Late Cretaceous bimodal volcanic rocks in Shuanghu induced by lithospheric delamination beneath the Southern Qiangtang, Tibet

Abstract

<p>The southern Qiangtang (SQT) in central Tibet is characterized by sporadic Late Cretaceous magmatic rocks whose petrogenesis and tectonic implications remain unclear. Here, new zircon U–Pb ages and Hf isotope compositions, together with whole-rock element and isotope data have been collected for the Duoma volcanic rocks from the SQT, central Tibet. Zircon U–Pb dating indicates that the Duoma volcanic rocks crystallized at ca. 98 Ma. The basalts of the Duoma volcanic rocks have low SiO₂ (51.49–53.36 wt%), high total alkalis (4.65–6.25 wt%), and display OIB-like isotopic signatures (⁸⁷Sr/⁸⁶Sr)_<i>i</i> = 0.7051 to 0.7058, ε_Nd(<i>t</i>) = +0.04 to +1.10), overall similar to intra-plate alkali basalts. Moreover, basalts exhibit slight depletion of high-field-strength elements (HFSEs; e.g., Nb, Ta, and Ti), but enrichment of light rare earth (La_N/Yb_N = 6.7–7.6) with no negative Eu anomalies (Eu/Eu* = 0.91–0.99). These geochemical features, together with their high La/Nb (1.56–1.70) and Zr/Hf (44.70–45.87) but low La/Ba (0.08–0.09) and Ca/Al (0.43–0.65) ratios, suggest that the Duoma basalts originated from partial melting of garnet clinopyroxenite that was metasomatized by depleted mantle-derived melts. The coexisting rhyolites have high SiO₂ (69.78–69.92 wt%) and exhibit significant enrichment in large-ion lithophile elements (LILEs) and depletion in HFSEs (Nb, Ta, and Ti). Furthermore, the Duoma rhyolites are characterized by the decoupled Nd (ε_Nd(<i>t</i>) values ranging from 4.4 to 11.1) and Hf (ε_Hf(<i>t</i>) values ranging from 0.7 to +2.7) compositions, that character indicates that they were probably derived from the partial melting of the lower continental crust. The compositional bimodality and heterogeneous source of the Late Cretaceous volcanism indicate that they were formed in an extension setting, presumably triggered by lithospheric delamination beneath the SQT.</p>