Contribution of continental subduction to very light B isotope signatures in post-collisional magmas: Evidence from southern Tibetan ultrapotassic rocks

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 δ11B (−9.0 to −2.5‰) values, the TYXR post-collisional ultrapotassic rocks with much lower B/Nb (0.05-0.85) and δ11B (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 δ11B 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 δ11B values than MORB (e.g., the Armenia post-collisional mafic rocks). Given the low δ11B 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.