International Geologiical Congress - Oslo 2008

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UHP-03 Ultra-high pressure metamorphism: Minerals, microstructures and nanoscale observations - Part 1

 

Garnet-pyroxene exsolution microstructure in orogenic peridotite predates ultra-high pressure metamorphism in Su-Lu

 

Dirk Spengler, Kyoto University (Japan)
Masaaki Obata, Kyoto University (Japan)
Takao Hirajima, Kyoto University (Japan)
 

 

Garnet (Grt) lamellae that occur in cm-scale porphyroclastic ortho- and clinopyroxene (Opx and Cpx) enclosed in orogenic Grt-peridotite from Xugou, Su-Lu, China, are 20-100 m in width, several mm in length, regularly distributed and widely spaced. Thin sections perpendicular to the [001] axis of pyroxene suggest that Grt precipitated parallel to the (100) and (010) planes of pyroxene, consistent with an origin by exsolution. A second generation of exsolution in Opx is evident from Cr-spinel lamellae that are a few m in width, several hundred m in length, regularly distributed sub-parallel Grt and narrowly spaced. Both exsolved Al phases in Opx are associated and partly interlaced with Cpx lamellae similar in size. We interpret this microstructure in Opx to have formed from a precursor pyroxene by successive exsolution that involved Al, Cr and Ca. Steep Ca isopleths (CMAS system) and Cr-Al systematics (FMASCr system) constrain both generations of exsolution occurred during cooling or from a metastable state after cooling.

Porphyroclastic Opx has flat compositional profiles with 0.20 wt% Al2O3 content. The peridotite matrix assemblage composed of Grt + Opx + Cpx + olivine ± phlogopite records a strain-induced recrystallization and lacks precipitates in mineral cores. Matrix Opx cores have 0.12 wt% Al2O3 content. Geothermobarometry on Grt-Opx mineral pairs indicate 5.6 GPa / 760 °C and 6.3 GPa / 840 °C for the exsolved and recrystallized microstructure, respectively. These pressure-temperature estimates derived from the same mineral phases preserved in two structurally different positions overlap with the range in metamorphic condition reported for the peak UHPM in the region (5-7 GPa and 780-870 °C).

The lack of precipitates in recrystallized minerals sets a relative time limit for the exsolution process to have taken place, at or prior to UHPM. The exsolution of Cr-spinel, however, requires cooling as isothermal compression would increase the Cr# in Opx. Consequently, the exsolution microstructure preserved in porphyroclastic minerals at Xugou formed prior to but preserves a mineral-chemical equilibration similar to the local UHPM. Comparable microstructural and petrological observations recently reported from the western Scandinavian Caledonides show that destruction (recrystallization) of the Grt-Pyx exsolution microstructure is consistent with prograde metamorphism during continental plate subduction. Acceptance of a similar interpretation for the Xugou peridotite would imply sampling of the Xugou mantle fragments from the hanging wall occurred at deeper depths levels (170 km) than of peridotite bodies in the Scandinavian Caledonides (110 km), before subduction of continental lithosphere in both orogens culminated at 200 km depth. Similar observations in mantle fragments from both orogens strongly suggest that mineral microstructures in orogenic peridotite preserve more information on the evolution of the SCLM than previously recognized.

 

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