Early Jurassic I-type garnet leucogranite in the Siguniangshan pluton, eastern margin of the Songpan-Ganze terrane (NE Tibet), and its tectonic implications

Abstract

For the first time, the highly evolved I-type garnet leucogranite was discovered in the Siguniangshan pluton, eastern margin of the Songpan-Ganze terrane, northeastern Tibet. New zircon U-Pb dating and in situ Hf isotope analyses of the garnet leucogranite and the country rocks suggest that continuous postorogenic magmatic activity, including at least three successive magmatic events (~204 Ma, ~196 Ma and ~185 Ma), occurred and persisted for ~20 Myr during the Late Triassic to Early Jurassic in the Siguniangshan pluton. Integrating these results with those of previous studies, the geochemical analyses and petrographic features reveal that intense crystal fractionation played the primary role in producing highly evolved melts (high silica contents, A/CNK > 1.1, pronounced depletions in Ba, Sr, and Eu, extremely high Rb/Sr ratios, low Nb/Ta ratios and markedly “non-chondritic” Y/Ho and Zr/Hf ratios), while hydrothermal alteration played a secondary role during the final phase. Geochemical calculations suggest that a high degree of mineral fractionation (~90%) was required, which agrees with the occurrence of the vein-like garnet leucogranite dike and the low crystallization temperature. Considering the hydrothermal alteration, the degree of mineral fractionation required might be lower than the calculated result. The initial garnet leucogranite melt was possibly derived from partial melting of the Neoproterozoic basement or early-formed intrusions. Rapid uplift and extra shear heating along the decollement promoted partial melting. After intense crystal fractionation and slight hydrothermal alteration, a very small amount of highly evolved residual melt intruded the early-formed biotite granites as a dike.