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The nature and timing of post-collisional crustal thickening and its link to surface uplift in the eastern Lhasa block of the southern Tibetan plateau remain controversial. Here we report on Cenozoic magmatism in the Wuyu area of the eastern Lhasa block. The Eocene (ca. 46 Ma) trachyandesites and trachydacites show slight fractionation of rare earth elements (REE), slightly negative Eu and Sr anomalies, and relatively homogeneous Sr-Nd and zircon Hf isotopes (87Sr/86Sr(i) = 0.7050-0.7063, εNd(t) = -0.92 to -0.03, εHf(t) = +2.6 to +4.8). Previous studies have suggested Neo-Tethys oceanic slab break-off at 50-45 Ma; thus, the Wuyu Eocene magmatism could represent a magmatic response to this slab break-off and originate from relatively juvenile Lhasa crust. The Miocene (ca. 15-12 Ma) dacites and rhyolites have adakitic affinities, e.g., high Sr (average 588 ppm), Sr/Y (29-136), and La/ Yb (30--76) values, low Y (4-12 ppm) and Yb (0.4-0.9 ppm) contents, and variable Sr-Nd and zircon Hf isotopes (87Sr/86Sr(i) = 0.7064-0.7142, εNd(t) = -11.7 to -3.7, εHf(t) = -3.2 to +4.5). Their more enriched Sr-Nd-Hf isotopes relative to the Eocene lavas indicate that they should be derived from mixed Lhasa lower crust comprising juvenile crust, ultrapotassic rocks, and probably Indian lower crustderived rocks. This study has also revealed the transformation from Eocene juvenile and thin crust with a thickness of <40 km to Miocene mixed and thickened crust with a thickness of >50 km. Combined with published tectonic data, we suggest that both lithospheric shortening and magma underplating contributed to eastern Lhasa block post-collisional crustal thickening. Given the spatial-temporal distribution of eastern Lhasa block magmatism and regional geology, we invoke a post-collisional tectonic model of steep subduction of the Indian plate and subsequent westward-propagating plate break-off beneath the eastern Lhasa block, which caused the surface uplift. [ABSTRACT FROM AUTHOR]

The role played by felsic igneous magmatism in crustal differentiation is of importance for the evolution and habitability of Earth system. However, the petrogenesis of felsic rocks and their genetic links to mafic magmas remain controversial, hindering our understanding of crustal differentiation processes. Cenozoic post-collisional felsic potassic rocks occur widely in the well-known Himalaya-Tibet orogen and are intimately associated, in space and time, with mantle-derived ultrapotassic rocks, which thus provide a clear case study to investigate the genesis of felsic rocks and their genetic relationship with mafic rocks. Here we report zircon U Pb ages and Hf O isotopes, mineral major elements, and whole-rock elemental and Sr-Nd-Pb isotopic compositions for the TangraYumco high-silica potassic rocks (trachytes) in central Lhasa block of southern Tibet. The results, combined with literature data of the TangraYumco ultrapotassic rocks, can help better illuminate their genesis and the role of ultrapotassic rocks in their generation. The trachytes erupted in the Miocene (~13 Ma), coeval with ultrapotassic lavas in this area. They have very enriched Sr-Nd-Pb-Hf-O isotope compositions similar to those of ultrapotassic rocks. However, they display more fractionated LREEs (light rare earth elements) than ultrapotassic rocks. The trachytes also show nearly constant Y and Yb contents with increasing SiO 2. This indicates that they were not derived by fractional crystallization from ultrapotassic magmas or by mixing between ultrapotassic and ancient crust-derived felsic magmas. Instead, given their lower LREE contents than ultrapotassic rocks and occurrence of clinopyroxene antecrysts, we suggest that the TangraYumco trachytes were likely produced by remelting of newly-formed, Sr-Nd-Pb isotopically enriched, and K-rich cumulates that crystallized from ultrapotassic magmas. Moreover, taking into account regional magmatism and tectonics, we propose that the TangraYumco-Xuruco post-collisional coeval magmatism and rift system represent a response to tearing of subducted Indian continental slab. Finally, in combination with the nature of post-collisional magmatism in the Lhasa block, we suggest that in addition to reworking of juvenile and ancient crust, underplating of post-collisional mafic magmas and remelting of their cumulates could be an important way to differentiate continental crust in collisional orogeny zones. • TangraYumco Miocene trachytes have a K-rich, isotopically enriched crustal origin. • The source components are cumulates that crystallized from ultrapotassic magmas. • TangraYumco coeval rift and magmatism are a response to Indian continental tearing. • Cumulate melting can differentiate continental crust in collisional orogenic zones. [ABSTRACT FROM AUTHOR]