Unravelling granulitic conditions in Early Triassic crustal xenoliths from NE-Qiangtang, Tibet.
Abstract
Unlike most "wedge" orogens, the structure and the composition of the
Tibetan Plateau lower crust has kept being largely inaccessible otherwise
than by geophysical studies and by the few scattered sampling of
xenoliths brought to the surface by volcanism. In the Nangqian basin (NEQiangtang
terrane), Eocene syn-contractional potassic lavas carry
partially molten foliated quartzo-feldspathic xenoliths. Zircon LA-ICPMS
U-Pb dating of these crustal xenoliths yields a Lower Triassic age of ca.
248.5 Ma, which is interpreted as the protolith crystallization age, with
no evidence for any metamorphic zircon growth or resetting. The rare
relict ferro-magnesian minerals found (clinopyroxene, amphibole,
phlogopite, garnet) are in textural and chemical disequilibrium, making
conventional thermobarometry methods inoperable. Nevertheless, the
xenoliths have retained unusual metamorphic features which can help
unravelling their pre-entrapment metamorphic history. Pure anorthite
"patches" with magnetite inclusions are interpreted as pseudomorphs
after garnet. Since these patches are often found in contact with
clinopyroxene and rutile, these minerals may together represent a former
high-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 crustal
thickness of the Tibetan Plateau was already acquired in Eocene times.
Moreover, corundum-bearing assemblages, interpreted as pseudomorphs
after phlogopite, are best explained by kinetic disequilibrium under
high-temperature granulitic conditions, suggesting a short-lived
metamorphic event. Taken together, this set of new petrological and
geochronological data suggests that crustal granulitic metamorphism may
not b e related to the onset of a general warming of the Tibetan crust
sustained until the present, but rather to the punctual injection of mantlederived
melts in the crust before Eocene eruption.