摘要：

To better constrain the petrogenesis and tectonic setting of the granitic plutons in the Guaizihu region, NW China, this study presents a new analysis of zircon U–Pb ages, whole-rock geochemistry and Hf isotopes of syenogranites. The syenogranites are medium- to coarse-grained, with moderate to high SiO 2 (72.55–74.27?wt%) and Al 2 O 3 (13.02–13.75?wt%) contents and moderate total alkalis (Na 2 O?+?K 2 O, 7.58–7.98?wt%), which indicate that the syenogranites exhibit calc-alkaline and peraluminous characteristics. The low Ce, 10?000 Ga/Al, and A/CNK values and the negative correlation of SiO 2 and P 2 O 5 confirm that the syenogranites are of I-type origin. The whole-rock geochemistry demonstrates that the magma was generated by partial melting and experienced fractional crystallisation. Specifically, the correlations of Rb with Ba and Sr and the weak negative Eu anomalies indicate that the plagioclase has undergone fractional crystallisation. In addition, Eu anomalies and La/Sm–La relations show that both fractional crystallisation and partial melting contributed to the magmatic process. The positive εHf(t) values (0.11–9.78); enrichments in Rb, Th and other LILEs; depletions in Nb, Sr and other HFSEs; and the ratios of geochemical elements indicate that the syenogranites were generated from the partial melting of juvenile crust. The granites are inferred to have formed in a post-collisional extensional environment. Additionally, the U–Pb zircon ages (284.6?±?0.84?Ma and 269.4?±?1.2?Ma) of syenogranites suggest that final closure of the Paleo-Asian Ocean occurred before 285?Ma. KEY POINTS Granites in the Guaizihu region are dated at 284.6?±?0.84?Ma and 269.4?±?1.2?Ma, and the Paleo-Asian Ocean, where the Engger Us Ophiolite Belt is located, closed before 285?Ma. Granites are medium- to coarse-grained and highly fractionated I-type syenogranites. Granites have been derived from magma generated by the partial melting of juvenile crust followed by fractional crystallisation. Granites were formed in an intraplate extensional environment related to a post-collisional event.

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