U–Pb zircon dating, geochemical and Sr–Nd–Hf isotopic compositions of Motuo quartz–monzonite: Implication for the genesis and diversity of the high Ba–Sr granitoids in orogenic belt

Abstract

Early Paleogene granitoids in Southern Lhasa subterrane have been widely investigated and many petrogenesis and geodynamic models have been proposed in the past few years. However, contemporaneous granitoids in the Motuo tectono-magmatic belt, southeast Lhasa terrane, are still limitedly studied. Here we present the petrology, zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Hf isotope data of the Damu and 52K quartz–monzonite in the Motuo area. LA-ICP-MS U–Pb zircon dating shows that they have magma crystallization ages of 49 and 69Ma, respectively. The Damu quartz–monzonite (SiO2=63.76–68.33wt.%) is high-K calc-alkaline (K2O=2.54–4.02wt.% with K2O/Na2O=0.59–1.09) and metaluminous to weakly peraluminous (A/CNK=0.99–1.07). The 52K quartz–monzonite (SiO2=61.12–66.12wt.%) shows slightly higher K2O contents (3.80–5.28wt.% with K2O/Na2O=1.03–1.45) and metaluminous series (A/CNK=0.96–1.00). The analyzed samples are characterized by high Ba (850–2573ppm), Sr (534–986ppm) contents, and fractionated REE patterns ((La/Yb)N=22–72 and (Sm/Yb)N=4.55–8.24). These geochemical features are comparable with those of high Ba–Sr granite. They display weakly evolved Sr–Nd–Hf compositions (whole-rock (87Sr/86Sr)0=0.7068 to 0.7086, εNd(t)=−4.20 to −3.41, and zircon εHf(t)=−5.2 to −0.9). Geochemical and Sr–Nd–Hf isotopic data reflect that the Damu and 52K quartz–monzonite represent residual magma from AFC processes of lithospheric mantle-derived mafic melts. The over-thickened lower crust in the eastern Lhasa terrane had been delaminated during ca. 83–70Ma, which led to the replacement of ancient lithospheric mantle by the juvenile lithospheric mantle. The juvenile mantle wedge in the study area was suspected to be metasomatized by melts that were derived from the foundering arc root, rather than the subducted sediments. Thus, the early Paleogene high Ba–Sr magmas from the SE Lhasa terrane may provide evidence for recycling of continental lithosphere by density destruction.