Geochronology And Geochemistry Of Sangri Group Volcanic Rocks Of The Southern Lhasa Terrane: Implications For The Early Subduction History Of The Neo-tethys And Gangdese Magmatic Arc | 18649
Journal of Earth Science & Climatic Change
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The Sangri Group volcanic rocks are distributed along the southern edge of the Lhasa Terrane, on the northern side of the
Indus-Yarlung Zangbo suture zone. It consists of the Mamuxia and Bima formations and has long been considered to be
Late Jurassic to Early Cretaceous in age. In this presentation, for the first time zircon LA-ICP-MS U-Pb ages, whole-rock major
and trace element geochemistry, as well asSr-Nd isotope data for the Bima Formation volcanic rocks in Sangri County, Tibet
is described. Two samples collected from the Bima Formation volcanic rocks yield zircon U-Pb ages of 195?3 Ma and 189?3
Ma, respectively. These data suggest that the Bima Formation volcanic rocks formed during the Early Jurassic and not the
Later Jurassic-Early Cretaceous as previously reported. The volcanic rocks of the Bima Formation are composed dominantly of
basalt and andesite enriched in LILEs and LREEs and depleted in HFSEs, showing typical characteristics of arc volcanic rocks.
They also show positive εNd(t) (3.19-7.02) values and low initial
Sr (0.703182-0.705489) ratios, similar to the MORB of
the Indus-Yarlung Zangboophiolites, indicating that the Bima Formation was derived from a depleted mantle wedge that was
metasomatized by subducted oceanic slab-derived fluids. The magmas subsequently experienced juvenile crust contamination
and fractional crystallization during ascent. The magmas of the Bima formation volcanic rocks were thus generated by the
northward subduction of Neo-Tethys beneath the southern Lhasa Terrane as early as 195 Ma. Considering they were coeval
with a large volume of Late Triassic-Early Jurassic felsic intrusion within the Gangdese arc, it is concluded that the Gangdese
magmatic arc was initiated at a juvenile continental margin during Late Triassic-Early Jurassic, although a possible intra-
oceanic arc setting cannot be excluded.
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