Geochronology,Geochemistry,Petrogenesis And Geodynamics Of Early Cenozoic Granitoids In Chacangka,Southern Tibet

Studies of granitoids can provide important knowledge regarding the composition of continental crust and the geodynamic processes of continental growth.Nevertheless,the mechanisms of formation of granitoids during continental collision remain uncertain.In this paper,we present new data on zircon U–Pb–Hf isotope and whole-rock Sr–Nd isotope compositions and major-and trace-element data for granitoids from the Chacangka area of the central Lhasa Terrane,southern Tibet.The Chacangka quartz monzonites crystallized at ca.61 Ma,whereas the monzonites and granites were emplaced at ca.51 Ma.The quartz monzonites are peraluminous and have high⁸⁷Sr/⁸⁶Sri ratios(0.7129–0.7131),and their low?_Nd(t)(-7.65 to-7.10)and zircon?Hf(t)(-9.3 to-7.5)values are more depleted than those of ancient crustal basement beneath the central Lhasa Terrane,which suggests that they originated from a mixed source of juvenile and ancient lower crust.Compared with the quartz monzonites,the monzonites have much higher contents of Mg O(2.34 to 3.62 wt.%)and values of Mg#(42.3 to46.3)and zircon?_Hf(t)(–6.9 to 1.9)and show more depleted Sr–Nd isotopes,indicating that more juvenile materials were involved in their source.The granites have Sr–Nd isotope compositions that are similar to those of the coeval monzonites but have higher Si O₂(70.36-71.48 wt.%)contents,lower Ca O(1.21–1.61 wt.%)and Mg O(0.55–0.70wt.%)contents,and lower values of Mg#(30.8–31.7),suggesting that they were differentiates of the monzonites.The elemental and isotopic compositions of the Chacangka granitoids record a marked transformation from 61 to 51 Ma,probably reflecting breakoff of Neo-Tethyan oceanic slab.Neo-Tethyan slab breakoff not only induced the upwelling of deep material and eruption of magma but also caused re-melting and destruction of ancient crustal basement of the central Lhasa Terrane.