Postcollisional potassium (K)-rich magmatic rocks have important implications for unveiling the deep geodynamic processes active during the evolution of orogenic belts and the surface uplift of plateaus. However, their petrogenesis remains highly controversial. Here we report on zircon U-Pb and mineral 40Ar-39Ar age, zircon Hf-O isotope, mineral composition, whole-rock major and trace element composition, and Nd-Sr isotope data for the postcollisional K-rich lavas in the Gemuchaka area of the central Qiangtang Block (central Tibet). Age dating suggests that these lavas were generated in the late Early Oligocene (ca. 30 Ma). Two series of rocks were recognized: silica-undersaturated and -saturated lavas. Both series are geochemically characterized by continuous variation trends. All rocks are enriched in light rare earth elements (LREEs) relative to heavy REEs (HREEs) and depleted in Ta-Nb-Ti, with negligible Sr and Eu and obviously positive Pb anomalies. The rocks have higher Th/La (0.2−0.8) and Nb/Ta (18.6−19.8) values than asthenospheric mantle-derived rocks but slightly variable values of εNd(t) (−6.0 to −4.6), (87Sr/86Sr)i (0.7076−0.7083), and zircon εHf(t) (−7.2 to +0.8) and δ18O (7.1−8.5‰). We suggest that the parental magmas of these K-rich lavas were likely derived by variable partial melting of lithospheric mantle metasomatized by subducting continental sediment-derived melts with the addition of minor asthenospheric components. Element modeling results suggest that the basic end members were likely generated by fractionation processes dominated by olivine and minor clinopyroxene from parental magmas, and some other evolved rocks were generated by crystal (olivine, clinopyroxene, Fe-Ti oxides) fractionation from basic magmas. Combining geological and Cenozoic magmatic rock data in the Qiangtang Block, we propose that the Gemuchaka magmatism was most likely triggered by the delamination of lithospheric mantle thickened by early (Paleocene−Eocene) intracontinental subduction and subsequent asthenospheric upwelling.
Postcollisional delamination and partial melting of enriched lithospheric mantle: Evidence from Oligocene (ca. 30 Ma) potassium-rich lavas in the Gemuchaka area of the central Qiangtang Block, Tibet
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Quan Ou, Qiang Wang, Derek A. Wyman, Chunfu Zhang, Lu-Lu Hao, Wei Dan, Zi-Qi Jiang, Fu-Yuan Wu, Jin-Hui Yang, Hai-Xiang Zhang, Xiao-Ping Xia, Lin Ma, Xiao-Ping Long, Jie Li; Postcollisional delamination and partial melting of enriched lithospheric mantle: Evidence from Oligocene (ca. 30 Ma) potassium-rich lavas in the Gemuchaka area of the central Qiangtang Block, Tibet. GSA Bulletin doi: https://doi.org/10.1130/B31911.1
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