Identifying the leucogranites in the Ailaoshan-Red River shear zone:
Constraints on the timing of the southeastward expansion of the
Tibetan Plateau
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Abstract
The uplift of the Tibetan Plateau significantly affected the global climate system. However, the timing of its uplift
and the formation of its vast expanse are poorly understood. The occurrence of two types of leucogranites (the
two-mica leucogranites and garnet-bearing leucogranites) identified in the Ailaoshan-Red River (ASRR) shear
zone suggests an extension event in the southeastern Tibetan Plateau. The age of these leucogranites could be
used to constrain the timing of uplift and southeastward expansion of the plateau. Petrography, geochronology
and geochemistry investigations, including Sr–Nd isotope analysis, were conducted on the two-mica leucogranites
and garnet-bearing leucogranites from the ASRR shear zone. LA-ICP-MS zircon U–Pb dating indicates that these
rocks were emplaced at ~27 Ma, implying that the Tibetan Plateau had already achieved maximum uplift prior to
the late Oligocene. It subsequently started to expand southeastward as a result of crustal flow. Compared to classic
metapelite-derived leucogranites from Himalaya, the two-mica leucogranites show high K2O/Na2O (1.31–1.92),
low Rb/Sr, CaO, lower 87Sr/86Sr ratios (0.7089–0.7164) and higher εNd(t) (8.83 to 3.10). This whole-rock
geochemical characteristics likely indicates a mixing source origin, composed predominantly of amphibolite
with subordinated metapelite, which is also evidenced by 87Sr/86Sr vs. εNd(t) diagram. However, The garnetbearing
leucogranites with high SiO2 contents (72.25–74.12 wt.%) have high initial 87Sr/86Sr ratios
(0.7332–0.7535) and low εNd(t) (16.36 to 18.98), indicating that they are derived from the source comprised
of metapelite and results of fluexed muscovite melting under lower crustal level, which is also evidenced by the
Rb–Sr–Ba systematics. These leucogranites formed from partial melting of the thickened lower crust, which
resulted in the formation of granitic melt that weakened the crust. The weakened crust aided the left-lateral strikeslip
movement of the ASRR shear zone, triggering the escape of the Indochina terrane in the southeastern Tibetan
Plateau during the late Oligocene.
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