B isotopes reveal Eocene mélange melting in northern Tibet during continental subduction

Abstract

Continental subduction is an important but poorly known means of transporting the Earth's crust into the mantle. Continental subduction has also been proposed to responsible for the formation of the Cenozoic intracontinental potassic rocks widespread in Tibet and uplift of the Tibetan Plateau. However, such ancient continental subduction only occasionally been identified and is controversial. In this contribution, we explore the use of boron and its isotopes, to provide new constraints on the origin of newly identified Eocene potassium-rich volcanic rocks in northern Tibet and potential subduction of continental crust. Our results indicate that the latest Eocene (ca. 34 Ma) potassium-rich volcanic rocks are characterised by moderate silica, adakite-like trace elements, enriched Sr-Nd-Pb and light boron isotope (δ11B = −20.12‰ to −7.95‰) signatures. This isotopically light signature of boron suggests a mélange source composed of mantle wedge peridotite and recycled dehydrated continental crust. The Eocene potassium-rich magmas were likely derived from low-degree partial melting of diapir mélange at high-pressure and medium temperature within subduction channel. Our work indicates potential obduction of the Tibetan plate over the Asian plate thus triggering intracontinental subduction before the end of Eocene. This continental obduction may have accommodated a huge amount of crustal material during Cenozoic rapid convergence between India and Asia. We conclude that during this process an isotopically light component is likely to have been transported into the mantle to form a distinct source region. B isotopes therefore have the potential to trace the continental crust recycling in large collisional orogens.