Unravelling granulitic conditions in Early Triassic crustal xenoliths from NE-Qiangtang, Tibet

National audience; Unlike most "wedge" orogens, the structure and the composition of theTibetan Plateau lower crust has kept being largely inaccessible otherwisethan by geophysical studies and by the few scattered sampling ofxenoliths brought to the surface by volcanism. In the Nangqian basin (NEQiangtangterrane), Eocene syn-contractional potassic lavas carrypartially molten foliated quartzo-feldspathic xenoliths. Zircon LA-ICPMSU-Pb dating of these crustal xenoliths yields a Lower Triassic age of ca.248.5 Ma, which is interpreted as the protolith crystallization age, withno evidence for any metamorphic zircon growth or resetting. The rarerelict ferro-magnesian minerals found (clinopyroxene, amphibole,phlogopite, garnet) are in textural and chemical disequilibrium, makingconventional thermobarometry methods inoperable. Nevertheless, thexenoliths have retained unusual metamorphic features which can helpunravelling their pre-entrapment metamorphic history. Pure anorthite"patches" with magnetite inclusions are interpreted as pseudomorphsafter garnet. Since these patches are often found in contact withclinopyroxene and rutile, these minerals may together represent a formerhigh-pressure assemblage that developed at 1.2-1.5 GPa and 620-730°C.Such pressure record would indicate that 80% of the present-day crustalthickness of the Tibetan Plateau was already acquired in Eocene times.Moreover, corundum-bearing assemblages, interpreted as pseudomorphsafter phlogopite, are best explained by kinetic disequilibrium underhigh-temperature granulitic conditions, suggesting a short-livedmetamorphic event. Taken together, this set of new petrological andgeochronological data suggests that crustal granulitic metamorphism maynot b e related to the onset of a general warming of the Tibetan crustsustained until the present, but rather to the punctual injection of mantlederivedmelts in the crust before Eocene eruption.