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Collisional zones are commonly considered as important regions for crustal reworking, but the reworking mechanism remains debated. The well-known Himalayan-southern Tibetan orogen, built by India-Asia collision and convergence, has the thickest continental crust on Earth and is therefore an ideal region for studying crustal reworking during collisional orogenesis. Here we revisit the Miocene high-silica potassic rocks (trachytes) in the Konglong area of the central Lhasa block, southern Tibet. Integrated studies of geochronology, mineral compositions, bulk-rock major- and trace-element geochemistry, and Sr-Nd-Pb-Hf-O isotopes unequivocally indicate that the Konglong trachytes formed by mixing between enriched mantle-derived ultrapotassic and thickened ancient crust-derived magmas. Combined with post-collisional magma mixing recently identified in the southern Lhasa block, we suggest that magma underplating and subsequent mantle-crust interaction (i.e., the matter and energy transfer from the mantle to the crust) has been a common and important crustal reworking process in southern Tibet during Indian continental subduction. This process may be related to Indian plate flat subduction and subsequent foundering during the post-collisional stage. In combination with the nature of Cenozoic magmatism in the Himalaya block, we suggest that in addition to partial melting of the subducted continental crust, magma underplating and subsequent crust-mantle mixing beneath the obducted continent has also played an important role in crustal reworking of the collisional zone. • Konglong Miocene trachytes formed by mixing of crustal and ultrapotassic magmas. • Magma underplating and mantle-crust interaction induced Lhasa crustal reworking. • Indian plate flat subduction and foundering controlled crustal reworking. [ABSTRACT FROM AUTHOR]

The question of whether subducted continental crust can be recycled into post-collisional magmatism in continental collisional zones remains controversial. Post-collisional mantle-derived ultrapotassic rocks are widespread in western part of central Lhasa block (WCL) of the Tibet-Himalaya orogen and show arc-type trace-element signatures and very enriched Sr Nd isotopes, which have been explained by the recycling of subducted Indian continental crust. However, due to the lack of late Cretaceous mafic magmatism in the WCL, the nature of the ancient subcontinental lithospheric mantle (SCLM) beneath the WCL has not been well constrained. It is therefore still unclear whether the enriched components of these ultrapotassic rocks were derived from subducted Indian continental crust or inherited from the ancient SCLM. Here we report the mafic dykes from the TangraYumco area in the WCL, which formed at ∼90 Ma (whole-rock 40Ar 39Ar, and zircon and titanite U Pb ages). The diabase-porphyrites and diorite-porphyrites are similar to intraplate Nb-enriched basalts and magnesium adakitic rocks, respectively. This rock association reveals the lithospheric foundering process. Furthermore, the diabase-porphyrites generated by the interaction between the ancient SCLM and asthenosphere provide important constraints on the nature of the ancient SCLM, which shows distinct Nd isotopes to the post-collisional ultrapotassic rocks. In addition, the NW Lhasa block was in a rear-arc environment since the late Cretaceous and thus this SCLM was not significantly influenced by the ongoing northward subduction of the Indus-YarlungZangbu Neo-Tethys ocean. Finally, we propose that the post-collisional ultrapotassic rocks cannot be sourced from the ancient SCLM, but instead were derived from a relatively depleted mantle (e.g., juvenile lithospheric mantle) metasomatized by subducted Indian continental crust. Thus, this study confirms the recycling of subducted continental materials in continental collisional orogens. • The ca. 90 Ma mafic dykes in central Lhasa block of southern Tibetan plateau reveal the lithospheric foundering process. • The diabase-porphyrites formed by asthenosphere-ancient lithospheric mantle interaction constrain the nature of this SCLM. • This study confirms the recycling of subducted continental materials in continental collisional orogens. [ABSTRACT FROM AUTHOR]

Understanding the subduction and recycling of continental crust is crucial for reconstructing the long-term evolutionary history of Earth's mantle and crust. The Himalaya-Tibet orogen is arguably the world's best natural laboratory for investigating these processes. Cenozoic post-collisional ultrapotassic volcanic rocks are common in the Lhasa block of southern Tibet and they can provide important clues to crust-mantle interaction in a well-characterized continental collision zone. Understanding the sources and processes that generated these lavas can contribute to our understanding of the thermal and compositional characteristics of the deep mantle and geodynamic processes in this region, including Indian continental subduction. In this contribution, we report Sr-Nd-Pb-O-B isotope and elemental chemistry data for post-collisional (13-11 Ma) ultrapotassic rocks from the TangraYumco-Xuruco rift (TYXR) in the Lhasa block. The arc-like trace-element signatures and markedly enriched Sr-Nd-Pb-O isotope compositions indicate that these mafic rocks originate from a mantle source containing recycled crustal components. Unlike pre-collisional (∼64 Ma) ultrapotassic rocks in the Lhasa block with arc-like B/Nb (0.85-1.89) and δ 11 B (−9.0 to −2.5‰) values, the TYXR post-collisional ultrapotassic rocks with much lower B/Nb (0.05-0.85) and δ 11 B (down to −20.5‰) values resemble Miocene K-rich volcanic rocks from western Anatolia. These western Anatolian rocks have been formed by either progressive dehydration of a stalled slab or deep-subducted continental crust. However, some TYXR samples have lower B/Nb ratios than MORB, consistent with a fluid-starved source. These markedly negative δ 11 B in conjunction with low B/Nb cannot be explained by the addition of melts from oceanic sediments, which generally yield lower B/Nb but higher δ 11 B values than MORB (e.g., the Armenia post-collisional mafic rocks). Given the low δ 11 B of Indian upper continental crust and its similar Sr-Nd-Pb isotopic signatures to the post-collisional lavas, it is clear that the post-collisional ultrapotassic rocks in the Lhasa block contain a significant component derived from subducted Indian continental crust. Combined with the temporal evolution of regional magmatism, tectonics and geophysical data, we propose that the break-off and tearing of subducted Indian continental slab induced post-collisional magmatism in the Lhasa block. Our case study provides evidence that continental subduction contributes to very light B isotope compositions of post-collisional magmas, which suggests that B isotopes have the potential to discriminate between oceanic subduction and continental subduction. • Post-collisional ultrapotassic rocks in southern Tibet have very low B/Nb and δ 11 B. • B isotopes of these ultrapotassic rocks support contribution from Indian continental crust. • Melting of subducted Indian continental crust and interaction with overlying mantle generated these rocks. • B isotopes have the potential to discriminate between oceanic and continental subduction. [ABSTRACT FROM AUTHOR]