Origin of the ca. 90 Ma magnesia-rich volcanic rocks in SE Nyima, central Tibet: Products of lithospheric delamination beneath the Lhasa-Qiangtang collision zone

Abstract Bulk-rock major and trace element, Sr–Nd–Hf isotope, zircon U–Pb age, and zircon Hf isotopic data of the Late Cretaceous Zhuogapu volcanic rocks in the northern Lhasa subterrane provide a new insight into tectonic processes following the collision of the terrane with the Qiangtang zone. SHRIMP zircon U–Pb dating reveals that the Zhuogapu volcanic rocks crystallized at ca. 91 Ma, postdating the development of a regional angular unconformity between the Upper Cretaceous and the underlying strata in the Lhasa–Qiangtang collision zone. Compared to the Andean arc-type andesites and dacites, the Zhuogapu volcanic rocks are characterized by higher MgO of 2.78–5.86 wt.% and Mg# of 54–64 for andesites and MgO of 2.30–2.61 wt.% and Mg# of 55–58 for dacites. Eight andesite samples have whole-rock (87Sr/86Sr)i of 0.7054–0.7065, εNd(t) of − 3.2 to − 1.7, and εHf(t) of + 3.8–+ 6.4, similar to those of the three dacite samples with (87Sr/86Sr)i = 0.7056–0.7060, εNd(t) of − 2.7 to − 2.2, and εHf(t) of + 5.6–+ 7.0. Thirteen analyses from a dacite sample give positive zircon εHf(t) of + 5.6 to + 8.7. These signatures indicate that the Zhuogapu Mg-rich andesites were most likely derived from partial melting of a delaminated mafic lower crust (including the lowermost crust straddling the northern and central Lhasa subterranes) that led to the generation of the Zhuogapu primary melts with adakitic signatures and small negative εNd(t). Such melts subsequently experienced interaction of melt-asthenospheric mantle peridotite followed by the modification of highly fractionated magmas in shallow crustal magma chamber. Hornblende-controlled fractionation results in the change of geochemical composition from Mg-rich andesitic to Mg-rich dacitic magmas. Field observations, together with geochronological and geochemical data, indicate that the Zhuogapu Mg-rich volcanic rocks and coeval magmatism in the northern Lhasa subterrane may be the result of thickened lithospheric delamination following the final Lhasa–Qiangtang amalgamation.