Petrogenesis and tectonic setting of the Queershan composite granitic pluton,eastern Tibetan Plateau: Constraints from geochronology,geochemistry and Hf isotope data

The Queershan composite granitic pluton is located in the north of the late Paleozoic Yidun arc collision-orogenic belt, eastern Tibetan Plateau. The main rock types are coarse-grained porphyritic alkalic-monzonite granite with minor fine-grained porphyritic monzogranite and granodiorite distributed in the eastern and southwestern regions. Here we report their zircon U-Pb ages and geochemical data. The intrusive contact relations indicate that granodiorite was formed earlier than the alkalic-monzonite granite (105.9±1.3 Ma) and monzogranite (102.6±1.1 Ma). These suggest that the Queershan composite granitic pluton was formed through three-stage magmatic events. The alkalic-monzonite granite (105.9±1.3 Ma) and monzogranite (102.6±1.1 Ma) are characterized by high SiO₂ (73.5%–77.7%), K₂O+Na₂O (6.9%–8.5%), Ga/Al ratios (2.6–3.4) and low Al₂O₃ (11.8%–14.5%), CaO (0.25%–1.5%), MgO (0.18%–0.69%), negative Ba, Sr and Eu anomalies, showing A-type granite affinities. The granodiorite exhibits lower SiO₂, P₂O₅ and K₂O+Na₂O contents, but higher Al₂O₃, CaO and MgO contents than alkalic-monzonite granite and monzogranite, showing I-type granite affinity. ¹⁷⁶Hf/¹⁷⁷Hf ratios of the alkalic-monzonite granite and the monzogranite are 0.282692–0.282749 and 0.282685–0.282765, respectively, and with similar ? _Hf(t) values (?0.56 to 1.43 and ?0.87 to 1.90 respectively). They also present similar T _DM2 model ages (1.04–1.22 and 1.07–1.2 Ga respectively), indicating they may be sourced from a similar rock source, mostly like Kangding Complex. The homogeneity of the Hf isotopic compositions and the absence of the MMEs demonstrate that little depleted mantle materials have contributed to the source. We propose that the Mesoproterozoic crust materials of the Yangtze Craton exist beneath the Yidun arc terrane and support it was a dismembered part of the Yangtze Craton. The A-type granites of Queershan composite granitic pluton are most probably related to the closure of the Bangong-Nujiang Tethys ocean.