systematic variation. Consistency and high symmetry in topotaxial orientation suggest lamellae type exsolution in the system garnet-pyroxene has well defined crystallographic relationships that are apparently independent on the host phase (garnet, pyroxene) and the precipitate size. Orientation data inferred from low temperature precipitates is consistent with those from high temperature precipitates implying the topotaxial relationship is non-discriminative for the origin of the microstructure.
Pressure-temperature estimates reported from mineral-chemical equilibria in global UHPM belts show that the temperature conditions of less than 1000 °C in modern continent-continent collision zones do not resemble those reported from reconstructed garnet-pyroxene solid solution phases that are greater 1000 °C. This implies that either pressure or temperature conditions in UHPM belts with majoritic garnet precursors have been drastically underestimated, or that the majoritic garnet precursors in UHPM belts are relicts from earlier cratonic history of mantle rocks. From this discussion we conclude that garnet-pyroxene exsolution microstructures in orogenic peridotites preserve more information on the cratonic lithosphere evolution than previously recognized.
|
