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Magnesium and Lithium Isotopic Evidence for a Remnant Oceanic Slab Beneath Central Tibet
Author(s) -
Tian HengCi,
Teng FangZhen,
Hou ZengQian,
Tian ShiHong,
Yang Wei,
Chen XinYang,
Song YuCai
Publication year - 2020
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2019jb018197
Subject(s) - geology , geochemistry , mantle (geology) , zircon , magmatism , partial melting , subduction , oceanic crust , crust , adakite , eclogite , tectonics , paleontology
The voluminous Eocene postcollisional potassium‐rich magmatism in central Tibet is crucial in understanding the deep geodynamic process and related uplift history of the Tibetan Plateau. Here, we present an integrated study of whole‐rock Sr‐Nd‐Mg‐Li isotopes and zircon U‐Pb ages on two suites of lavas from Duocaima and Nariniya in the northern Qiangtang block. The Duocaima high‐K calc‐alkaline trachyandesites were formed at ~35 Ma and are enriched in light rare earth elements and large ion lithophile elements. They have slightly lower δ 26 Mg (−0.41‰ to −0.33‰) and δ 7 Li (+1.0‰ to +2.6‰) relative to the mantle. Together with their high 87 Sr/ 86 Sr (i) (0.7066–0.7067) and low ɛ Nd (t) (−1.96 to −1.61) values, they most likely originated from the lithospheric mantle previously metasomatized by melts from subducted carbonate‐bearing sediments. Model calculation suggests that ~7–11% sediments were added into the mantle via melting. By contrast, the contemporaneous Nariniya adakitic trachytes have higher δ 26 Mg (−0.13‰ to −0.02‰) and δ 7 Li (+3.3‰ to +5.4‰) values, as well as higher Pb/Ce and Ba/La ratios, which most likely resulted from the thickened lower crust that was metasomatized by fluids released from downgoing Songpan‐Ganzi oceanic slab. Our combined isotopic study suggests the existence of a remnant oceanic slab beneath central Tibetan Plateau during the late Eocene and implies that the initial uplift of central Tibet was before ~38 Ma.