Westward-younging high-Mg adakitic magmatism in central Tibet: Record of a westward-migrating lithospheric foundering beneath the Lhasa-Qiangtang collision zone during the Late Cretaceous

Magmatic records with high Mg and low Y signatures provide important insights into the deep mantle dynamics that dictate the evolution of ancient orogenic belts. This paper reports the presence of felsic intrusive dikes with high Mg and low Y signatures intruding the Baingoin Batholith (central Tibet) at ca. 94 Ma. These dikes show adakitic signatures, including high Sr and low Y and Yb contents and high Sr/Y and (La/Yb)N ratios. The adakitic dacitic dikes can be grouped into high Mg (Mg# = 48-52) with positive zircon εHf(t) (8.3-12.4) and whole-rock εNd(t) (0.27) and low Mg (Mg# = 37-45) with positive zircon εHf(t) (7.2-13.5) varieties. These distinct geochemical features, along with geological observations, show that the Baingoin high-Mg adakitic dacitic dikes were most likely derived from the partial melting of a foundering juvenile lower crust and subsequent melt-mantle reaction, whereas the low-Mg adakitic dacitic dikes can be interpreted as resulting from the partial melting of a thickened lower crust that did not delaminate. The generation of these adakitic rhyolitic dikes can be attributed to hornblende-dominated fractional crystallization of the adakitic dacitic melts. A decrease in Y and heavy rare earth element contents of magmatic records from 120-100 Ma to 95-80 Ma within the Lhasa-Qiangtang collision zone suggest an increase in crustal thickness with time. The volumetrically minor westward-migrating high-Mg adakitic magmatism can readily be interpreted as the consequences of the westward-migrating foundering of the lithosphere triggered by Rayleigh-Taylor instabilities along the Lhasa-Qiangtang collision zone during the period 94-80 Ma.