U–Pb zircon dating, geochemical and Sr–Nd–Hf isotopic compositions of mafic intrusive rocks in the Motuo, SE Tibet constrain on their petrogenesis and tectonic implication

Abstract

Mafic intrusive rocks are widely exposed in the Motuo tectono-magmatic belt, southeast Lhasa terrane. LA–ICP-MS U–Pb zircon dating shows that they have magma crystallization ages of 69 and ca. 50Ma. These mafic intrusive rocks are characterized by variable SiO2 (44.60–57.60wt.%), high Al2O3 (17.19–20.86wt.%), and low MgO (1.85–5.38wt.%) with Mg# of 31–55. Their chemical composition is comparable with low-MgO high-Al basalts to basaltic andesites. They are enriched in LILEs (Rb, Ba, K) and LREE and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), with weakly evolved Sr–Nd–Hf compositions (whole-rock (87Sr/86Sr)0=0.7064 to 0.7086, εNd(t)=−3.41 to +1.22, and zircon εHf(t)=−3.8 to +6.4). The mafic rocks were derived from partial melting of metasomatized lithospheric mantle. Geochemical and Sr–Nd–Hf isotopic data show that they were insignificantly modified during magma emplacement. We provide a different secular evolution of the lithospheric mantle beneath the eastern part relative to the central part of the southern Lhasa terrane. Sr–Nd isotopic composition of the Motuo Late Cretaceous–Eocene mafic rocks argues that they were derived from partial melting of a relatively homogeneous and depleted lithospheric mantle. We propose that the Late Cretaceous delamination resulted in the replacement of ancient lithospheric mantle by the juvenile homogeneous lithospheric mantle in the eastern Lhasa terrane. The foundered ancient materials may subsequently re-fertilize the lithospheric mantle not only in the eastern Lhasa terrane but also in the surrounding areas.