3 years ago

Leucogranites in Lhozag, southern Tibet: Implications for the tectonic evolution of the eastern Himalaya

Leucogranites in Lhozag, southern Tibet: Implications for the tectonic evolution of the eastern Himalaya
Petrogenesis of the Himalayan leucogranite is strongly influenced by P-T-X H2O conditions which are associated with the tectonic evolution of Himalayan orogen. In this article, we present petrological, geochronological and geochemical results of the Lhozag leucogranites that crop out alongside the South Tibetan Detachment System (STDS) in the east of Himalaya. Zircon U–Pb dating revealed three episodes of leucogranitic magmatism in Lhozag at 17.8 ± 0.1 Ma, 15.1 ± 0.1 Ma, and 12.0 ± 0.1 Ma, respectively. The Lhozag leucogranites show relatively low εNd(t), zircon εHf(t) and high initial 87Sr/86Sr ratios, which are similar to the High Himalayan Crystalline Series (HHCS), indicating they were derived from the HHCS. The characteristics of relatively high Na2O, Rb, Rb/Sr ratios and low CaO, MgO, TFe2O3, TiO2, Sr contents indicate that both the ca. 18 Ma Lhozag tourmaline leucogranites and ca. 15 Ma Lhozag two-mica granites were derived from the fluid-absent muscovite-dehydration melting of metasediments. The opposite geochemistry characteristics of the ca. 12 Ma Khula Kangri two-mica granites imply that these granites are derived from the fluid-present melting of metasediments. Four Khula Kangri two-mica granite samples with relatively lower TiO2, TFe2O3, MgO and CaO contents, higher Rb concentration and Rb/Sr ratio could be evolved from the Khula Kangri two-mica granites with relatively lower Rb/Sr ratios. The melting behaviors of the Lhozag leucogranites varied from fluid-absent muscovite-dehydration melting to fluid-present muscovite melting, implying there were P-T-X H2O variations in the deep crustal. The tectonic evolution would give rise to variation of P-T-X H2O variation, and subsequent transformation of melting behavior. Our new results display the transformation of melting behaviors of the Lhozag leucogranites, which implies the tectonic evolution from earlier N–S extension to later E–W extension in the eastern Himalaya at ca. 12 Ma.

Publisher URL: www.sciencedirect.com/science

DOI: S0024493717303250

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